Research and analysis

Volumetric Modular Construction research

Published 26 November 2024

Applies to England

Report finalised July 2022
Published 26 November 2024

Authors:

Jennifer Brennan, Clare Vokes, Terry Massey
Harlow Consulting

Mila Duncheva, Wojciech Plowas, Andrew Livingstone, Alasdair Reid, Julio Bros Williamson, Robert Hairstans
Edinburgh Napier University

1. Executive summary

Research context

Building on a pledge to support Modern Methods of Construction (MMC) in its Autumn Budget of 2017, the 2019 Conservative party manifesto reaffirmed the target of 300,000 homes a year by the mid-2020s and made explicit commitment to support MMC. The need to accelerate housebuilding is a critical driver for greater adoption of Modern Methods of Construction (MMC). Responding to this need, the Department for Levelling Up, Housing and Communities (DLUHC) has a programme of work to support MMC, including through its housing funding and land programmes, and to tackle strategic barriers, notably the lack of product standardisation across the industry and the difficulties in obtaining product warranties - and therefore insurance and mortgages.

This policy commitment towards greater use of MMC, however, is coupled with the recognition that the existing regulatory framework was set out in relation to traditional construction. There may therefore be aspects of the system of regulation which are not tailored to MMC, or gaps which could result in issues with building quality, performance, and safety.

The aim of this research is therefore to investigate potential risks to safety and performance posed by volumetric construction and identify potential options for mitigating those risks.

The scope of this research is limited to one type of MMC – offsite manufactured volumetric construction. Volumetric construction is a type of offsite construction, whereby fully enclosed, six-sided building modules – i.e., floor, four walls and ceiling – are manufactured offsite in a factory and assembled onsite, most commonly by stacking and/or joining together. Modules can also be five-sided, without a floor or roof. Once site installation is complete, in most cases, buildings will need only limited additional work to be considered complete and ready to use.

This research has identified risks that may occur across each stage of the modular construction lifecycle, and cross-cutting risks which underpin every stage. There is not enough evidence to estimate the frequency with which these risks occur, and it should not be construed that this report is stating that all these risks occur on every volumetric construction project. While the scope of this research relates to risks in volumetric construction, it is emphasised that many of the risks identified are also present in traditional construction.

Key findings

Comprehensive pre-project planning is essential to volumetric construction. Any decision to use volumetric should be made at an early stage to allow sufficient time for design and enable early engagement with the supply chain. This does not always happen; clients may take the decision to use volumetric too late, which can have ramifications for the remainder of the project delivery.

The primary risk at the outset is lack of clear ownership and accountability. Often insufficiently defined, this risk can trigger a ripple effect of significant deficiencies in process and project management affecting the whole programme or project. Rather than a joined-up process, there can be gaps; ownership of the whole process from start to finish is often missing – there may be multiple owners in the design team, in the factory, in logistics and onsite. This can be true even within vertically integrated organisations. This risk is common in traditional construction as well, but the risk is more prominent in volumetric construction at the critical interface between offsite and onsite, when modules from the factory are transported to site for assembly.

Liability and accountability can accordingly be “sliced and diced” – problems, associated risk and responsibility for resolution are passed from organisation to organisation, and/or person to person. This can also partly stem from contracts which are not suited to volumetric construction; standard contract forms were developed for traditional construction and do not typically allow for c.70% of the value to be in the manufacturing package of the programme.

A successful volumetric construction programme relies on a collaborative and integrated approach to project management. Risks include lack of effective information sharing and absence of key personnel from the outset, for example a structural fire engineer. Expertise in structural fire engineering is required to align with the design and manufacturing/construction teams at the earliest point in the process, or there is a risk that the design will not fully consider all required fire safety details.

A notable advantage of volumetric construction is that factory manufacturing processes are predominantly underpinned by comprehensive quality assurance, testing, and inspection procedures. This means that any defects or issues can be readily identified and corrected before modules are transported to site. Analysis of the issue at source can prevent it from recurring, enabling a culture of continuous improvement.

However, there is a risk prior to this stage, stemming from insufficient expertise in Design for Manufacture and Assembly (DfMA). Design errors/flaws can also feature in traditional construction, but in volumetric construction the impact may be more wide-reaching, as defects stemming from design issues can be ‘built in’ at the point of manufacture, and subsequently replicated across all units if defects are not identified during the process of assembly onsite. As with quality and design flaws in traditional construction, over the longer-term - if not mitigated - this can create risks for building structural integrity, performance, and safety.

A highly critical risk is the point of interface between offsite and onsite. This is a specific risk in volumetric construction; critical impacts which can arise if risks are not mitigated include potential damage during transit which is either not identified or not rectified, and errors during installation. Skills shortages and gaps onsite contribute to errors in installation and assembly, potentially creating quality issues and defects – this is also a significant risk in traditional construction. Typical site tolerances differ from typical manufacturing tolerances which may result in misalignment, with the potential for unintended gaps to be created between modules. A further risk is that poor-quality installation may be undetected (and therefore, not remedied) due to limited access onsite for inspection once modules are connected.

Product and component testing for volumetric construction is subject to gaps, notably where suitable tests do not exist (in respect of new/innovative products), or the testing conditions do not consider the modular context. The risk therefore is that products which have been deemed compliant may not have been subject to the right test conditions.

Furthermore, there is no standard fire test methodology which is directly applicable for volumetric construction; testing that takes place is likely to be sub-optimal. While there is insufficient evidence to suggest whether a fire is more or less likely in a modular building compared with a traditionally constructed building, the event of a serious fire is likely to result in more serious consequences in a modular building if the choice has been made to use combustible elements in the voids and cavities through which fire and smoke can travel quickly – unless these risks are mitigated via appropriate design, manufacture, and installation. Further research is critical to have a clearer understanding of how materials (and combinations of materials/components) behave in modular buildings in the event of a fire, to feed into development of a standard fire testing methodology for volumetric construction.

There are very few industry standards directly applicable to volumetric construction, meaning that companies and practitioners often design and manufacture bespoke systems that cut across multiple standards. Different approaches in volumetric construction in relation to warranties, accreditation and certification create confusion. The resulting lack of uniformity and fragmentation that exists points to a need for commonality.

Respondents to this research broadly agree that there are gaps in the regulatory framework in relation to volumetric construction. Current Approved Documents were developed for traditional forms of construction, and do not explicitly focus on volumetric construction or other types of MMC. There is a need, according to respondents, to tighten wording of existing regulatory documentation, with additional details/clarifications and signposting to new technical guidance specifically developed for all forms of MMC.

Post-building completion, there is a potential risk that occupants, or builders without the relevant skills and understanding of the volumetric building, could make changes via repair and maintenance that compromise the integrity of the building. However, post-occupation phases of volumetric buildings and repair and maintenance issues do not appear to be major areas of research, and so it is not possible to understand the full extent of this potential risk. This issue is coupled with a lack of readily accessible long-term data about volumetric building performance which could provide insurers and lenders with greater confidence and reassurance about the longevity of the asset. Respondents point to the need for greater transparency and robust data; notably digital records of buildings to provide clear evidence of building methods, materials, testing, inspection and guidance for repair and maintenance.

There is also a need for further, on-going research into materials behaviour/long-term structural integrity of volumetric construction, connections to underpin robust stability for high rise steel volumetric construction and the risk of progressive collapse. Structural engineering research in modular buildings appears to be increasing, but there are still gaps in knowledge and an incomplete understanding of these critical elements.

In summary, many of the risks – and opportunities to mitigate them – are most prominent in the initial phases of any volumetric construction programme, namely procurement, design, and approach to project management. Issues at this stage which are not addressed, can provoke and/or magnify additional risks at subsequent stages of the programme. There is then a further risk that the same issues are repeated across different programmes. There is a clear opportunity to effectively manage many of the risks through the implementation of an end-to-end process underpinned by quality assurance, checks and balances, supported by a common standard and a regulatory framework with greater relevance for volumetric construction (and other forms of MMC).

Digital tools and technologies offer the opportunity to mitigate a wide range of risks in multiple ways. Harnessing digitally underpinned solutions effectively can enhance project management, process monitoring, design, inspection, crane pathways and information sharing – to name only a few.

The tables that follow summarise the risks identified through this research, which are explained in detail in the body of the report. Risks are described for each stage of the typical project lifecycle, Risks are categorised into those deemed to be elevated in, or specific only to volumetric construction, compared with traditional construction, and those which are applicable to traditional construction as well as volumetric construction. The detailed evidence in this report enables understanding and management of potential risks in volumetric construction in order that they can be effectively mitigated.

1. Procurement and project management

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Standard contract forms not applicable to MMC Lack of integrated project management and defined processes	Supply chain resilience: risk of insolvency Lack of early engagement of supply chain (including key roles: structural fire engineer, building control)

2. Design

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Shortage of skilled DfMA designers Perceived gaps in design standards and codes	Lack of integration between key individuals could undermine design Insufficient time for design; early design freeze is a critical success factor Potential for conflict between energy efficiency and fire safety design requirements Design defects: risk that defects are ‘built in’

Stages 1: Procurement and project management and 2: Design are critical; any issues here will have a significant ripple effect on all the remaining stages.

3. Manufacture

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Design variants incompatible with mass customisation Damage prior to transit which is undetected	Deviation from original design Unauthorised product substitution Health & safety risks

4. Transportation

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Damage and/or water ingress in transit Lack of process ownership between factory and site; ambiguity relating to responsibilities and accountability

5. Site installation

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Interface between offsite and onsite: misalignment Mismatch in tolerances, typically used in the factory and typically used onsite, creating issues for installation Damage incurred to modules onsite, which may or may not be detected and remedied Limitations of access for inspection/onsite	Skills and knowledge gaps onsite Ineffective crane and lifting operations onsite

6. Occupancy and beyond

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Risk of disproportionate damage in the event of fire or flooding; modules may be difficult to access to replace or repair	Discrepancies between ‘as designed’ and ‘as built’ Limited occupant and building performance evaluation Potential for risk introduced as a result of repair & maintenance

Cross-cutting themes – underpinning the whole lifecycle

• Key issue (applicable to traditional construction as well): Lack of oversight; absence of ‘joined up process’ without clear task allocation, ownership, or accountability. Results in the liability baton being passed around and the risk that issues can be undetected or are identified but it is not clear who has responsibility to take action; may lack ‘cradle to grave’ quality assurance process
• Gaps in relation to volumetric construction in the regulatory framework and in design guidance
• Standard fire testing and product testing not fit for purpose for volumetric construction
• Gaps in knowledge still being researched: behaviour/reaction of different materials in volumetric construction in the event of fire
• Serious fires in modular buildings likely to result in more serious consequences in a modular building if choice made to use combustible elements in the voids and cavities through which fire and smoke can travel quickly, unless risk has been mitigated via effective design/install
• Gaps in knowledge still being researched: behaviour/long-term structural integrity of modular construction; high rise steel volumetric construction; progressive collapse
• Absence of one clear common standard for volumetric construction
• Skills and knowledge gaps and shortages
• Supply chain fragmentation and resilience; lack of integration and collaboration
• Confusion in relation to multiple warranties, accreditation, and certification

Research recommendations

These recommendations relate to volumetric construction; investigation of risks in traditional construction were outside of the research scope. Nonetheless, DLUHC and industry may wish to consider where these recommendations may apply to traditional as well as volumetric construction. Recommendations are described in more detail in section 14.2.

Key recommendation for government, industry, and industry bodies

1. Adopt a systems approach to ensure integrated programme management with clearly defined processes

Recommendations for government

2. Develop one common Standard for volumetric construction
3. Consider amendments to Approved Documents, and develop additional technical guidance specific to volumetric construction
4. Develop a standard fire test methodology and British Standard for fire testing
5. Ensure an adequate product testing regime for volumetric construction
6. Undertake further research in relation to structural integrity
7. Collect long-term data to build comprehensive knowledge and understanding of modular buildings
8. Develop a digital database of buildings

Recommendations for industry and industry bodies

9. Collect long-term data to build comprehensive knowledge and understanding of modular buildings
10. Look for ways to educate clients and improve skills
11. Consider how to harness digital tools and technologies to help mitigate risks

2. Introduction

2.1 Research context

In 2017, the UK government set out its programme of reform for the housing market, including an annual target to build 300,000 homes per year by the mid-2020s (Department for Communities and Local Government (DCLG), 2017). At the time of writing, there remains a consistent shortfall between this target and the actual number of homes constructed per annum, attributed to multiple factors including low productivity, skills gaps, and shortages and more recently, the impact of the COVID-19 pandemic. Building on a pledge to support Modern Methods of Construction (MMC) in its Autumn Budget of 2017 the 2019 Conservative party manifesto reaffirmed the target of 300,000 homes a year by the mid-2020s and made explicit commitment to support MMC.

The need to accelerate housebuilding is a critical driver for greater adoption of MMC. Responding to this need, the Department for Levelling Up, Housing and Communities has a programme of work to support MMC, including through its housing funding and land programmes, and to tackle strategic barriers, notably the lack of product standardisation across the industry and the difficulties in obtaining product warranties and therefore insurance and mortgages).

This policy commitment towards greater use of MMC, however, is coupled with the recognition that the existing regulatory framework was set out in relation to traditional construction. There may therefore be aspects of the system of regulation surrounding the Building Regulations which are not tailored to MMC, or gaps which could result in issues with building quality, performance, and safety.

2.2 Research aim and objectives

The aim of this research is to investigate potential risks to safety and performance posed by volumetric construction and identify potential options for mitigating those risks. Such risks may stem from the technical design, execution (particularly onsite) or the interface between factory and site; the latter issue being particularly relevant to building control bodies (local authority building control and approved inspectors).

The key objectives are to:

• investigate the key risks and issues identified by the Building Regulations Advisory Committee (BRAC) stemming from offsite volumetric in existing sources based on research into current industry practices in the lifecycle of offsite volumetric;
• use evidence collected to identify potential options to reduce identified safety risks, particularly regarding records maintenance and dissemination; and
• consider necessary changes to the regulatory framework to achieve the potential options.

The scope of this research is limited to one type of MMC – offsite manufactured volumetric construction (Category 1 within the national MMC framework (MHCLG, 2019)) – because this is where the BRAC perceives potential risks to be greatest, compared with other types of MMC (BRAC is an advisory non-departmental public body sponsored by the Department for Levelling Up, Housing and Communities. Their role is to advise the Secretary of State in England on making building regulations and setting standards for the design and construction of buildings).

A broad range of risks have been identified – and it should be noted that some of these risks are specific to volumetric construction, but many also apply to traditional construction. Some risks, if they occur, will have a more significant impact than others. It is also important to note that the likelihood and impact of risks can differ, depending on the various different types of volumetric systems and uses.

The research conclusions highlight the risks in volumetric construction which have potential to become systemic, due to the likelihood of repetition, and the risks in volumetric construction which are deemed to be elevated, in comparison with traditional construction, or relevant only to volumetric construction. This is also highlighted in the text throughout the report where applicable.

2.3 Volumetric construction

Volumetric construction is a type of offsite construction, whereby fully enclosed, six-sided building modules – i.e., floor, four walls and ceiling – are manufactured offsite in a factory and assembled onsite, most commonly by stacking and/or joining together. Modules can also be five-sided, without a floor or roof. Once site installation is complete, in most cases, buildings will need only limited additional work to be considered complete and ready to use.

Modules can be pre-fitted to varying degrees of completion, from a basic structure to fully finished with all services. Typically, modules are fitted with plumbing, heating, electrics, doors, windows, and internal finishes before being transported from the factory to be installed onsite. The proportion of a building’s construction that takes place offsite or near-site using any of the seven categories of MMC (described in the MMC Definition Framework (PDF, 3.3MB) is measured as a percentage of the total to calculate its Pre-Manufactured Value (PMV)). Volumetric construction (Category 1) typically offers high PMV as much of the structure can be constructed offsite.

Potential benefits of volumetric construction cited in the literature include:

• faster speed of delivery (Ferdous et al., 2019);
• potential to reduce capital cost (Navaratnam et al., 2019);
• better quality of finished product, underpinned by consistency and quality control in the factory setting (Kamali et al., 2019);
• less time spent onsite, which can reduce the health and safety risk (Jabar et al., 2013); and
• produces less waste (Lacey et al., 2018; (Kamali and Hewage, 2016).

The concept of manufacturing components offsite and assembling them onsite is not new, but in recent years has become increasingly technology driven. The use of volumetric construction has become more commonplace in recent years, notably for building projects which lend themselves to a repetitive approach, such as school/college buildings, hotels, student accommodation, apartments, and hospitals. There are strong drivers influencing greater adoption of volumetric construction in relation to the need for higher volume output in home building and the impetus of the low carbon agenda.

Advantages of volumetric construction can only be fully realised where projects are implemented effectively. Understanding the risks of volumetric construction and how they can be mitigated is therefore an essential part of maximising the benefits of this type of construction.

2.4 Methodology

This research has been informed by:

• an evidence review of published literature and data (academic literature and module connection/product/materials testing/modelling data, grey literature, industry guidance, regulatory approaches, technical manuals – the cut-off point was July 2021). A wide range of keywords were used to search for relevant sources, given that many different terms can be used to indicate volumetric construction and MMC, for example modular construction, modular integrated construction, modular building systems.
• An evidence review of unpublished sources shared by respondents to the online consultation and industry stakeholders interviewed for this research (product/systems manuals and guidance, unpublished internal research undertaken by volumetric manufacturers relating to product safety, fire testing and product testing);
• semi-structured in-depth interviews (63) with industry stakeholders, product/systems manufacturers, trade and professional bodies, consultants, building control, insurers (fieldwork took place between April and November 2021);
• an online call for evidence (achieving 53 responses – live during November and December 2021). This was not part of the original scope of the research and was undertaken as an additional means of gathering more detailed evidence about whether respondents perceived any requirement to amend Building Regulations and Approved Documents;
• input throughout the research from a dedicated Steering Group (see Appendix 1); and
• visits to a steel frame manufacturing plant and a site installing homes and apartments (taking place during the second half of 2021).

Evidence has been synthesised and analysed into the key themes set out in this report. Where reference is made to ‘respondents’, this refers to synthesis of depth interviews and the responses to the online call for evidence.

Where quotes have been included, these come from either in-depth interviews or from the call for evidence. Individual respondents have not been identified.

The researchers are very grateful to all those who have provided their time and expertise to inform this research.

2.5 Research limitations

When reading this report, it is important to note:

• It is not possible to estimate how often these risks actually occur, as there is insufficient evidence to provide a robust assessment of the likelihood of risks occurring. The research has sought to identify all potential risks which may affect volumetric construction. Risks can be viewed subjectively, and perceptions of likelihood and impact varies between different stakeholder types – depending on their experience and use of different types of volumetric systems across different contexts e.g., low rise, high rise. While all identified risks have the potential to occur, it should not be assumed that they always will occur.

• The scope of this research is risks in volumetric construction; it was not part of the remit to identify risks in traditional construction. However, it should be taken into consideration that, as already stated above, many of the identified risks apply to traditional construction as well as to volumetric construction. Furthermore, there is a high level of confidence among the majority of industry research respondents, that while volumetric construction brings its own set of particular risks (or the potential to have greater consequences by comparison with traditional construction), that these can be effectively eliminated or mitigated via a structured set of actions (discussed in more detail in chapter 14).

• There are gaps in the existing literature for a number of reasons. Firstly, volumetric construction is only one type of MMC, and therefore is likely to have different risks and benefits than other MMC types – however, much of the identified published literature considers MMC as a whole, rather than specifically dealing with volumetric construction. A number of sources reference volumetric construction projects in other countries, where different climates/weather events, cultural contexts or regulatory systems could mean findings were less directly relevant for the UK. Finally, there is a lot of emphasis within the literature about perceived advantages and benefits of MMC, relative to traditional construction, with less research by comparison into potential risks or issues.

• Volumetric construction techniques and technologies are likely to evolve, which could help to mitigate or eliminate existing risks, but equally new risks could emerge over time. Additional research may therefore be required to maintain a robust knowledge and understanding of risks and how they can be mitigated.

2.6 About this report

Following this introductory chapter, risks are described in relation to the typical lifecycle of a volumetric construction project – from procurement through to design, manufacture, transportation, site installation and to occupancy and beyond (chapters three to eight). Cross-cutting themes – i.e., factors which have relevance for multiple aspects within this typical lifecycle – are presented in chapters 9 to 13, spanning fire safety, products and structure, warranties, standards and certification and digital technologies. Research conclusions and recommendations are presented in chapter 14. The bibliography is contained in the annex.

Please note, the terms ‘volumetric’ and ‘modular’ are used interchangeably throughout the report.

3. Procurement and project management

3.1 Overview

Comprehensive pre-project planning is essential to volumetric construction; the decision to use volumetric must be taken at an early stage to allow sufficient time for design and establish an integrated approach to project management (i.e., all the relevant expertise involved from the outset). This does not always happen; clients may take the decision to use volumetric too late, which can have knock-on effects for the remainder of the project delivery,

The primary risk at this stage is lack of clear ownership and accountability. If this is not clearly defined from the outset, it can trigger significant deficiencies in process and project management throughout the project, potentially resulting in risks to quality and safety. It is typical to see multiple layers and levels of authority and approval, rather than ownership of a joined-up process. Critically, if the process is not joined up, with a lack of clarity as to who takes ownership at each stage, this can have significant ramifications in volumetric construction.

3.2 Risks – procurement and project management

3.2.1 Lack of clear ownership and accountability

Respondents state that effective project and process management is essential for any form of construction, not just volumetric, but that it is typical to experience multiple layers and levels of authority and approval in modular construction.

The process is not joined-up; it is not clear who is responsible throughout the project.

It is uncommon to see ownership of the whole process from start to finish – there are multiple owners in the design team, in the factory, in logistics and onsite. This can be true even within vertically integrated organisations. Liability and accountability are therefore “sliced and diced” – problems, associated risk and responsibility for resolution are passed around.

The golden thread is all very well, but where does it start and finish?

While this risk also applies to traditional construction, the volumetric supply chain can be more complex and is heavily reliant on effective co-ordination and collaboration between stakeholders, which may introduce a greater degree of risk.

Responsibilities should be clear for every stage and exist for every stage, otherwise there are repercussions.

3.2.2 Lack of integration and collaboration

Volumetric construction projects rely on multiple professionals with specialist skills to work together. There is a risk of lack of coordination, particularly if key individuals and/or organisations become involved with the project too late. Key roles that can be absent from the start of the project (or at pre-project planning stage) are those of the structural fire engineer, design lead and building control. This can undermine effective communication between others, inducing issues from project conception (Mignacca et al., 2020).

Lack of integration can also arise from insufficient understanding of the key stakeholders, and their interdependencies. The roles played by all stakeholders and how they affect each other should be understood to help identify and manage risk, particularly in relation to defect management (Heravitorbati et al., 2011). The effective implementation of volumetric construction can achieve savings in construction programme time, improve quality control, sustainability, and productivity, and reduce project lifecycle costs. However, this must be underpinned by effective collaboration between and integration of stakeholders with conflicting interests, requirements, value systems, and needs (Wuni and Shen, 2020). The existing supply chain can often be fragmented, with poor traceability, and lack of real-time information (Wang et al., 2020).

Low supply chain integration can create problems, as different suppliers of components commonly have different levels of integration with requirements of volumetric manufacturing, notably precise tolerances. Where suppliers and manufacturers build established working relationships, there is high integration with the manufacturing process. Issues are more prevalent among suppliers of standard components (i.e., not specific to volumetric construction), where their exposure to and understanding of modular is limited, making it more likely that the materials do not align with volumetric manufacturing requirements. This could lead to risk if there is insufficient time to source alternative materials.

Fragmentation with the supply chain can be apparent across the construction sector as a whole.

3.2.3 Procurement

The term ‘design and build’ refers to a procurement route, whereby a main contractor is appointed to both design and build the works. Design and build commonly requires the client to commit to the design at a relatively early stage – and so should lend itself well to volumetric construction – but the risk with this approach is that design work can be commenced with design decisions being finalised before both the decision to build using volumetric construction is made, and the volumetric manufacturer is identified. This can create project risk if the decision to use volumetric is taken too late – potentially not allowing sufficient time for design (also see section 4.2.2).

However, “under single-stage procurement, clients risk commissioning a design which, when tendered, is found to be unnecessarily difficult or even impossible to build” (Finnie et al., 2018). This refers to single stage tendering, whereby sufficient information is provided from the client at tender stage to enable a comprehensive tender to be developed and submitted. If there is insufficient client knowledge and understanding of volumetric, the risk is that the specification is not clearly defined and is not suitable for a volumetric solution.

Design decisions taken too early and/or design flaws can also be a risk in traditional construction.

3.2.4 Contracting

Standard contract forms in construction were not designed for volumetric construction; there is no standard form to allow for c.70% of the value to be in the manufacturing package of work. Even when optional clauses are implemented to make the contract form more applicable to volumetric construction, these are rarely used by industry. A letter of intent, followed up with the JCT Design & Build contract, is the most commonly used contract to procure offsite manufacturing. However, this places design and time performance liability on the main contractor, which “raises the suggestion that this duplication of liability adds an additional layer of cost to the project” (Griffiths et al., 2018). This can create uncertainty as to liability between the main contractor and the manufacturer, as well as adding a premium cost onto the main contractor of responsibility for timely performance of the offsite manufacturer.

While perceived to be more collaborative, the NEC4 Supply Contract also has gaps in relation to volumetric construction – namely the absence of an installation clause (optional or standard) and insufficient provision for testing of the product when installed onsite, e.g., installation test, cold commissioning, or other final structural and performance tests.

The absence of a standard approach enabling high levels of value in manufacturing is a particular risk for volumetric construction.

3.2.5 Perceptions of higher costs: impact on the supply chain

Higher capital costs for volumetric products and systems in comparison to the cost for loose materials delivered to a building site are often cited by clients as a challenge inhibiting the use of volumetric (Duncheva, 2019; Homes for Scotland, 2015). However, decision-makers may not be comparing like-for-like in their cost comparisons, as these calculations do not capture the whole-life value of construction projects, including reduction in preliminaries, defects remediation and maintenance costs (Pan and Sidwell, 2011). The Construction Industry Research and Information Association (CIRIA) established a methodology linking Pre-Manufactured Value (PMV) to reduction of defects during the liability period of contracts (Jansen van Vuuren and Middleton, 2020). However, case studies and other data to demonstrate lower whole life costs are very limited, and there remains a strong perception among potential housing clients that volumetric (and MMC generally) is more expensive than traditional construction.

This perception creates risk within the volumetric supply chain; manufacturers need a consistent pipeline of work and factory throughput to maintain cashflow. Pausing production in between projects while maintaining equipment and paying salaries leads to a cashflow risk for manufacturing businesses. This creates a risk that members of the volumetric construction supply chain could go into administration; high insolvency rates can be more common in the MMC sector generally compared with traditional construction. There are likely knock-on effects if a supplier goes bust; increased project costs, programme delays and a greater risk of quality issues/defects arising from unauthorised product substitution.

The risk of insolvency and its knock-on effects is exacerbated by payment mechanisms which are not suitable for volumetric construction projects. Traditionally clients pay for work in phases when it is complete and inspected onsite. However, volumetric manufacturers invest heavily in procuring materials to manufacture products for the entire project, meaning the traditional payment approach creates a cashflow risk (Jalil et al., 2015).

Among eight volumetric manufacturers surveyed in 2015, three have since announced insolvency (Duncheva, 2019). Globally important players such as Katerra in the U.S.A (formerly the largest CLT manufacturer globally) are no longer in operation (Bryant, 2021). This risk is heightened by changes to the Corporate Insolvency and Governance Act 2020 (CIGA), whereby if a company that a volumetric/MMC manufacturer is supplying to announces insolvency, the manufacturer remains liable as an unsecured creditor for the delivery of the contract.

High risks of insolvency also lead to ownership risks for clients, who may not be able to recover components classified by the insolvency manager as property of the manufacturer rather than the client. Even if recovered as loose materials and partly manufactured components, there is no certainty that a different manufacturer will be able to use the same materials and components, leading to possible significant financial losses because of the need to re-manufacture everything or attempt to complete the works traditionally onsite without readily available labour (Dawson, 2018).

The risk of insolvency affecting one or more organisations within a programme supply chain is present in traditional as well as volumetric construction, however it would be more challenging to ensure continuity in a volumetric construction project – for example if a bespoke product/system was to be manufactured.

3.3 Risk mitigation – procurement and project management

3.3.1 Clarity of process – a standard and fully integrated approach

Respondents agree there is an urgent need for processes, roles, and responsibilities to be clearly defined, with granular detail and systems in place for controlling workflows and sharing information, which is essential to the success of a project, via tools such as Building Information Modelling (BIM) (Abdelmageed, S. and Zayed, T., 2020). In particular this should include means to ensure what has been designed is what is ultimately manufactured and installed.

Respondents emphasise the importance of having clarity within the process as to roles, responsibilities, ownership, and accountability – throughout the whole of the project. Ideally, one role would have overall ownership to ensure the “liability baton” is not simply passed on. A standard for the volumetric construction process has been proposed as a solution, which would be similar to the ISO 9001 Quality Management Standard (QMS) but be bespoke to volumetric (or to all forms of MMC), and/or link into the Construction Design Management regulations. This should be fully integrated, from planning and design, manufacturing, transportation through to onsite installation and be underpinned by clearly defined quality assurance processes.

Evidence from insurers representing leading insurance companies across a number of European countries suggest there should be a mandatory requirement to have a written Risk Register, kept updated throughout the project, which identifies all critical objectives of the project, impacts of identified risk factors and mitigations developed on a case-by-case basis and underpinned by a framework and roadmap for the integration of BIM into management of risks (Rohloff et al. 2021).

3.3.2 Client knowledge and understanding

Clients need a clear understanding of the difference between volumetric and traditional construction – notably the vital importance of early design freeze. Design freeze refers to the point in the project when an official ‘stop’ is implemented, i.e., after this point there can be no further changes to the design. After this point, the agreed design will be the one which is built with no further changes (discussed in more detail in section 4.2.2), impact on procurement and early engagement of the supply chain. Early contractor involvement is essential for effective project management of volumetric projects. Use of Building Information Modelling (BIM) can help to implement early contractor involvement (Goodier et al., 2019).

3.3.3 Integrated supply chain

It is vital to have an integrated project team – which includes the client – from the outset, at pre-planning stage. Early engagement of the supply chain enables potential partners to add value to the workplan while mitigating the risk of poor performance (Howe et al., 2016). Integrated Project Insurance (IPI) offers a framework for mitigating this risk of poor team integration through multi-disciplinary design and sharing of risks among the multidisciplinary team.

Respondents emphasise the need to include key roles at pre-planning and design phases of volumetric construction projects (and all forms of MMC) – in particular the roles of structural fire engineer, architect, designer with specific skills in Design for Manufacture and Assembly (DfMA). A minority of respondents suggest Building Control Bodies (BCBs) should be present at least by the point of early-stage manufacturing. A fully integrated project management team should also include all major trades, so that the designers understand which tradespeople will take ownership of connections onsite, how work will be undertaken, and the level of finishing required for units to enable tradespeople to complete their tasks.

In addition, Radio frequency identification (RFID) may improve supply chain efficiency by providing item-level identification and real-time information, as demonstrated in a study involving pre-cast concrete (Wang et al., 2017). In a 2016 study (Halil et al., 2016), improved trust between stakeholders was found to contribute towards lower costs and higher overall quality.

3.3.4 Contracts and standard forms for volumetric construction

There is a need for a suite of contracts and standard forms specifically for volumetric and MMC generally, with wording to address interface management and ensure clarity regarding liability. Such a suite could be adapted from existing documentation.

The Named Sub-Contractor process within the JCT Intermediate Contract can provide an intermediate solution to this risk because:

• “The client can be in dialogue with the offsite manufacturer and pay for design and works before a main contractor has been appointed;
• the design responsibility remains with the offsite manufacturer;
• an advanced payment arrangement can be included; and
• it is a recognised process which is already in place though the JCT suite of contracts” (Griffiths et al., 2018).

3.3.5 Minimising risks relating to insolvency in the supply chain

From a manufacturer’s perspective the solution of scheduled payments for completed works within the procurement agreement will help to manage cashflow and thus could help to prevent insolvency.

A further possible solution is to encourage the wider use of Project Bank Accounts (PBA) by public and private sector clients. The Construction Leadership Council stated in 2018, “An advantage from PBAs is that payment times for the supply chain were less than 30 days. This minimised the risk to the employer from insolvency in the contractor’s supply chain.” (CLC, 2018).

From a client’s perspective, the following steps are recommended (Dawson, 2018) – these refer to MMC but are applicable to volumetric:

• undertake detailed financial checks as part of the manufacturer procurement process (depending on the procurement model this could be the responsibility of the main contractor);
• put a bond in place that can be used by the client in case of issues with the delivery of completed MMC products to site;
• schedule payments for completed MMC products including vesting certificates to transfer ownership of the completed products to the client, and state that the manufacturer is responsible for safe storage (if the products are stored on their premises);
• store the completed MMC products owned by the client separately from other materials and components and label clearly as belonging to the client; and
• allow the client’s representatives to inspect items that belong to them i.e., when contractually the client takes ownership of volumetric units and relocate the items if deemed necessary.

In addition, if the client has full production drawings, materials specifications, and contact details of key suppliers they may potentially enable an alternative manufacturer to continue to the same requirements. This will depend on the alternative manufacturer having access to manufacturing equipment with the same capabilities as the original manufacturer, as well as being able to source the required materials on time. However, even if a client follows all the recommended steps and maintains access to all relevant information, it is likely that it would be very difficult to keep to the original programme. Thus, in practice, it is likely that the risks of the impacts of insolvency can only be partly mitigated. This underlines the need for inter-operability and standardisation in volumetric construction (and MMC generally).

3.4 Regulatory framework

The Building Regulations set out standards which must be achieved in construction of buildings. They are method-blind; all buildings regardless of type or mode of construction must adhere to Building Standards.

Approved Documents provide guidance as to how compliance with Building Regulations can be achieved – but there is no obligation to implement the approaches set out in the Approved Documents as long as Building Regulations are satisfied.

Approved Documents are reviewed and updated on a regular basis; at the time of writing the current suite spans:

• Structure: Approved Document A
• Fire safety: Approved Document B
• Site preparation and resistance to contaminates and moisture: Approved Document C
• Toxic substances: Approved Document D
• Resistance to sound: Approved Document E
• Ventilation: Approved Document F
• Sanitation, hot water safety and water efficiency: Approved Document G
• Drainage and waste disposal: Approved Document H
• Combustion appliances and fuel storage systems: Approved Document J
• Protection from falling, collision and impact: Approved Document K
• Conservation of fuel and power: Approved Document L
• Access to and use of buildings: Approved Document M
• Overheating: Approved Document O
• Electrical safety: Approved Document P
• Security in dwellings: Approved Document Q
• High speed electronic communications networks: Approved Document R
• Infrastructure for charging electric vehicles: Approved Document S
• Material and workmanship: Approved Document 7

This approach applies to England; there are separate Building Regulations in Wales and Northern Ireland, while Scotland has its own Building Standards.

3.4.1 Perspectives about changes to the regulatory framework

There are opposing views among respondents regarding potential changes in the regulatory framework. A minority of respondents strongly believe there is a need for a new Approved Document developed specifically for volumetric construction.

The majority of respondents, however, question the need for an entirely new/separate Approved Document, on the grounds that that this would take a long time to develop and would risk duplicating existing information. The pace of modular could exceed the pace of regulatory change and therefore a more dynamic solution was proposed - wording of existing documentation to be tightened, for example with additional clauses to add further detail in relation to volumetric and/or to make it clear where existing clauses in Approved Documents are not applicable to volumetric construction.

There is no reason to create new Building Regulations – the principles are the same regardless of the mode of construction.

Respondents also suggest that additional technical guidance specific to all forms of MMC, not just volumetric, be developed, to supplement existing Approved Documents, including detail on processes which need to be followed. Based on evidence from interviews and the online consultation, additional content and revisions to existing documentation would need to include details on:

• first principles of design;
• change control;
• sequencing of works and temporary arrangements;
• interfaces: ground/module; external through wall construction;
• inspection regime;
• testing requirement (products, fire, thermal performance);
• maintaining digital records of installation of critical fire stopping details and cavity barriers;
• maintenance and management; and
• adaptation.

Building Regulations are very clear. What we want in industry is clear technical guidance and clarity on the process to be followed.

4. Design

4.1 Overview

It is critical to ensure design and manufacturing/construction teams are integrated from an early stage in the process; a ‘golden thread’ which runs from design through to manufacture through to installation. To achieve this effectively, early design freeze is essential and must be underpinned by relevant expertise in Design for Manufacture and Assembly (DfMA). Design freeze refers to the point in the project when an official ‘stop’ is implemented, i.e., after this point there can be no further changes to the design. After this point, the agreed design will be the one which is built with no further changes. A key risk is that poor design will affect the quality of manufactured and installed modules; essentially meaning that defects are ‘built in’ and replicated across all units, rather than being isolated to one-off instances (Rohloff et al., 2021).

4.2 Risks – design

4.2.1 Lack of integration

If there is an information gap between relevant individuals (e.g., designer, architect, fire engineers, structural engineer, manufacturer, trades) this may hinder the manufactured modules from being wholly consistent with the design specification, or consistent with the design specification but otherwise defective. There is a risk that inconsistencies render the modular components unusable or are used regardless despite issues – which may or may not be identified at installation stage. Again, the golden thread of information is essential as all relevant parties need to have a clear understanding of the design, any limitations, and impacts of even the smallest changes.

The risk of inconsistencies or issues in design can contribute to defects which are replicated across all manufactured modules and not identified onsite, i.e., defects are ‘built in’. This can also occur in traditional construction.

4.2.2 Insufficient time available for design

The requirement for a front-loaded design phase with volumetric construction relies on the input of multiple parties, final decisions and extended timelines compared to other building methods. A publication in the Journal of Cleaner Production identified the top five significant constraints as “extensive coordination required prior to and during construction,” “need for additional project planning and design efforts,” “increased transportation and logistics considerations,” “requirement for an early commitment,” and “higher initial cost than conventional construction method.” (Hwang et al. 2018). All of these are associated with the front-loaded design phase in volumetric construction.

The point in time at which the decision to use volumetric construction is made is a risk to the financial, commissioning time, and construction programme aspects of construction projects (Gibb and Isack, 2003). Research shows approximately 50% of clients make the decision to use volumetric construction when the concept design has been outlined, or later. Even in best practice procurement case studies, a late decision to proceed with volumetric construction was cited as one the main reasons for final project delays (Oliver, 2018). Early design freeze is a critical success factor for volumetric construction; late decisions can limit the time available for DfMA and result in a longer lead-in time for offsite manufacture and construction, risking making its use no longer cost-effective to implement (Homes for Scotland, 2015).

Clients can lack knowledge and understanding of traditional as well as volumetric construction; early design freeze is also relevant in traditional construction.

4.2.3 Shortage of skilled DfMA designers

Respondents to this research perceive a shortage of designers with the relevant skills for DfMA, creating substantial risk for volumetric construction. As new construction methods and materials are introduced, the skills gap in the use and implementation procedures may persist and lead to DfMA risks in the design stage with subsequent implications in manufacturing, construction, and in-use (CCC, 2019).

This is a risk specific to volumetric construction where DfMA skills and experience is required, unlike in traditional construction.

4.2.4 Perceptions of gaps in design standards and codes

Current design practices for modular buildings are based on the conventional design guides of traditional buildings. These are often not suitable for modular buildings, as they possess different characteristics. For example, the process of manufacture and assembly of modular units generates short-term loading which may influence the load-transfer mechanism (Thai et al., 2020).

Evidence in the literature points to difficulties in implementing technologies for volumetric construction due to a lack of design guidelines, strong inter-module jointing techniques, and sufficient understanding of the structural behaviour, global stability, and structural robustness of modular buildings (Liew et al., 2019).

It should be noted that there are various versions of design guidance produced by professional bodies and trade associations – for example the Royal Institute of British Architects (RIBA) produced DfMA Overlay to the Plan of Work (RIBA, 2021), first published in 2016 and updated in 2021, which includes comprehensive mapping of core DfMA tasks. Design guidance/case studies are also produced by the Supply Chain Sustainability School, Structural Timber Association, Steel Construction Institute (SCI), and the Institution of Structural Engineers (IStructE).

However, design management standards do not typically make explicit reference to volumetric construction (or other forms of MMC). Research undertaken by BSI reviewed existing and draft British and European Standards, industry guidance, European Technical Approval Guides, European Organisation for Technical Approvals guidance, European Assessment Documents, and Publicly Available Specifications (PAS). This work found significant gaps in relation to volumetric construction and MMC generally – i.e., either not cited in many documents or if standards have been produced, they are now out of date. Detailed guidance is often missing in relation to interfaces connecting systems e.g., module to module, pods to floors. Detail is also missing when it comes to clear specification of tolerances and accuracy necessary for integration of modules manufactured offsite; tolerances stated in current standards do not always align with volumetric construction (BSI Group).

This is a risk which is specific to volumetric construction.

4.2.5 Design defects

Insufficient skills in DfMA and limited use of design guidance which is available can both contribute to a final design which is not entirely fit for purpose for volumetric construction. As stated in section 3.2.1, where the process is not joined up there can be a lack of clarity as to ownership/responsibility for different aspects – for example in design it may be unclear how responsibility is shared between the architect, engineer, or systems designer. Structural defects can be a particular risk when volumetric construction design applicable for low rise, is applied for tall buildings. Other potential issues, carrying associated risks to building structural integrity, performance, and occupant safety, include:

• materials and components may not interface effectively;
• design does not take logistics into account to ensure modular components/modules sizes, materials and weights will meet with transport requirements and regulations;
• discrepancies between manufacturing and assembly tolerances are not identified. The cost of rectifying these problems can be prohibitive (Wuni et al., 2019).

Evidence from interviews suggests that defects may be harder to detect in volumetric construction – partly because of the difficulties in access for quality checking and inspection – and, as such, are less likely to be identified and resolved before the conclusion of the project. Moreover, there could be defects due to damage in transit which are not visible when the volumetric modules arrive onsite (also discussed in section 6.2.1). Therefore, there is a risk that defects are “built-in” to the structure.

A minority of respondents believe there is a risk that energy efficiency/net zero building requirements may displace fire safety considerations, for example. if combustible insulation is used in the voids. This risk is described in more detail in section 9.2.2 but is referenced here, given that the risk can be mitigated at design stage.

As stated in section 4.2.1, the risk of inconsistencies or issues in design can contribute to defects which are replicated across all manufactured modules and not identified onsite, i.e., defects are ‘built in’. This can also occur in traditional construction.

4.3 Risk mitigation – design

4.3.1 Design standards for volumetric construction (and MMC)

Respondents suggest greater specificity is needed in design guidance for volumetric construction, which could form part of an overarching Publicly Available Specification (PAS) for MMC or just for volumetric. Clear and detailed design guidance should span all standardised modular connections, composite design of modular units using lightweight and high strength materials, stability analysis of high-rise modular building, robustness design, modelling of global and local imperfections, compartmentation and fire safety consideration, durability, and future maintenance (Liew et al., 2019).

RIBA guidance suggests a future direction of travel could include Platform Design for Manufacture and Assembly (P-DfMA) to encourage interoperability to a common standard, “creating an ecosystem of, for example, interchangeable components, sub-assemblies, and pre-assemblies from different suppliers. While clients are still tied to one platform, they nonetheless have choice” (RIBA 2021). However, this must be appropriately tested and proven for all potential combinations.

4.3.2 Early decision-making

Good practice research finds 20% of clients make the decision to use volumetric or other forms of MMC before the specific project has been considered. This provides opportunities for the design to be developed specifically for volumetric, with sufficient lead times for the design freeze required prior to starting manufacturing; a critical success factor to realise the benefits in practice (Gibb and Isack, 2003). Recent research found “a significant relationship between early design decisions and subsequent resource efficiency potentials of [MMC] across lifecycle phases” (Kedir and Hall, 2021). It is critical that repeat design concepts have been factored in at the point of the design freeze.

4.3.3 Lean principles

One method of reducing the occurrence of defects in the construction process is to adopt ‘Lean principles.’ Lean principles have been widely applied to improve the productivity and efficiency of construction operations, while simulation augments Lean theory by allowing its benefits and issues to be analysed quantitatively before actual implementation. Lean recommendations include implementing Total Quality Management (TQM) to reduce presence of defects (Goh and Goh, 2019).

4.3.4 Education and training

Education for clients and procurement teams about the risks that may be encountered at the design stage would help to ensure the critical risks can be mitigated – for example through allowing sufficient time for design, actioning an early design freeze, and working with appropriately qualified designers experienced in DfMA.

Greater provision of training courses in DfMA is essential; not just to tackle skills gaps within the existing designer population, but to make more training opportunities available to address skills shortages. Although contractors may be chosen for their knowledge of volumetric construction, some firms and particularly smaller contractors should consider re-training and updating their knowledge to meet the needs of the clients (CITB, 2019). A minimum level of competence in DfMA may also be introduced by the relevant professional bodies. CPD courses may be another option, but a more direct and focused approach incorporating hands-on sessions and practical training will be required to fully address the issue.

5. Manufacture

5.1 Overview

Factory manufacturing processes for volumetric construction are predominantly underpinned by comprehensive quality assurance, testing, and inspection procedures. This means that any defects or issues can be readily identified and corrected before modules are transported to site. Analysis of the issue at source can prevent it from recurring, enabling a culture of continuous improvement. The factory environment can also protect modules from exposure to weather during fabrication, but this must continue through transportation and while stored onsite, until fully installed.

With appropriate quality control processes and independent third-party certification, defects in manufacturing should be less likely to occur. However, this is not guaranteed. Defects in manufacturing can still occur, however - for example if quality control is deficient, or if materials are substituted therefore deviating from the original design. Manufacturing can also be deficient if the original design was not fit for purpose, as discussed in chapter 4.

The key risk at this stage, is that defects are not identified and are accordingly replicated across all the modules produced. Defects would then be either identified at site, whereby modules would need to be re-manufactured (incurring delays and additional costs), or not identified and assembled – leading to risks to the building’s structural integrity, performance, and safety.

5.2 Risks – manufacture

5.2.1 Deviation from design

Stringent quality control processes in a factory environment are a critical success factor for volumetric construction, and if not in place or inadequate, this could result in poor quality outputs which deviate from the original design. This is not necessarily a risk due to the manufacturing process, but a risk which can occur during the manufacturing process due to factors such as poor-quality control, design miscommunication or coordination issues among the supply chain. A study of two volumetric projects which compared designed to as-manufactured and as-installed dimensional deviations in Mechanical, Electrical and Plumbing (MEP) modules using a laser scanned model overlaid over the as-designed BIM model showed deviations in the range of -25mm to +20mm. This may not come across as a significant deviation compared to other construction systems, however in volumetric with tolerances in the range of -3mm to +3mm these can lead to misalignment of connections onsite, or deviation from the building footprint or height when accumulated across several modules (Rausch et al. 2020).

The automated manufacturing process is reliant on high precision in measurements and tolerances typically by means of dedicated software and systems for module fabrication. Unintended errors could also stem from IT system issues, operator error, or the risk of cyber-attacks (Rohloff et al., 2021), but it is not clear from the evidence how likely this is to actually occur.

This risk is not unique to volumetric construction but could occur with any use of Computer-Aided Design (CAD) or similar software in traditional construction. However, the impact may be greater in volumetric construction which requires very precise tolerances.

5.2.2 Design variants incompatible with mass customisation

There is a risk of not realising the benefits of volumetric manufacture due to too many design variants, which may be incompliant with mass customisation principles, applied within platform-based volumetric module configurations. Many variations in window sizes, cladding finishes and module dimensions when combined, can result in unmanageable variety of details that need to be complied with during the manufacturing process, and can result in time, quality and supply chain issues (Popovic et al. 2021).

The risk in regard to manufacturing is specific to volumetric construction, but management of variations/details can also affect traditional construction.

5.2.3 Unauthorised product substitution

Interview evidence suggests that product substitution is rife across the construction sector as a whole and can result in significant consequences. Product substitution in volumetric construction is a concern for insurers, but respondents note that the risk appears more prevalent in traditional construction – as there is greater quality control in a factory setting. However, the consequences of product substitution may be more significant for volumetric construction as the design can be fixed around particular types of components. For example, a project may need three types of different components from three separate suppliers with no contingency to source alternatives. If the supply chain is not able to provide a factory with the consistent level of the correct products specified, there is a risk that a substandard product is used instead, rather than halt the production line and introduce delay into the project. Insolvency in the supply chain can be another reason for failure to receive specified products or components.

Failure to stick to materials specifications when manufacturing - after the system has been certified by an independent third-party - can create risks for quality, structural integrity, performance, and safety. Respondents note that more recent shortages of materials after the EU Exit have intensified the risk of unauthorised product substitution.

This issue may also be a risk during site installation.

While the risk of unauthorised product substitution is deemed to be more common in traditional construction, the risk is potentially more significant in volumetric construction as inappropriate products substituted which deviate from the original design, may introduce issues which could be replicated across a high volume of units, rather than isolated incidents of unauthorised substitution.

5.2.4 Damage before transit

After modules are produced, they may be stored for a period of time or despatched to site soon after completion. If quality control in the factory is deficient, there is a risk of damage to the modules before transit – for example if exposed to weather rather than stored inside, or if incorrectly packaged/lifted for transit.

This risk in relation to potential damage to modules is specific to volumetric construction.

5.2.5 Health & safety

Working in manufacturing facilities alongside mobile plant and large equipment has several health & safety risks. Typically, these are well captured and managed by the factory managers and their staff. Accidents and near misses can still happen, for example (Duncheva et al. 2017):

• manual handling – strain due to carrying too heavy a load;
• lifting operations – failure of the load or of the crane; or
• interface with factory vehicles, especially forklifts – forklift driving forward with a load that blocks their view and colliding with a person.

Risks in the factory environment are considerably lower in comparison with the site environment, i.e., traditionally constructed buildings onsite.

5.3 Risk mitigation – manufacture

5.3.1 Quality assurance

Risks of manufacturing defects are significantly mitigated where there is a comprehensive quality assurance plan and quality control processes and checks in place. Case study evidence emphasises the importance of inbound and outbound quality checks to ensure any modules with defects are not transported to site and installed. Quality assurance would need to feature prominently in any standard process which is developed for volumetric construction (also discussed in section 3.3), and should encompass the full cradle to grave approach, rather than simply manufacture in isolation.

Both facilities considered in Johnsson and Meiling’s study (2009) used semi-automation and mechanisation extensively alongside manual production; hence simply using more or different manufacturing equipment will not mitigate this risk. Instead, these findings indicate a positive culture towards quality, in which problems are captured early and not passed down the building process line, provides the greatest mitigation: “Working with identification of defects and deviations is here demonstrated to have potential to support improvements of process and product design, thus favouring both clients and company in the long run” (Johnsson and Meiling, 2009).

5.3.2 Factory inspection

Regular factory production inspection is essential; this is typical of all volumetric manufacturers interviewed for this research, particularly where their products/systems have been accredited by an independent third party. Quality controls and inspections should also factor in air/temperature monitoring, mould prevention and moisture content testing, to prevent unintended damage to materials or modules. Evidence from respondents suggests inspection is taking place on a self-regulated basis, but there is no common ‘standard’ to dictate how often this should occur, who should undertake it or a framework of common aspects for inspection across all factory environments. A theme that emerged is that Building Control could take ownership of inspections, but concerns were strongly expressed about how this would be resourced and managed in practice.

5.3.3 Process to identify and mitigate unauthorised product substitution

This is a common issue across the whole of the construction sector, suggesting a need to review how quality assurance and compliance processes are implemented in factories and onsite. Contingency plans should be factored in to source alternative components of a similar specification if so required. Accreditation approaches can be a means of mitigating this risk; for example, BOPAS (Buildoffsite Property Assurance Scheme (BOPAS) – discussed in more detail in chapter 12), as part of its accreditation scheme, undertakes six-monthly surveillance visits to help ensure that there is no substitution of materials unless they are of equal approved and verified quality.

6. Transportation

6.1 Overview

Transportation is a critical part of the volumetric construction lifecycle; any damage to modules could incur significant cost and time delays to the project. If damage is not identified upon arrival onsite, there is a risk of installation of modules with defects, bringing about longer-term risks for the structural integrity, performance, and safety of the building.

6.2 Risks – transportation

6.2.1 Damage in transit

Fully finished volumetric modules must be transported from the factory setting to the site. If damage is incurred during transit, this can compromise the quality of individual modules or the completed building, depending on the extent of the damage, when it is detected, and remedial action(s) taken. The risk increases if damage affects fire performance and is undetected at the point of site installation. Research has shown that there are risks of damaging volumetric units in transportation and during lifting operations, specifically above large windows, and door openings. Data from Sweden showed that 10% of defects captured in audits resulted from transportation in lifting (Johnsson and Meiling 2009). This risk was reiterated in Scottish case study interview results (Duncheva 2019).

Accidental impact can also occur, for example, through damage to the structure from emergency braking during transportation. The order and method of loading is important and can create risk of damage if this is flawed. Packaging must be strong to prevent the risk of damage and cracks to volumetric modules, which can also result from issues with the structural system used – for example a rack is used, or a structure specifically designed to hold the portal frames.

Overseas imports are at an increased risk of damage due to the longer transit period; delays also contribute to this increased risk. Delays may arise due to factors outside of the manufacturer’s control such as congestion and weather conditions (Wuni and Shen 2019).

Water entry during transportation is a further risk which could cause long-term damage. If installed successfully, heavy-duty tarpaulins can provide temporary waterproofing for volumetric modules, but these may be a source of moisture build-up if used for a prolonged period (risks of water and moisture damage are discussed in more detail in section 10.2.3).

Evidence from insurers from the UK, Germany, Italy, France, and Russia identified the following risk factors in relation to transportation:

• improper stacking of volumetric units;
• lack of stacking tools appropriate for the volumetric units;
• long transit distances;
• inadequate transport road conditions (including the radius of gyration of the road and the limit of the bearing capacity of bridges);
• transport vehicles not meeting safety requirements, or being inadequate for the size and weight of prefabricated components (or volumetric units/pods); and
• lack of adequate measures to secure the modules in a safe position during transportation.

(Rohloff et al., 2021)

A further potential risk is the ensuring the availability of appropriately skilled and experienced logistics organisations; as the demand for volumetric construction rises, respondents note that this in turn increases demand for the logistics supply chain. If a less experienced logistics team is used, this increases the risk of damage to volumetric units during transit.

The risk of module damage is specific to volumetric construction. Components transported to traditional construction sites also run the risk of damage, but the impact of module damage would have more severe consequences in volumetric construction.

6.2.2 Process ownership between factory and site

Research respondents emphasise the importance of maintaining the golden thread throughout the manufacture, transportation, and site installation stages. There is a risk that roles and responsibilities are not clearly defined contractually, making it difficult to ascertain who takes ownership and accountability from a contractual perspective, and in turn, who is responsible for identifying and remedying any damage. It is not always clearly defined within contracts.

It should be clear whose responsibility it is to ensure proper handling of the modules. [Depending on the contract terms,] It is the factory’s responsibility to produce the modules, and the contractor’s responsibility to assemble them onsite, however it can be a bit grey in between.

This risk is specific to volumetric construction, due to the need for the interface between offsite and onsite.

6.3 Risk mitigation – transportation

6.3.1 Defined processes and contractual responsibilities

Respondents point to examples of risk assessments undertaken at the earliest possible stage of the project; initial scoping and planning should include plans for transportation, thinking about module material, size, weight, distance, and route to be travelled, as well as protection against exposure to cold, heat and moisture. A cradle to grave quality assurance process should set out appropriate protection measures for different types of systems and materials – for example anti-corrosion measures. Scheduling should avoid the need to store large volumes of modules onsite, where they could be exposed to damage from external events. It is not clear how often modules are actually stored onsite for any prolonged period of time, prior to installation/assembly.

Contractually, responsibilities should be clearly defined as to who takes ownership in the event of module damage discovered upon arrival onsite, prior to or post installation, and the actions to be taken in consequence.

6.3.2 Use of appropriate systems for transportation and storage

Damage from emergency braking can be mitigated for buildings using a portal structural system, as this this means each module will have independent structural stability, without the need for additional temporary structural stiffeners for transportation. For buildings adopting the racking approach, individual modules may require temporary racking stiffeners that can be removed when the module has reached its final location.

6.3.3 Process upon arrival at site

Site quality checks and inspections are essential to ensure modules arrive onsite in the same condition as they left the factory, with appropriate remedial actions taken if this is not the case. This approach needs to be underpinned by skilled and knowledgeable individuals undertaking the inspections. This process should be defined in a cradle to grave quality assurance process.

7. Site installation

7.1 Overview

The most critical risk at this stage is the interface between offsite and onsite; if there is misalignment or error, for example in relation to processes, tolerances, or sequencing. The onsite team needs to understand the original design and have the skills and knowledge to install the modules as specified. If errors are introduced at site installation stage, this can undermine the intended design and result in quality, performance, and/or safety issues if not detected and rectified. Remediation of errors can incur additional costs and create programme delays.

The use of cranes is pivotal in volumetric construction, requiring intensive pre-planning considering factors such as site layout, number/type of cranes, lifting operations planning, weather conditions to name only a few. Plans which do not factor in all relevant considerations can contribute to risks of accidents onsite and/or damage to modules.

Respondents point to the need for a digital database; a means of collating digital records about how buildings were constructed, materials used, testing and warranties and other information relevant for building repair and maintenance. This is relevant for all forms of construction, not just volumetric.

7.2 Risks – site installation

7.2.1 Interface between offsite and onsite

Insufficient coordination and communication between the offsite and onsite elements of a volumetric construction project could have severe consequences. If onsite processes are more aligned to traditional construction rather than volumetric, particularly if coupled with gaps in skills and knowledge of onsite installers, then this can result in defects, structural problems and risks to safety and building performance.

At the outset, there is a risk of incompatibility between the foundations built onsite, and the modules. This could affect the stability of the building if not identified and remedied. As stated earlier, if there is a lack of ownership and/or contractual definition of roles and responsibilities across the entire process, it may not be clear who is responsible for remedying identified issues.

There are often problems with interfaces – module to module, ground to module. They are somehow cobbled together onsite, but people onsite don’t necessarily know what happens in the factory.

This risk is specific to volumetric construction, due to the need for the interface between offsite and onsite.

7.2.2 Dimensional and geometric variability

Dimensional and geometric variability in volumetric construction can stem from manufacturing processes, flexing, warping and damage of components during transportation and installation and must be carefully managed (Lawson et al. 2014). There is significant variance in tolerances in construction; within volumetric construction module production tolerances can be extremely small in comparison with typical site tolerances which rely on traditional construction techniques (i.e., larger tolerance magnitudes).

The risk, therefore, is that different tolerances can be used by groundworks and offsite contractors. In more severe cases, miscommunication in drawings can result in misalignment between traditional and volumetric systems. This risk is not widely discussed in literature, but its importance has been captured via direct site experience in Scotland (Duncheva, 2019). In a volumetric case study, a miscommunication regarding the location of the mains services connections was of only a few millimetres and mitigated easily. If a similar error happens on a different project but cannot be easily addressed onsite, this could require the re-transportation of the modules to the factory for re-work, or re-manufacturing of all impact modules. This would also necessitate re-work or re-construction of the groundworks – depending on the responsibility for the error, as it would not be captured until modules are delivered and installed onsite. This risk also highlights a further risk around lack of control over onsite alterations which deviate from the original design.

In taller buildings, stacking modules on top of another may result in significant inaccuracies of the structure in respect with the ground datum. Due to an increased number of stacked modules in high-rise buildings, the accumulative tolerance error in successive module placements may potentially become larger (Liew et al., 2019). The geometric and positioning tolerances in modular buildings may result in additional eccentricities loads on the structural components in the modules. It is reported that the geometric and positioning tolerance of modular buildings of 10 storeys is approximately h/350, which is 60% higher than the tolerance for conventional buildings that consider the positioning tolerance only (Lawson et al., 2012). This reinforces earlier points about the need for design principles to be appropriate.

The stability of the structure depends on the connections between modules and connection to the structural core, [where there is one] and small misalignments in bottom floors could translate into large deviations at the top, more severely with greater building height

(Rohloff et al., 2021).

Achieving precise tolerances is required in both traditional and volumetric construction.

7.2.3 Skills onsite

Skills gaps and shortages are well documented in the construction sector (Farmer, 2016; Brennan and Vokes, 2017). Lack of appropriately skilled and knowledgeable workers onsite can undermine the key advantages of volumetric construction, such as quality of the finished product and speed of project completion. Construction subcontractors who typically work with traditional methods may lack exposure to the materials, systems and assembly techniques required in volumetric construction. This risk is exacerbated with the use of more innovative systems as installers onsite are less likely to have any experience of them.

Skills gaps present a risk throughout the entire lifecycle of a volumetric construction project but are included here as this issue can be particularly problematic at installation stage and may result in flaws such as incorrect sequencing, inaccuracies in installation, and knock-on risks for fire protection. For example, if there are issues with joints where modules interface, this can be a significant risk to continuity of passive fire protection. Respondents interviewed for this research have pointed to numerous examples of jointing being missed completely or poorly finished.

Other examples of potential issues cited in literature include:

• seam material not securely fastened;
• drainage where required not installed on time;
• incorrect mounting of flexible connectors; and
• inaccurate positioning of starting point which can lead to assembly failure.

To be fit for purpose, training may need to be bespoke to particular types of systems, making it difficult to access nationally recognised qualifications which may lack relevant detail for installation of volumetric construction.

The risk of skills gaps and shortages is equally if not more apparent in traditional construction. In both volumetric and traditional construction, skills issues can result in quality issues and defects in the finished product.

7.2.4 Cranes and lifting operations

Volumetric construction relies on the correct and safe use of cranes, which are pivotal to transportation of the modules to their ultimate destinations. A review of literature relating to crane operations and planning in modular construction (Hussein and Zayed., 2021) suggests crane layout planning needs to identify:

• the optimum number of cranes by crane type (tower or mobile), model and locations;
• locations of supply points and allocation to demand points (to minimise onsite transportation time);
• operating lifting radius and crane capacity to bear the heaviest modules; and
• how operational planning may need to flex, given site layouts are dynamic based on progression of construction tasks - this does not appear to be widely researched in the literature.

Other key considerations for crane operations include the likely weather conditions and general surrounding environment and lift path planning. Longer paths or poor planning creates the risk of accidents onsite and/or programme delay. In the absence of on-going review of lift path planning (given the dynamic nature of the site), there is the risk of collision, particularly once modules are installed and may become obstacles for upcoming modules (Kang et. al., 2009).

Effective scheduling of crane tasks is also essential to avoid to risk of collision. This is not always clearly defined, and can be based on the crane operator’s experience, rules of thumb set for the site or First Come First Served (FCFS) i.e., lifting tasks take place in the order of receipt. This creates a risk of “sub-optimal solutions” (Hussein and Zayed, 2021).

Cranes must be stable, underpinned by effective supportive systems for mobile cranes, design of foundation of tower cranes and design of lateral support system of tower cranes (Sohn et. al., 2014). Instability of any kind inevitably creates a significant risk to safety onsite. Hussein & Zayed’s 2021 review found that effective planning should factor in the:

• crane location relationship with the design of crane supporting systems and design of crane foundation;
• weight of the modules;
• jib length of tower cranes;
• boom length of mobile cranes;
• self-standing height of the tower crane;
• ground-bearing capacity of crane locations; and
• loading capacity of lifting cables and hooks.

The extent to which all of these critical factors are incorporated into planning decisions for crane operations is not clear from the literature or interviews. Ineffective management of crane operations could result in accidents (minor, major or fatal), programme delays and/or increased programme costs (Hussein and Zayed, 2021).

A further risk is that of the potential for accident/injury if lifting operations are not effectively planned or implemented. Fard et al., (2017) studied the causes of accidents in modular construction (the scope spanned 125 accidents). Results showed that the most common injury was “fracture”, which took place due to “falling” because of “unstable structures”; structures may be exposed to extreme weather events before the whole structure is made stable. As a result of larger lifting loads the likelihood of lifting failure can increase, from operator error or equipment failure (Sertyesilisik et al., 2010; Hairstans, 2019).

Lifting operations and the associated logistics onsite are a key risk. Lifting a module which is not secured properly or has loose materials is unlikely – but would be a major incident if a whole module falls. It’s less likely but the impact would be severe.

Cranes are commonly used across the construction sector as a whole, but their use is more extensive in volumetric construction compared with traditional.

7.2.5 Damage incurred onsite

Any object that is required to be manoeuvred, must be able to withstand any resulting forces from this action. Stresses should be considered at design stage. Therefore, structural elements must be designed with the lifting and transportation in mind. Failure to do so can result in Ultimate Limit State (ULS) failure i.e., structural collapse, or a Serviceability Limit State (SLS) failure where the module will not collapse but may show signs of excessive deflection (Hairstans, 2019; Srisangeerthanan et al., 2020).

Torsional buckling can result from lifting procedures which are not fit for purpose (inappropriate temporary storage and poor structural rigidity during transit are also risk factors). Respondents generally deem this risk to be low, but if it did occur impacts could be severe. Torsional buckling of the volumetric module can result in an ULS failure or a SLS failure within members or connections forced open or cracks in wall linings and damaging the airtight seals. This could potentially lead to water ingress or moisture damage, which would have an impact on the building once constructed (Lawson and Richards, 2010).

Any damage incurred onsite as a result of errors during assembly and module connection (or if errors had been undetected before leaving the factory) would require repair or potentially even removal/replacement of entire modules. The risk of undetected damage (more likely in the event of skills and knowledge gaps onsite) is that defects would be ‘built in’.

Damage incurred onsite can result in structural risk to modules, but traditional construction quality issues/defects can also create risk for structural integrity.

7.2.6 Building control and inspection

Compliance with the building regulations for mid-rise steel volumetric construction typically requires design for notional element removal, in combination with tying modules together at their upper structural level to prevent disproportionate collapse. Building inspections need to carefully inspect the tying of components at the upper structural level, but there is a risk of not being able to access all connections for inspection after all modules have been connected.

A further risk in building control and inspection of volumetric projects is the high variety of connections, in terms of type and specification. If the building control inspectors are not familiar with the correct installation of each of these connection types, there is a risk that either modules that are not well connected will be approved, or that sophisticated innovative connections will not be approved out of caution. This was revealed by a systematic review into the risks of modular construction looking mainly at the Asian context (Wuni et al. 2019).

Evidence from insurers suggests they perceive risks in relation to a perceived lack of:

• quality inspection methods;
• technologies to test the quality of connections;
• quality acceptance method and standard system;
• catalogue of building parts and components; and
• proper testing and certification of materials and accessories used for component installation (Rohloff et al., 2021).

However, it is not clear from the evidence how frequently these risks are actually observed.

This risk in relation to connections is specific to volumetric construction.

7.3 Risk mitigation – site installation

7.3.1 Defined processes, with clear roles, responsibilities, and accountability

Strongly echoing mitigation strategies from previous chapters, the need for clearly defined processes is re-emphasised for this stage of the lifecycle of volumetric construction projects. It is particularly important at this stage to have a minimum standard process for the handover between offsite and onsite, incorporating quality checks of modules upon arrival at site and procedures for storage of modules when onsite, including regular assessment of the risk of water ingress and checking for moisture content (discussed in more detail in section 10.2.3). Building services require further planning for contractors to achieve the required sequencing needed that achieves DfMA (Fraser et al., 2015). Projects need to have input throughout – and particularly at site installation - from experienced structural engineers, with relevant knowledge and understanding of volumetric construction.

7.3.2 Compliance check

A minority of respondents suggest the need for a new role or a defined step in the process at the interface between offsite and onsite – to provide a compliance check. It should be noted, however, that concerns were flagged about having just one Compliance Lead/Manager – as this would place significant demands and pressure on one role, with potential knock-on effects on PPI costs. This would focus on checking the modules before they leave the factory, as they are packaged for transit and upon arrival at site – including checking for movement of elements fitted in the factory, moisture content/water ingress or any other damage and storage of modules. It should be noted that many of the organisations interviewed for this research already have this in place within their own in-house processes and quality assurance procedures. However, the exact steps taken vary between organisations; there is no defined industry standard setting out the minimum steps which must be taken which comprise a ‘compliance check’ to ensure risk is mitigated. Such an approach would need to be incorporated within an overarching quality assurance process as previously described, i.e., defined at a macro level. This compliance check could be repeated in different ways at other key points in the process – notably to ensure no deviation between what is designed and what is built, but also to define any remediation and responsibilities for this (mirroring contractual terms), where relevant.

7.3.3 Building records – database

There is a clear consensus among respondents about the need for some form of database (relevant as a mitigation throughout the construction lifecycle; highlighted here due to the potential risks that arise at the transition between offsite and onsite) to store digital records of all buildings – regardless of their mode of construction. Respondents suggest that any such database would need to include, as a minimum:

• information about how the building was constructed;
• materials, products, systems, and components used;
• testing, accreditation, and warranties including fire tests undertaken and the results;
• guidance regarding repair and maintenance for occupants, building/facilities managers; and
• photographic evidence (date and time stamped) to provide an audit trail from the original specification, factory manufacture, preparation for transit, arrival onsite, installation, and completion.

Opinions are mixed among respondents as to the accessibility of such a database. A minority of respondents believe this should be publicly accessible to ensure transparency. However, concerns were flagged that members of the public may find the information difficult to understand – an alternative proposal being to create a ‘Homeowners’ Manual’ (or similar wording), to provide each occupant with relevant data about their own home. The majority of respondents believe building data should be available to relevant organisations such as Building Control Bodies (BCBs), firefighting/safety bodies, insurers, lenders, and building/facilities managers – to share with others as required.

7.3.4 Early engagement with Building Control

Early engagement with Building Control – ideally from the outset or at the point of early-stage manufacturing – should determine what the optimum inspection regime will be. There are mixed opinions emerging from interviews, with one school of thought suggesting that there should be a minimum inspection regime requiring BCBs to inspect factories at pre manufacturing stage and during manufacturing, rather than just at installation stage. This would also require BCB involvement at the design stage, hence the need for early engagement.

However, an alternative point of view emerging from the interviews is that greater involvement of BCBs is unrealistic, on the grounds that there is insufficient resource already within BCBs. It was also suggested that BCB staff would require more education/training in volumetric (and MMC generally).

Regardless of the inspection approach, which is ultimately defined, the majority of respondents agree there is a need for better education and information about volumetric (and MMC) within BCBs. A small number of BCBs were highlighted as “leading the way” due to their strong knowledge of volumetric and desire to build robust inspection regimes, but overall, the perceived lack of resource within BCBs was a concern for respondents.

7.3.5 Tolerance strategy

Project management should include the development of a tolerance strategy to proactively manage risk of misalignment onsite. This should make use of digital tools and technologies to improve accuracy and reliability; Shahtaheri et. al., (2017) proposes the use of 3D imaging and sensing technologies to monitor and control tolerance strategies throughout production, transportation, and installation including onsite.

It is important to control the combined geometric and positional deviations in the construction of modular buildings. It is recommended that tolerance issues are considered as part of the structural design, including consideration for structural movement over time. The reinforced concrete core walls or any laterally bracing systems should be constructed to similar accuracy to avoid unacceptable gaps among the modules, causing subsequent installation problems onsite (Liew et al., 2019).

In a cross-laminated timber (CLT) case study, a specialist contractor was hired to ensure the tolerances of the in-situ floor slab were of the same range as that of the CLT system (in millimetres rather than the traditional centimetres) (Duncheva, 2019). This approach and quality control help to prevent misalignment between in-situ and offsite manufactured components. Another solution is increased use of 4D BIM simulations (3-dimensional construction simulations including time as the 4th dimension) and BIM-enabled clash detection software to capture misalignment before modules are manufactured.

The table below sets out examples of risks and mitigations (content taken from Shahtaheri et. al., (2017)).

Table 1: Examples of risks related to dimensional and geometric variability and corresponding risk responses in construction

Risks related to dimensional or geometric variability	Examples of risk responses - reactive strategies	Examples of risk responses - proactive strategies
Component(s) too large	Trim parts, exchange with another component, ream holes, warping and racking	Splicing and lapping at joints, pipe cut-lengths, strict production tolerances, produce smaller component and fill-in gaps
Component(s) too small	Fill in the gap: spaces, shim plates, grouting, caulking	Splicing and lapping at joints, pipe cut-lengths, strict production tolerances
Component(s) not level	Grouting, shim plates, spacers	Strict production tolerances, self-levelling technologies, flexible and adjustable connections
Excessive geometry changes and misalignments	Warping and racking, discard component and replace	Flexible or adjustable connections, increasing relative stiffness to withstand larger loads, rigging strategies, temporary bracing strategies
Onsite fit up and assembly conflicts	Warping and racking, force-fit into place	Flexible or adjustable connections

Content reproduced from Shahtaheri et al., 2017

Technologies can be deployed to manage dimensional and geometric variability – such as 3D fixturing and jig systems, laser cutting and robotic assembly (Shahtaheri et al., 2017) during production. Further digital tools such as laser scanning can be used for inspection of production compliance with specified tolerances, however it is still often difficult to achieve alignment once onsite, in light of varying site conditions (Shahtaheri et al., 2017).

7.3.6 Risk management strategy for crane operations

Evaluation of risks on different performance measures of safety and productivity and risk mitigation strategies relating to crane operations should take into account the dynamic nature of the site, as well as weather conditions which are impossible to control. Many strategies discussed in the literature do not take this into account, instead providing “static solutions” (Shahtaheri et. al., 2017). There should be clearly defined and tested procedures for lifting and rigging, underpinned by regular inspections of lifting tools and machinery.

7.3.7 Research into digital tools and technologies

Further research is recommended into the use of digital technologies to mitigate risks in crane operations, such as positioning and tracking systems, visualisation of lift paths, optimisation of sequencing of crane tasks, use of motion sensors and laser scanning. This is discussed in more detail in chapter 13.

7.3.8 Training and qualifications

The education and training of the workforce has significant implications for curriculum design for further and higher education providers. Evidence from interviews indicates that it is critical that new vocational and degree MMC construction programmes should integrate with computer science, mechatronics, and manufacturing principles.

Skills gaps in the existing (and future) workforce partly stem from a transition from traditional to volumetric construction; those trained in traditional approaches may lack the necessary understanding for effective onsite installation of volumetric modules. Manufacturers are typically training their own installers (either directly employed or sub-contracted), but this may only be applicable to their own particular product/system. As new products and systems are introduced over time, this points to a need for regular upskilling, best suited to Continuing Professional Development (CPD) courses which are easy to access and to update.

Given the critical importance of correct, accurate installation onsite, for volumetric construction, it may be necessary to consider formal accreditation for installers, however this could be difficult to implement in practice because of the range of different products and systems in use (which are continuing to evolve).

8. Occupancy and beyond

8.1 Overview

Building performance evaluation is essential to gain long-term data about volumetric buildings, specifically thermal performance, interaction of different materials and components, structural resilience and response to any water or fire damage. There is a potential risk that occupants, or builders without the relevant skills and understanding of the volumetric building, could make changes via repair and maintenance that compromise the integrity of the building. However, post-occupation phases of volumetric buildings and repair and maintenance issues do not appear to be major areas of research, and so it is not possible to understand the full extent of this potential risk.

Long-term data about volumetric building performance as well as initial structural and architectural design details would better inform processes for safe disassembly, as well as providing insurers with greater confidence and reassurance about the longevity of the asset. Respondents point to the need for greater transparency; digital records of buildings to provide clear evidence of building methods, materials, testing, inspection and guidance for repair and maintenance.

8.2 Risks – occupancy and beyond

8.2.1 Limited occupant and building performance evaluation

Clients and other stakeholders of the building procurement process may omit or disregard the need for building performance evaluation or post-occupancy evaluation (POE), or both. Such studies are critical in the understanding of volumetric construction given their approach, use of materials, and thermal and structural innovation. Learning from the building and user behaviour can enhance the performance of volumetric buildings (Fifield et al., 2018), therefore the risk is that errors are repeated over time if key learnings are not captured and acted upon.

This risk is also applicable to traditional construction, however, having access to more long-term performance data for volumetric buildings would help to build an evidence base for insurers, lenders etc. who are seeking reassurance about longevity of volumetric buildings.

8.2.2 Potential risks introduced by repair and maintenance

Research into the risks introduced by occupants themselves – i.e., who want to make changes to the internal structure of modular homes – or risks inherent in extending modular homes, along with the risks associated with contractor awareness (i.e., whether building contractors would know how to work on repairing or retrofitting a modular building) appears to be exceptionally scarce. A minority of respondents have flagged concerns that long-term building resilience or performance could be compromised by different replacement components or incorrect choice of materials affecting how the building behaves. Changes to modules or extensions undertaken by people without the relevant skills and knowledge could unwittingly cause damage. This risk is also prevalent in traditional construction but is likely to be more serious in volumetric or other lightweight framed construction.

A handful of studies explore the repair and maintenance implications of volumetric construction – in multi-occupancy and social housing – for in-house maintenance teams responsible for routine repair and asset management. Volumetric buildings can present challenges to maintenance teams if they do not understand how the building was constructed and any implications for repair and maintenance as a result, highlighting the importance of having a trained workforce and the potential risks which may be engendered if maintenance teams do not possess the requisite skills (Kempton, 2010; Kempton and Syms, 2009).

Post-occupation phases of volumetric buildings and repair and maintenance issues do not appear to be major areas of research (Jang et al., 2021; Abdelmageed and Zayed, 2020). Much of the research which relates to repair and maintenance consists of stakeholder and industry views about the enhanced quality of modular products manufactured in controlled factory environments, and the perception that modular components have reduced failure rates, both minimising the need for subsequent repair and maintenance and reducing whole-life costs of buildings (NHBF, 2016; KPMG, 2016).

Research undertaken by Arup identified the potential for failure modes in adhesively bonded brick slip systems, arising from deterioration of the adhesive and/or its interfaces. It is unclear whether adhesive-only brick slip systems are being regularly tested to check how they will avoid this failure mechanism – or even if a relevant product test or standard exists. Furthermore, there are different accreditation regimes which do not align – a minimum 60-year design life assessment for a module chassis, compared with a 25-year BBA certificate for a brick slip. This is not specifically a volumetric construction issue and warrants further investigation as there is a risk of serious injury in the event of failure of the adhesive/interfaces.

There has been a modicum of research into the maintenance performance and quality of modular components. Research into the maintenance requirements of modular bathrooms, compared to in-situ bathrooms installed using traditional methods, has emphasised the superior quality of modular components, suggesting that such modular components have fewer maintenance issues and costs compared to traditional builds (Gibb and Pan, 2009; Pan et al., 2008). Other research conducted in South Korea, exploring the maintenance priorities of prefabricated residential buildings, suggests that repair and maintenance requirements of many modular components are low. In respect of ongoing maintenance, tighter build tolerances in the original build could mean it is more difficult to extract damaged components and replace them. (Lee at al., 2018).

This risk is also applicable to traditional construction.

8.2.3 Discrepancies between ‘as designed’ and ‘as built’

A critical aspect for safe and efficient dismantling and demolition is having access to the building documents, which should contain the original or most up to date building specification (Bradley, 2001; Kowalczyk and Dorsthorst, 2001; Storey and Pedersen, 2003). This information is used to gain an understanding of the original intended building use, the materials that will have been used, and any repair or maintenance carried out over its lifespan.

In the UK, houses are typically designed with a minimum 60-year design life, although many significantly outlive this, and over the course of this period the original documents can be difficult to source, are no longer in existence, or do not accurately reflect the building being disassembled (Rose and Stegemann, 2018). Typically, in the UK, pre-construction drawings are archived by local authority building control. As the name suggests, the pre-construction drawings are only up to date prior to the construction, and therefore the relevant local authority is unlikely to have updated these following any design changes (Rose and Stegemann, 2018). The most up-to-date drawings are most likely to be retained by the design team, however, these are not always reflective of the as-built structure, particularly if there has been repair and maintenance carried out during the building’s use phase.

When disassembling volumetric modules, risks in terms of time and cost will arise if original drawings cannot be sourced, if there are discrepancies in the as-designed and as-built construction, or if the building has been subject to repair and maintenance without updating the relevant drawings.

This risk is also applicable to traditional construction.

8.2.4 Risk of disproportionate damage in the event of fire or flooding; modules may be difficult to access to replace or repair

Insurers have raised concerns about potential disproportionate damage from water ingress or fire damage (including water damage as a result of tackling the fire) in modular buildings, compared with traditional construction, citing case study evidence to support these concerns. Modules can be difficult to access for repair or replacement once damaged. Insurers require long-term data about the costs for repairs, ability to source expertise to undertake the repair (i.e., does this require specialist expertise/knowledge), and ease with which they can be carried out, and about the behaviour of combustible materials in a volumetric building as well as the possible water damage in flood risk zones.

This risk of disproportionate damage is specific to volumetric construction, i.e., the perception is that damage would be more severe in a volumetric building compared with a traditionally constructed building.

8.3 Risk mitigation – occupancy and beyond

8.3.1 Data capture – long-term performance of volumetric buildings

Respondents suggest negative insurer perceptions of volumetric construction are predominantly because of an absence of long-term data into resilience of volumetric buildings and how they behave in the event of flooding or fire, which can result in reduced longevity of the asset. Insurers seek evidence about the cost and ease with which repairs can be carried out, and about the behaviour of combustible materials in a volumetric building as well as the possible water damage in flood risk zones. Insurance can be obtained, but it may be more cost prohibitive compared with traditional construction. Long-term data needs to be captured about volumetric building performance. By implementing occupant related studies such as POEs, a deeper understanding of performance is obtained which evaluates fully the energy demand, carbon impact, and occupant satisfaction levels (Popovic et al., 2021).

8.3.2 Deconstruction planning

Development of a deconstruction plan in line with what has been specified by the SEDA design guide is one option to mitigate risks at the point of disassembly (Morgan and Stevenson, 2005). Developed by the structural engineer, the deconstruction plan should be held with the legal deeds of the building, its Health and Safety file, and the maintenance file. The plan should contain information on the material inventory, building elements, and instruction on how best to deconstruct the components. Over the course of the building’s life, the deconstruction plan should be regularly updated with any structural alterations made to the asset.

9. Fire safety

9.1 Overview

While there is insufficient evidence to suggest whether a fire is more or less likely in a modular building compared with a traditionally constructed building, the event of a serious fire may result in more serious consequences in a modular building if the choice has been made to use combustible elements in the voids and cavities through which fire and smoke can travel quickly – unless these risks are mitigated within the design, manufacture, and installation.

It is essential that fire performance and the requirements particular to modular construction are well understood from the outset and factored into the design. Risks can be introduced if there is no fit-for-purpose fire strategy, in the event of lack of continuity of structural and general fire engineering expertise throughout the project and where standard fire testing does not work effectively for volumetric products and systems.

Respondents broadly agree that there are gaps in the regulatory framework in relation to volumetric construction – it should be noted that Approved Document B is currently under review.

Further research is critical to have a clearer understanding of how materials (and combinations of materials/components) behave in modular buildings in the event of a fire.

9.2 Risks – fire safety

9.2.1 Understanding of the characteristics of modular buildings

Risks arise for fire safety if there is insufficient knowledge and understanding of the characteristics of modular buildings.

The fire resistance of steel-based modular construction derives from four important aspects of performance (Lawson et. al., 2012):

• the stability of the light steel walls is a function of the load applied to the wall and the fire protection of the internal and external face of the wall of the module;
• the load capacity of the module floor is influenced by the thermal-shielding effect of the ceiling of the module beneath;
• the elimination of fire spread by fire barriers placed between the modules (to prevent the spread of smoke or fire in the cavity between the modules); and
• the limiting of heat transfer through the double-leaf wall and floor-ceiling construction of the modules.

Evidence from interviews has identified the following key risks:

• while there is insufficient evidence to suggest whether a fire is more or less likely in a modular building compared with a traditionally constructed building, the event of a fire is likely to result in more serious consequences in a modular building if the choice has been made to use combustible elements in the voids and cavities through which fire and smoke can travel quickly – unless these risks are mitigated within the design, manufacture and installation;
• once a fire takes hold in a modular building, it may be very difficult to prevent total building destruction as the fire can take hold and spread quickly through the cavities if they are not properly fire stopped; and
• firefighters may not understand how a modular building could behave in the event of a fire, leading to life safety issues during firefighting.

Combustible materials

There is a need to consider the types of materials used in volumetric construction; it is typical to use combustible materials for insulation. Fibre reinforced composites (FRCs) have been used extensively in volumetric construction because of a high strength-to-weight ratio, ease of application and flexibility in manufacturing (Rohloff et al., 2021). Composites can be manufactured from a combination of materials, all of which may react differently to fire (Composites UK, 2019). Timber frame or composite panels may use insulation materials such as polyurethane (PUR) or expanded polystyrene (EPS) – both of these are highly combustible.

Combustible materials can introduce risks which have an impact on all parts of a building’s fire strategy, in particular flame spread across internal surfaces, external fire spread and space separation. Where combustible materials form large parts of a fire compartment’s surface area and can contribute as a source of fuel (either because it is exposed by design or may become exposed prematurely during a fire), they can affect the fire dynamics within the structure. Compared to non-combustible materials this may lead to higher heat release rates (HRRs), increased compartment gas temperatures, higher incident heat fluxes to structural elements, prolonged fire duration and more combustion outside of openings causing more severe external flaming (STA, 2020).

There is a risk to the building structure if the implications of building with combustible materials and allowing it to contribute as a source of fuel are not fully understood by the design team and considered at the outset of a project through a qualitative design review (QDR) process (STA, 2020). This risk applies to all buildings not just volumetric but, as previously stated, the event of a serious fire may result in more serious consequences in volumetric buildings.

Structural considerations

Modular buildings are constructed with individual modules connected through inter-component connections. This typical configuration introduces cavities and voids being formed between modules and within modules – which are potential pathways for fire and smoke to spread in the building (Nguyen et al., 2020).

Another characteristic of modular buildings is the cellular structure. The fire resistance of modular buildings should be investigated at individual module scale as well as a whole system (Lawson, 2014); i.e., local fire safety refers to fire resistance of an individual module, while global fire safety refers to the prevention of spread of fire from module to module. In any building, compartmentalisation and the use of fire-resistant materials are intended to constrain the spread of fire and smoke; effectively preventing the fire from becoming too large so that occupants can escape/be rescued, and firefighters can tackle the fire. Research conducted for the Ministry of Defence in 2021 highlights how modular construction differs from traditional construction given there is no continuous floor spanning the length and width of the building:

Typically, modular construction comprises of a set of modules where each module has its own fire resisting ceiling. These ceilings provide protection to the floors of the modules above. It is important to note that a common modular construction method is to terminate these ceilings at the module wall lining. This results in a discontinuous ceiling line. This in turn results in a series of vertical and horizontal voids (or gaps) that are formed between each module

(Arup, 2022 – unpublished draft)

If fire gets into a void without effective compartmentalisation, the fire can continue to spread while firefighters are focused on rescue. This risk can be exacerbated if combustible materials are used in the voids. Fire compartmentation could also be undermined by drilling the modules in order to install additional services; this presents a risk if not addressed during the design macro-phase (Arup, 2022).

Fire stops and other preventions for the inter-module cavities must be properly designed considering all load-transfer components (including connections) are located in this space (Paneni, 2018). There is a risk of collapse in the event of a fire if fire stops are incorrectly installed. Limited access to the cavities after assembly of the adjacent modules also make it difficult for inspection and further maintenance throughout the building’s lifespan (Nguyen et al., 2020).

The fire resistance of the building can be compromised as significant thermal expansion of each module may trigger failures at rigid connections (Paneni, 2018).

This risk is specific to volumetric construction as it relates to having a dedicated knowledge and understanding of the characteristics of modular buildings in order to achieve fire safety.

9.2.2 Potential for conflict between fire safety and energy efficiency requirements

Successive research studies identify a conflict between sustainable/green building design and fire safety compliance (Meacham and McNamee, 2020; Roberts et al., 2016; Gollner et al., 2012). This body of research indicates how a range of sustainable design features in buildings may inadvertently increase the risk of fire. Although it should be emphasised that this literature is not specific to volumetric, numerous findings remain directly applicable to sustainable design features of volumetric construction. An example was cited by one respondent who pointed to the choice between using PIR insulation and mineral wool; the former is more energy efficient but is flammable, whereas mineral wool offers better protection against fire and must be used in greater quantities to achieve the desired level of thermal insulation.

A key risk with fire safety specific to volumetric steel residential buildings is the conflict in requirements between acoustic performance and fire performance. Volumetric steel frames tend to be tied together and are also tied to the ground floor slab and any structural core elements, allowing impact sound to travel through the structure. To resolve this, complex details are often designed.

Evidence from respondent interviews suggests it is easier to demonstrate sustainability credentials within volumetric construction compared with traditional construction. Furthermore, combustibility concerns about the use of timber can lead to increased use of materials such as concrete, undermining potential for embodied carbon savings.

This risk is also applicable to traditional construction.

9.2.3 Standard fire testing not fit for purpose in volumetric construction

Standard fire testing does not fully address the specific requirements of volumetric construction. Standard approaches, based on British Standards/Eurocodes, typically look at individual details and do not test connections e.g., how a module is connected to a floor, nor do they test entire volumetric systems taking the height of the building into account. Research undertaken by BRE has identified that for MMC generally there are “modes of failure which cannot be predicted or observed based on standard fire tests” (BRE, 2022 – unpublished draft), notable gaps in standard tests and assessment being:

• interaction between components which is not addressed in standard fire testing; and
• connection performance in fire scenarios, particularly pertinent for light steel frame systems where the types of joint typically used to form the connection between floor joists and edge or ring beams (self-drill, self-tap screws, rivets, spot welds) may have insufficient strength or ductility to accommodate the large deflections.

Evidence from fire consultants and the firefighting community suggests suitable fire tests do not exist for all forms of volumetric construction. In particular, concerns have been flagged about testing of single units, rather than units operating together as a system, notably in high rise buildings. It is not clear from existing tests how protection will be maintained – i.e., if there is failure in one module, how this affects the others. Walls could be tested in isolation – potentially presenting an unrealistic scenario as walls typically have sockets, light switches, and pictures.

Other specific issues flagged during interviews in relation to fire safety product testing:

• reliance on fire tests in non-load bearing situations, when loadbearing may mean failure at a faster rate; and
• most fire protection products are tested between two concrete plinths, but in practice are more typically flanked by steel frame and plasterboard or cladding board.

There is very little fire related test data for volumetric type assemblies, either because the assemblies are not covered by the existing British Standards/Eurocodes, or because assembly manufacturers are extrapolating fire tests for stud systems from conventional construction.

We don’t have a suite of fire tests to prove performance [for volumetric]. There needs to be a suite of fire tests typical of the end use application.

The lack of one standard fire test methodology is a further concern for industry stakeholders. Test houses are given the specification of what to test, rather than following a standard approach. Furthermore, tests that do exist are not always suitable for the end use application in the context of a modular building.

Evidence from interviews points to a further risk around transparency of fire test data. There is limited fire testing capacity in the UK; many manufacturers need testing to be undertaken in other countries. Fire testing is typically deemed to be ‘commercial in confidence’, and results are not commonly shared or made public.

Research undertaken for the Ministry of Defence in 2021 highlighted that third-party accreditation schemes, certifying products to achieve a certain performance, should not automatically be taken to mean that a product or its installation is compliant, and therefore should not take away the need to supply evidence to robustly demonstrate compliance i.e., performance in the event of a fire (Arup, 2022). The Ministry of Defence research also cautions against any potential adverse effect of protective membranes used to wrap modules during transportation, which could affect their fire performance. This potential risk may not be well understood from existing fire tests.

It should be noted that a fire test and performance standard was created by BRE in 2014 applicable to volumetric construction. The Loss Prevention Standard (LPS) 1501 was created to define ‘fire test and performance requirements for innovative methods of building construction.’ One of its objectives was to cover fire performance of innovative building systems – including modular buildings – ‘not addressed under recognised standards and codes for building construction with respect to fire performance.’ It outlines a test method to determine if there are any weak areas within modular building systems that could inhibit or prevent them achieving prescribed fire performance.

Respondents interviewed for this research believe this is the only fire test standard that has direct applicability for modular construction. The procedures were modified and included as a revised approach in the Annex B of BRE’s Building Performance Standard (BPS) 7014. However, there has been very limited take up of this standard; anecdotal feedback suggests it was not widely publicised and that it is expensive to implement, acting as a barrier to its adoption.

It should also be taken into consideration that relevant design guidance for fire safety is developing and evolving.

Design guidance for fire resistance and structural fire safety has been recently produced by the Steel Construction Institute (SCI) and Structural Timber Association (STA). SCI guidance (see for example, guidance P375 and P424 available from the Steel Construction Institute) includes an overview of the fire design of steel and composite structures, criteria to be met, approaches to determine fire resistant structures and fire safety engineering (see chapter 13 for more detail). The STA has undertaken extensive research in partnership with industry representatives and academics, to produce its Fire Safety in Use Guidance suite of documentation, including a pattern book of EN tested timber frame systems (STA, 2021a) and guidance on cavity barriers and fire stopping (STA, 2021b) for timber frame buildings up to 18m in height. This is expected to evolve over time, to incorporate bespoke or modified systems once they have been validated.

This risk is specific to volumetric construction.

9.2.4 Understanding of behaviour of materials used in volumetric construction during a fire

It is not always well understood how different types of materials (and combinations of them) used in volumetric construction will behave in the event of a fire. The extent of knowledge and understanding is reliant on the breadth and depth of research undertaken, and whether it is available in the public domain. Much of the research is undertaken or commissioned at individual manufacturer level and may be bespoke to their own particular products or systems.

Steel

Research published in 2020 states that light gauge steel frame (LSF) floor-ceiling systems are “increasingly used in buildings without a full understanding of their thermal and fire resistance performance” (Steau and Mahendran, 2020). Testing undertaken of LSF floor-ceiling systems in 2020 to understand insulation failure times and time-temperature profiles reached the following conclusions in relation to fire performance:

• the use of cavity insulation is detrimental (high hot flange and gypsum plasterboard temperatures);
• the use of even half-filled rockwool cavity insulation led to increased plasterboard temperatures on the fire side, which could cause premature gypsum plasterboard fall-off and reduced fire resistance (observed in the fire tests); and
• there is a need to improve LSF floor-ceiling configurations (relevant for thermal as well as fire performance) (Steau and Mahendran, 2020).

Anecdotal evidence suggests the fire performance of the different fixings and bolts used within light gauge steel frame volumetric construction once in situ is less well understood.

Research into fire performance of modular panels conducted in 2021 reiterated the need to have a better understanding of LSF and fire safety; “the link to how it [modular building system] performs under fire is still not understood comprehensively” (Perera et al., 2021). However, findings from assessment of fire performance of conventional and LSF modular wall panels (including 16 modular wall configurations with single and double fire resistance plasterboard linings and three different insulations: rockwool, glass fibre and mineral wool. The structural fire resistance time was determined using the established Load Ratio (LR) vs critical Hot-Flange (HF) temperature correlation) via numerical analyses found no noticeable difference in the structural fire resistance time between the modular and the corresponding mapped conventional LSF wall configurations. Furthermore, modular wall panels experienced enhanced insulation fire resistance. The research also concluded there was no significant influence from the choice of insulation material on structural fire resistance (between rockwool, glass-fibre and mineral-wool) (Perera et al., 2021).

The same research from 2021 pointed to the need for enhanced understanding of Fire-Resistant Levels (FRLs) for LSF wall panels, because currently industry practice specifies structural, integrity and insulation FRLs for single skin wall panels. However, stacking two volumetric units back-to-back creates a double skin LSF wall panel for which there is no robust assessment of the FRL (Perera et al., 2021).

Research in 2021 points out “Certain walls may not need to be fire resisting for compartmentation reasons (i.e., a wall between two bedrooms within an apartment) but in an LSF building they need to be fire resisting in order to protect the structural framing within the walls. That level of complexity is often missed, meaning that certain walls are not designated as being fire resisting when in fact, they should all be fire resisting for structural reasons” (CROSS UK, 2019).

Perceptions of behaviour of Timber and Cross-Laminated Timber (CLT)

CLT is included here as it can be used for volumetric construction.

It should be noted that this section refers to the term ‘self-extinction’ in relation to fires in timber buildings – evidence from this research does not support the use of this term, on the basis that it can create a misleading perception that fire will eventually go out without requiring intervention from firefighters. Rather than exclude any references to ‘self-extinction’, this report points to examples of where the phrase is being used, specifically to highlight that it is a term which is being commonly used in research/literature, which creates a risk in itself regarding perceptions for fire safety for timber.

One of the primary concerns regarding behaviour of laminated timber in fire is the possibility of delamination caused by the elevated temperatures. If separation of lamellae occurs as the thermal wave penetrates the CLT, then fresh timber is exposed and there is a rapid increase in mass loss rate (and associated flaming combustion) (Emberley, R., et. al., 2017). It should be noted that this delamination behaviour depends on the type of adhesive used to manufacture the CLT and can be avoided if the CLT product uses adhesives which are more heat resisting. There are standards in the USA which required CLT products to pass product standards which control for this behaviour (ANSI/APA PRG 320-2019, Standard for Performance-Rated Cross-Laminated Timber) – typically referred to as ‘glue line integrity’ but there are no similar standards in existence in the UK or Europe.

As stated above, respondents from the firefighting and fire research community raise strong concerns about the use of the term ‘self-extinction’ in some of the literature in relation to timber, on the basis that this could create an expectation among occupiers that a fire in a timber building will eventually go out on its own – which it will in some circumstances, but in many other circumstances it will continue to smoulder and will not extinguish without firefighter intervention. This is noted here to highlight the potential risk that research referring to ‘self-extinction’ could perpetuate misleading views about the behaviour of timber in modular buildings in the event of a fire.

The applicability/relevance of a guidance-based route for timber to compliance with current Building Regulations depends upon the structural fire performance objectives:

• provision of adequate time: the structure having a reasonable likelihood of surviving the full duration of a fire is not a prerequisite for compliance with Regulation B3(1)
• an adequate likelihood of surviving burn-out: unless the structure is prevented from contributing as a source of fuel, applying the fire resistance guidance in, for example, ADB cannot be said to result in a structure that can satisfy Regulation B3(1)

It should be noted that existing guidance documents do not appear to be sufficient to address all relevant factors which are affected when using structural timber elements in buildings.

Partial protection implies that the fire resistance classification from BS EN 13501-2 is achieved through a combination of contributions from the lining (often protection) material and substrate (e.g., sheathing, or structural element) (STA, 2020). There is a risk that the design phase does not consider potentially changing fire dynamics, the risk that fresh fuel could become available during the late stages of a fire and the corresponding risks posed to firefighters as a result.

Encapsulation implies that sufficient protection is provided to the underlying structure/substrate to mitigate the onset of pyrolysis for the full duration of the relevant fire resistance period. This is commonly addressed through the specification of linings achieving demonstrated k2 classifications per BS EN 13501-2. Where encapsulation is adopted as the design solution, further consideration needs to be given to the overall fire strategy’s ‘defence-in-depth’ to assure that premature failure of any protective lining does not lead to disproportionate damage/collapse of the overall structure (STA, 2020).

Exposed structures and self-extinction concern the cessation of flaming combustion of the structural elements either because they have been exposed to a fire from the outset or have become exposed throughout the duration of a fire due to a partial protection solution. The STA guidance notes that demonstrating self-extinction is considered a prerequisite for compliance with Regulation B3(1) in cases where the performance objective is an adequate likelihood of surviving burn-out and encapsulation is not proposed, and that demonstration of self-extinction would form part of a performance-based route to compliance [STA, 2020). Again, this is included to reinforce the risk in relation to the use of the term ‘self-extinction’ in literature and other documentation. If this becomes part of commonly used terminology, it is setting an expectation that the fire will eventually go out on its own.

Structural backstop - irrespective of the solution outlined above, the (residual) structural elements must be capable of supporting the load either for the duration of the fire resistance period or for the full duration of a fire, as relevant to the route of compliance. This will typically involve a demonstration that the structure can support the loads for an accidental loading combination, as set out in BS EN 1990 and BS EN 1991-1-2 (STA, 2020).

Research undertaken for the Ministry of Defence finds that Cross Laminated Timber (CLT) can achieve significant fire resistance because of its mass, but when designing a fire strategy, careful consideration should be given to:

• a risk of failure of CLT in a fire at the glue line between lamellas;
• when exposed CLT is used there will be an increased compartment fire load; and
• junction details between CLT panels or between floors and walls and other elements of structure (Arup & DIO, 2021).

Research published in 2017 concluded that “a proper physical understanding of CLT’s mechanical response and failure modes in fire is needed to enable confident structural fire design and analysis of ever taller CLT buildings.” The same research found that instability failures – rather than material failures – were likely to be the defining structural fire failure mode for CLT compression elements in buildings and recommended further research into the reduction of elastic modulus in CLT when heated, “since stiffness (rather than strength) is critical for maintaining structural stability” (Wiesner et al., 2017) (structural stability is discussed further in section 11.2).

This risk is specific to volumetric construction.

9.2.5 Fire safety regulatory requirements

Evidence from interviews indicates that Approved Document B does not address all situations or modes of construction, and this is recognised accordingly in the Manual to the Building Regulations. A separate Technical Review of Approved Document B is in process, and its conclusions will be used to inform any amendments specific to volumetric construction.

Building Regulations in England and Wales set out minimum expectations focused on life safety. Building Regulation B3(1) states: “The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period.” This is clarified in Approved Document B (ADB): “For defined periods, loadbearing elements of structure withstand the effects of fire without loss of stability.” Whilst speaking of periods of time, neither the wording of Regulation B3(1) nor the clarified intention in ADB explicitly define the duration of structural stability required in the event of fire. The structural fire safety performance objectives for a building can vary in function of the consequences of fire induced collapse. Implicitly, the fire resistance paradigm and the guidance documents/codes that reference fire resistance periods include limitations (STA, 2020), such as, where the structure is allowed to contribute as a source of fuel, the required fire resistance of the elements in terms of having a reasonable likelihood of surviving burnout cannot be known in advance (OFR, 2020).

For higher consequence buildings, the affording of fire resistance to elements of structure is a proxy for an objective of the structure having a reasonable likelihood of surviving the full duration of a fire (burn-out). Where structural elements contribute as a source of fuel, the affordance of fire resistance to structural elements does not assure that the structural system will have a reasonable likelihood of surviving burn-out. By extension, the application of guidance does not always assure compliance with the relevant parts of the Building Regulations, with alternative routes to compliance required where a structure burns but must ultimately survive burn-out (STA, 2020).

This risk is applicable also to traditional construction.

9.2.6 Gaps: skills, knowledge and clearly defined end-to-end process

Echoing findings from previous chapters, fire safety may be at risk if there are skills/knowledge gaps in relation to fire performance in volumetric construction, and also in the absence of clearly defined roles, responsibilities, and processes. As described in detail in sections 3.2.1 and 3.2.2, considerable risk is introduced if the right types of expertise are not involved from the outset, and if there is no clear ownership of the end-to-end process.

Evidence from interviews with manufacturers suggests that structural fire engineers are often involved too late in the process, are not involved throughout the process, or that generic fire strategies are provided by fire engineers who lack experience of volumetric construction. Evidence also points to a lack of clarity as to task ownership, if there are gaps between modules, it may not be clear within contractual documentation whose responsibility it is to ensure fire stopping in those gaps.

The problem with fire is that on the vast majority of projects, no one person or organisation is able to advise whether a robust end-to-end process has been followed. The fire engineer will set the requirements, but not police their application. A more joined-up process is required to ensure robust fire compliance, and this is especially important for volumetric projects where additional risks are present.

Respondents emphasise the importance of skilled installation, to ensure fire safety details are correctly fitted and therefore perform as expected. Fire safety issues can arise which are outside the control of the manufacturer, if incorrectly fitted or inadvertently damaged onsite. Factory fitted cavity barriers risk incurring damage during loading and unloading of modules and potentially during transit. If any such issues are not identified onsite when modules arrive, the risk is that defects are built in (described in more detail in section 7.2.3).

Prefabricated façades often consist of more than two layers to provide different performance such as thermal, acoustic, air and watertight, load bearing, fire, and aesthetic characteristics. The use of multiple layers in prefabricated façades often involve combustible components such as vapour membranes. In conventionally constructed buildings, façades are generally the last element to be installed and built as a continuous skin of the external envelope. In volumetric buildings, façades can be constructed offsite for each individual module and then the gaps are connected on the building site to make the connection seamless. However, significant dependence on the accuracy of lifting cranes to position modules may cause incorrect alignment – thus compromising the full performance of the fire barrier and possibly resulting in life safety risks (Nguyen et al., 2020).

We need a true warranty – product warranty is one thing, but this is meaningless if incorrectly installed.

This risk is applicable also to traditional construction.

9.3 Risk mitigation – fire safety

9.3.1 Integrated approach

Fire experts should be brought in at an early stage of the project to ensure there is an understanding of how to demonstrate and achieve required levels of fire resistance, including the requirement for and placement of cavity barriers and fire stopping. Respondents emphasise the need for a structural fire engineer to bring the right level of expertise. The behaviour and interaction of all modular components in a closed façade system should be carefully considered and investigated. Designers, manufacturers, and installers all require a shared understanding as to how fire compartmentation will be achieved. Fire barriers or fire stops should be installed correctly, and quality inspected within the module. It is also important that all modular units within the building are properly aligned and within specified tolerances (Nguyen et al., 2020). Integrating the mitigations into a systems approach through the building’s lifespan would collectively mitigate this risk. The process should consider how different elements of design may impact other areas – for example how insulation requirements to meet net zero/Future Homes Standard needs could have an effect on structural considerations and fire safety.

Continuity – the same fire consultant for module and building design, detailed approval of all standard module details and project specific interfaces, regular check visits from fire consultant to factory and site to inspect detailed work in practice, with detailed recording of fire stopping measures for each module manufacture and install.

9.3.2 Standard fire test methodology

A standard fire test methodology would define minimum criteria for fire testing applicable to a modular context. A standalone British Standard for fire testing in volumetric construction (and MMC generally) is required to ensure clarity of the requirements that need to be met and provide greater reassurance to insurers. Whole systems and combinations of products/components need to be tested, rather than just individual units/elements. It is important that tests are application specific and include module connections and all fixings in scope.

9.3.3 Regulatory framework

The on-going Technical Review of Approved Documents should consider recommendations in respect of volumetric construction; respondents to this research propose that additional technical guidance should be issued to plug existing gaps within Approved Documents while concurrently clarifying where they are not appropriate for modular construction.

9.3.4 Fire strategies for all volumetric construction projects

An effective fire strategy in volumetric construction needs to consider (Arup, 2022):

• structural design of the building;
• which building elements are structural and which are not;
• which structural elements are loadbearing and which non-loadbearing to apply the correct fire resistance criteria;
• how fire resistance is determined;
• installation of cavity barriers taking into consideration building materials, likely extent of building movement over time and assembly sequencing; and
• locations for cavity barriers/fire stopping (likely to be visualised and communicated using software such as BIM).

According to the STA, the most appropriate route to compliance for a mass timber building project should be reviewed at the inception stage. The STA promotes the undertaking of a QDR (Qualitative Design Review) for all projects involving mass timber construction, albeit acknowledges that this may be more formal and comprehensive for higher consequence versus lesser consequence buildings.

BS 7974 provides a structured process for highlighting and determining fire hazards, fire risks, design actions and mitigation measures. The QDR will typically involve the following main stages: (STA, 2020):

• review architectural design and selection of materials, including their suitability and fire properties, occupant characteristics and client requirements;
• establish functional objectives for fire;
• identify fire hazards and possible consequences;
• establish trial fire safety engineering designs;
• set acceptance criteria for the designs;
• identify the method of analysis; and
• establish fire scenarios for analysis.

9.3.5 Building records database

As previously cited in section 7.3.3, digital records of the building should be readily accessible to facilitate appropriate repair and maintenance which does not change the building dynamic, potentially exposing it, its occupants, and firefighters to greater risk in the event of a fire. Fire testing data should be shared – (this data needs to be readily accessible to firefighters and building/facilities managers, owners, and occupants. Respondents did not believe this kind of detailed data needs to be in the public domain (as they felt it was unlikely to be well understood by the general public) this can be limited to the type of construction, material(s), fixings, insulation materials, test undertaken and under what conditions, date, and results so that Intellectual Property is not compromised. The National Fire Chiefs Council and the London Fire Brigade have called for an open-source register for building construction techniques to enable firefighters to be aware of any abnormal patterns in fire behaviour or ways to fight fires.

Photographic evidence should be supplied of all aspects of manufacture and installation relevant for fire safety, such as fire stops, utility connections and fixing plates between modules – with a focus on the areas where onsite work can be covered up after modules are connected. Several examples were provided by respondents whereby dedicated software was used to record date/time of photographs logged against a code (e.g., a QR code) for each specific junction.

9.3.6 Further research

Further research including a programme of destructive fire tests with different types of insulation is required to investigate the consequences of product combinations and how risks can occur and thus be controlled. Performance of LSF and CLT systems in the event of fire is not fully understood. Provision of appropriate semi-rigid connecting designs will allow flexible movement between modules to prevent potential failures of the structure due to thermal expansion (Paneni, 2018). The tying action between modules also affects the robustness of structural fire safety design of prefabricated buildings. It is recommended that a minimum tying force of 35 kN and 50 kN should be used in robustness design of modular construction where scenarios such as loss of corner or intermediate support are assumed (Lawson, 2014).

10. Products

10.1 Overview

Product and component testing for volumetric construction is subject to gaps, notably where suitable tests do not exist (in respect of new/innovative products), or the testing conditions do not consider the modular context.

Insurers have raised concerns about potential disproportionate damage from water ingress in modular buildings, compared with traditional construction. Detailed information and product guidance is required to be followed regarding quality control conditions for materials, products, and components.

10.2 Risks – products

10.2.1 Product and component testing

Stakeholders consulted during this research raised risks in relation to product testing, with particular focus on energy efficiency and fire testing (see section 9.2.3). It was noted it can be difficult to test new and innovative products emerging onto the market, as suitable tests may not yet exist or be difficult to access. Stakeholder evidence also flagged how products used to assemble a volumetric module as a system can be tested individually, but there may be reliance on other products for them to perform at the expected standard. Any subsequent unauthorised product substitution could therefore compromise overarching quality, performance, or safety – notably if replacement components lack third-party certification.

The approach to product and component testing may have gaps; for example, taking the UK climate into account or testing for combinations of components rather than reliance on individual parts achieving compliance in isolation. Bespoke tests for a range of product and component combinations are expected to be costly and time consuming, and therefore may be overlooked if individual components are deemed compliant against existing standards. However, this results in an evidence gap with no investigation into how products behave together. Tests may be undertaken retrospectively to validate an existing design, but such test evidence is not readily available in the sector.

Respondents were asked to consider the value of having a minimum product assembly standard; there was limited appetite for this given the wide variety across product types. Respondents emphasise that the process is more important – i.e., ensuring competence of site installation, integrated project management, clear task ownership and accountability.

This risk is applicable also to traditional construction.

10.2.2 CE to UKCA marking rules changes for building products

The UKCA (UK Conformity Assessed) marking came into effect on 1st January 2021 and covers most goods which previously required CE marking. It does not apply to existing stock manufactured and CE marked prior to 1st January 2021. Goods will still require CE marking for sale in the EU as the UKCA marking is not recognised by the EU market. New goods which are for the market in Great Britain in scope of the UKCA legislation will need UKCA marking from 1st January 2023 (CE marking is accepted until this date).

As volumetric manufacturers typically purchase materials in bulk for the manufacture of an entire project, there is a risk that an inability to source one component due to a lack of a UKCA mark may result in a ‘bottleneck’ or halt of the entire production line until an equivalent product can be sourced and approved by the relevant insurance company as a suitable substitution.

This risk is applicable also to traditional construction.

10.2.3 Water/moisture ingress damages products (or entire modules)

Risks of water damage include:

• moisture trapped between and within components;
• water in gaps between modules, which may require equipment to draw out the water and dry them;
• inability or difficulty in testing for moisture due to access issues; and
• difficulty in accessing damaged components for repair, especially if a base floor module needs to be replaced.

Condensation and mould issues present a further risk; cavities within modular buildings and spaces between modules enable outdoor air to easily enter and travel through the building with the potential to cause direct condensation, in turn leading to mould growth.

Recent research points to the need to consider future climate changes which could increase moisture content of insulation for CLT. “This would not only cause moisture problems in the insulating materials but also degrade the thermal insulation performance of the wall; this would increase the building energy consumption. The risk of mould growth increased with climate change for all insulation conditions. Therefore, for the CLT building with modular construction to be free from moisture problems, it is necessary to predict and evaluate the hygrothermal performance considering both the wall composition and future climate conditions of the building site” (Chang et al., 2021).

If products are not stored and maintained correctly (as set out in manufacturer instructions), with provision made for moisture checks and actions to remedy any damage quickly, water damage to modules may have significant short and long-term effects, contributing to consequences such as mould growth and swelling or cupping of timber panels.

Research undertaken by Stora Enso sought to better understand the properties of CLT in relation to its moisture dynamics. Findings have been used to inform detailed guidance about moisture management and onsite storage of CLT products, which take into account humidity and temperature – both of which can directly affect the moisture content of timber, but neither of which can be controlled if materials are exposed to the weather and are not allowed to dry out as per manufacturer specifications. Risks can be introduced if materials are not handled and stored in accordance with manufacturer specifications. This research is not available in the public domain, but Stora Enso have shared their findings for the purpose of this study.

This risk applies to all forms of construction, not just to volumetric – however evidence from large insurance companies suggest the cost of repair to volumetric buildings may be disproportionately higher compared with traditionally constructed buildings – mainly because of potential difficulties accessing modules for the purpose of repair.

10.3 Risk mitigations – products

10.3.1 Product testing

Increasing capacity among existing UKAS accredited testing facilities for testing of a wide range of building products to UKCA standards will help to mitigate this risk, as well as investment in additional accredited facilities for the anticipated significant demand for testing.

10.3.2 Moisture management and storage onsite

Review of guidance notes for CLT shared by Stora Enso for the purpose of this research, reveals the importance of access to comprehensive instruction about moisture management and storage onsite. Key considerations include:

• length of time for optimum storage;
• how packages should be stored outside onsite, including any need to raise them off the ground (even when still in external packaging);
• whether/how external packaging should be opened while stacked in storage;
• ventilation requirements of the storage facility;
• moisture control planning and frequency of testing for moisture content; and
• processes for inspection (upon arrival onsite and while stored onsite).

Initial project planning should evaluate the likelihood and severity of moisture-related issues and the risk of water ingress, with accompanying risk mitigation strategies.

11. Structure

11.1 Overview

The key risk for consideration is the role played by inter-module connections (IMCs), notably the structural behaviour and impact on robustness of modular structures. In recent years, there has been increasing research into existing types of connections, investigation into new types of connections and factors affecting risk of progressive collapse. Research is also being undertaken into high rise volumetric steel structures. However, it is generally concluded in the literature that more research is needed to achieve a comprehensive understanding.

Risks of poor-quality installation may be exacerbated by limited access onsite for inspection once modules are connected.

11.2 Risks – structure

11.2.1 Role of inter-module connections

One of the main differences between traditionally constructed and modular buildings is the presence of additional inter-module connections (IMCs) which join modules together onsite. IMCs are shown in the literature to be critical in influencing the structural behaviours of modular buildings, however more research is needed to understand the structural behaviour of inter-module joints and to study the robustness of modular structures.

Research published in 2020 has found IMC stiffness has a significant effect on overall building response to the lateral loads, particularly the translational stiffness of vertical IMCs in the same direction as the lateral load – i.e., IMC shear stiffness was shown to be more important. The paper concluded that there is a need for further research into shear behaviour (Lacey and Bi, 2020). There is a risk at design stage if the suitability of the IMCs in respect of the building, its use, and materials, is not fully evaluated (Chua et al., 2020).

This risk of a lack of understanding of the IMCs and use of unsuitable IMCs is specific to volumetric construction.

11.2.2 Inappropriate inter-module connection affecting overall stability

In volumetric construction, the joining of individual modules needs special connectors that must be fast to install and robust enough to ensure structural integrity. Due to significant amount of inter-module connections, there are uncertainties in continuity of the structural components, with significant impact on the overall stability and sway behaviour of the building.

Resistance to horizontal and accidental loads becomes increasingly important with the scale of the building (Lawson et al., 2012; Liew et al., 2019). If the modules are connected at the corner joints only, the floor slab of each module acts as a discrete rigid diaphragm and relies on inter-module connection to transfer the lateral loads to the lateral bracing system. It is questionable whether discrete floor diaphragms can provide as much lateral resistance as compared to conventional construction methods, in which the entire floor slab acts as a continuous rigid diaphragm (Srisangeerthanan et al., 2020).

This risk is specific to volumetric construction.

11.2.3 Gaps in understanding – high rise steel volumetric construction

High-rise steel modular construction typically uses either:

• load-bearing steel modules, in which loads are transferred through the side walls of the modules;
• corner supported steel modules, in which loads are transferred via edge beams to corner posts.

Research in 2021 concluded that there is, as yet no comprehensive understanding of volumetric steel construction’s structural performance:

Despite the increasing adoption of modular construction worldwide, it is still largely limited to low-to-medium-rise buildings. One of the main reasons is the knowledge gap regarding the structural design of modular high-rises. Understanding the applicability to high-rise engineering projects is hampered by a lack of information about high-rise structures’ stability due to multi-directional forces

(Pan et al., 2021).

Research is increasing into different types of joining techniques for IMCs, and their applicability for buildings of different heights – for example tie rods, connectors, and bolts. Further research is essential to develop robust joining techniques that are applicable to different types of buildings.

This risk is specific to volumetric construction.

11.2.4 Design guidance for structural integrity

Research published in 2020 found that “the modular buildings possess considerable collapse resisting capacity and are able to offer high level of robustness compared to their conventional counterparts” (Alembagheri et al., 2020).

However, the literature generally concludes that further research is required into stability and robustness of modular steel construction, with a need for new design codes and guidelines (Nadeem et al., 2021; Alembagheri et al., 2020).

Modular structures structural performance and load-transfer mechanism significantly differ from conventional moment resisting frames. Therefore, the applicability of current design codes and guidelines for conventional structures on analysis and design of modular structures may be limited and further investigation is required to better understand the structural behaviour [of modular steel buildings] and develop design guidelines

(Nadeem et al., 2021).

The progressive collapse in buildings refers to the phenomenon that local damage of structural elements caused by abnormal loads results in global collapse of the structure (Kim, 2013). Progressive collapse of a building occurs when a primary structural element, like a column, fails (initially localised damage that eventually propagates due to extreme events such as fire, explosions, and impacts (Thai et al., 2020).

Under current building regulations, the loadbearing system of the building must provide a satisfactory level of structural robustness, or the ability of a structure to resist progressive collapse after localised structural damage due to accidental loads. Currently, the tie force method and the alternative load path method are commonly used in the robustness design of building structures. In the tie force method, the tie members (such as beams) and their connections must be capable of withstanding a certain level of tensile load, to ensure load redistribution from damaged parts of a structure to undamaged parts. The alternative load path method is used to evaluate structural robustness by examining the ability of a building to remain stable without violation of an allowable collapsed area, after removal of supporting elements (such as columns and a portion of load-bearing wall) (Luo, 2019).

“There are two main reasons for the uniqueness of modular building in resisting progressive collapse. First, the clustered beam or column may confront both the partial removal and the complete removal scenarios. These two removal modes have totally different response mechanisms. Second, the discontinuous floor cassettes have a notable influence on the load transfer paths. Thus, a possible unique scenario for a modular building is that a module column (1/4 of a cluster-column) suddenly fails, and horizontally, the module is only connected to a corridor unit, leading to a weak horizontal tying which may pose limitations on the alternative load path” (Ye et al., 2021).

Structural analysis data using an alternative load path method by nonlinear dynamic analysis shows that the main risks to be considered are penetration between stacking modules and buckling of corner posts and wall bracing (Luo et al., 2019).

A number of sources consider implications for progressive collapse when using volumetric steel MMC (Alembagheri et al., 2020; Lacey et al., 2018; Liew et al., 2019; Luo et al., 2019; Thai et al., 2020; Ye et al., 2021). Authors from Australia discovered that typical volumetric plate and bolt connections used there are robust enough to prevent progressive collapse when measures according to ‘(a) connections translational behaviour, (b) inclusion of rotational stiffness of the connections, (c) removal time, and (d) modules’ flexibility. Six different module loss scenarios were applied to the models. In all of them, the base-case model was robust and shows no sign of progressive collapse’ (Alembagheri et al., 2020). Evidence from interviews suggest respondents believe the risk of progressive collapse to be generally low, although a 2020 review of modular construction for high rise buildings stated that research on progressive collapse and robustness of modular buildings is limited because of their complex behaviour (Thai et al., 2020).

This risk of insufficient understanding of the stability and robustness of modular steel construction is specific to volumetric.

11.2.5 Thermal bridging – light gauge steel volumetric construction

Thermal bridging in light gauge steel volumetric construction is a risk typical of steel construction which may be exacerbated due to the onsite connection installation to the ground floor slab, between modules, at the roof level of the building and cantilevered building elements such as balconies, especially if installed by the building owner post-construction without knowledge of the volumetric envelope:

“For some buildings, steel elements may be required to penetrate the insulated envelope, for example canopies or roof members. Or the insulated envelope may be penetrated by connection details, such as balcony attachments or brickwork support brackets. These areas require careful consideration to minimise thermal bridging. There are three ways to reduce thermal bridging in steel construction:

1. Ensure the steelwork is within the insulated envelope (i.e., ‘warm frame’ construction).
2. Locally insulate any steelwork that penetrates the envelope.
3. Reduce the thermal transmittance of the thermal bridge by using thermal breaks, changing the detailing or by including alternative materials” (SCI 2018).

Thermal bridging can also occur in traditional construction.

11.2.6 Deficiency in connections within modules and between modules

A risk in building control and inspection of volumetric projects is the high variety of connection types and specification. If designers, construction teams or building control inspectors are not familiar with the correct installation of each of these connection types, there is a risk that either not well-connected modules will be approved, or that sophisticated innovative connections will not be approved out of caution (Wuni et al., 2019).

However, the primary responsibility should be held by those responsible for the procurement, design, and construction. There may be installation errors if installers lack the relevant skills and knowledge, as well as failing to participate in the systemic approach to safety and quality along with the clients, main contractors, and manufacturers. Whether the connections are between module or within modules, the installation of connectors beyond the design, specification, and engineering calculations and drawings can result in structural, thermal and fire risks. Tolerances for connections can be very tight, to achieve the precision required for installation. As described in earlier chapters, the need for on-going quality control and inspection throughout the process is essential for assessing the correct installation of connections.

This risk in relation to modules is specific to volumetric construction, but it should be noted that different types of defects are common in traditional construction, stemming from installation errors.

11.2.7 Limited access space for installation, inspection, and maintenance of structural, inter-module connections

Access space is the main concern for inter-module connections, for both installation and inspection purposes. As a result, commercial connections tend to sacrifice simplicity (sometimes even ductility) for more convenient access onsite (Ye et al., 2021). Building control require access to visually inspect construction components to ensure compliance with regulations. This includes inspecting the tying of components at upper structural levels. There is a risk of not being able to access all connections for inspection after all modules have been connected (Hayes, 2019). For modules which are erected without the use of scaffolding, it becomes virtually impossible to check all the connections. This creates a structural risk for the building if errors are made and not identified or resolved.

This risk is specific to volumetric construction.

11.3 Risk mitigation – products and structure

11.3.1 Further research

In volumetric-type buildings, the limited access space requires minimal numbers of connections and for these to be easy to install, and reliable once placed. To ensure perfect assembly, the access for modular connections could be made externally, allowing safe and easy access to the connection points (Ferdous et al., 2019). New forms of joining techniques could be explored; for example, connecting the columns of the upper module to the columns of the lower modules, as well as the connections affixed outside of the modules (Liew et al., 2019). Inter-module connections with intra-module sockets have been explored recently in the literature, indicating highly condensed and delicate connections may soon become commercially available to benefit the industry (Ye et al., 2021).

11.3.2 Modelling at design stage and design guidance

Global modelling of modular high-rise building with appropriate joint model and structural design consideration is critical to ensure the analysis can capture the global behaviour and to ensure its lateral stability (Liew et al., 2019). The building is laterally stable if the inter-module connection is designed properly to transfer the horizontal load via axial and shear forces. The design of floor slabs in each module needs to be stiff enough to act as a rigid plate (Liew et al., 2019).

Many recent research studies emphasise the need for more accurate design guidelines for modular construction (Nadeem et al., 2021).

11.3.3 Mitigations against progressive collapse

The main risks identified can be mitigated with considerations for:

• member capacity of corner posts;
• increase in the number of vertical ties;
• effective longitudinal bracing;
• inter-module connections with adequate rotational stiffness; and
• designing the edge modules stronger and stiffer than the intermediate modules. (Luo et al. 2019)

Progressive collapse resistance reduces as the height of the building increases, and additional research is likely to be needed into the robustness of modular buildings over 10 storeys in height (Chua et al., 2022); this is not to suggest there are no risks with buildings under 10 storeys in height.

12. Standards, warranties, accreditation, and certification

12.1 Overview

There are very few standards directly applicable to volumetric construction, meaning that companies and practitioners often design and manufacture bespoke systems that cut across standards in the BSI construction portfolio. This layer of complexity has been described as a significant barrier to innovation (BSI).

Different approaches in relation to warranties, accreditation and certification create confusion. The resulting lack of uniformity and fragmentation that exists points to a need for commonality.

12.2 Risks – standards, warranties, accreditation, and certification

12.2.1 Standards

Research undertaken by BSI found almost 1300 standards applicable to building products and regulations; few of these were developed for MMC. There are gaps where standards are not applicable to MMC or fall short of what the sector requires. Of the few standards directly applicable to volumetric construction, most are out of date.

The current suite of standards available to the construction industry is therefore largely inapplicable to volumetric construction; in some respects, standards have been ‘left behind’ by advances in MMC technology:

• all volumetric systems require work to be at a level of accuracy which is beyond tolerances stated in most standards;
• design management for volumetric involves challenges in transport and logistics, maintenance and retrofit all of which must be considered in different ways to those covered in current design management standards; and
• crucial to the functionality of all volumetric systems is connectivity; volumetric units or panels must be connected accurately with attention paid to, for example plumbing, electrics and fire stopping. This is an area which is not covered by standards for traditional construction.

This risk is specific to volumetric construction.

12.2.2 Confusion relating to warranties, standards, accreditation, and certification

Within volumetric construction, there is a wide range of warranties, standards and accreditations which could be potentially applicable to products, systems, and components. Different approaches can be taken by different providers, which creates confusion, as the examples described below illustrate:

Buildoffsite Property Assurance Scheme (BOPAS)

BOPAS is a risk-based evaluation (not a warranty) which aims to demonstrate to funders, lenders, valuers, and purchasers that homes built from non-traditional methods and materials will stand the test of time for at least 60 years i.e., two mortgage terms.

The scheme comprises:

1. The Lloyd’s Register process accreditation which covers:

• Competency management
• Procurement management
• Configuration management
• Process control
• Risk management
• Performance management and improvement

2. The (Building Life Plans) BLP durability and maintenance assessment which assures:

• Minimum 60-year durability
• No disproportionate maintenance
• Component lifecycle assessments
• Site-specific workmanship checks

3. An online database comprising details of assessed building methodologies, registered sites, and registered/warranted properties.

NHBC Accepts

Launched in 2020, NHBC Accepts is a review service for innovative products and systems; described as the route for acceptance of such products and systems for use in homes covered by all NHBC warranty and insurance policies. It is for NHBC developments.

• NHBC Accepts Registration - an innovative product or system has been assessed and NHBC consider it can meet NHBC Standards. It also demonstrates that, subject to appropriate design and installation, the system or product can be used in homes covered by all NHBC warranty products.

• NHBC Accepts - a service provided to and paid for by system owners (typically manufacturers) aiming to demonstrate that their system has been reviewed and accepted by NHBC.

• NHBC Warranty - provided on a completed home, including external works, substructure, superstructure, and finishes, for the benefit of the homeowner.

British Property Standard (BPS) 7014

Developed by BRE and launched in January 2021, BPS 7014 is the Standard for Modular Systems for Dwellings. It aims to provide a route to certification for modular systems used in the construction of residential buildings and contains requirements around the seven Basic Works Requirements of the Construction Products Regulation:

1. Mechanical resistance and stability
2. Safety in case of fire
3. Hygiene, health, and environment
4. Safety in use
5. Protection against noise
6. Energy economy and heat retention
7. Sustainable use of natural resources

It sets out to “provide a route to certification for offsite systems for use in the construction of residential buildings.” The BPS 7014 Standard can be applied to modular building systems and components for use in new build residential and in refurbishment of existing residential buildings. BPS 7014 does not consider site specific service installations, (including foundations), appliances covered by CE marking (such as waste disposal / hot water safety / drainage or heat producing appliances), internal services and finishes, facades, or mains services more than in designs stage (as they pass through the building envelope). Additionally, specific buildings have regulatory requirements to meet such as layout, designs assumptions and location, which are outside the scope of the Standard (BRE, 2021).

Checkmate

Checkmate provides a “System Type Approval process for offsite manufactured housing solutions, providing ten and twelve-year new home warranty policies and latent defects insurance for all types of residential and commercial developments.”

The assessment process includes a desktop overview of proposals and specifications also factory audits and site visits. Approval relies upon evidence of:

• a single point of responsibility for project design co-ordination;
• the project coordinator being responsible for ensuring compatibility of all individual construction elements;
• project design - drawings, plans, elevations and specifications;
• project method statements;
• installation manuals and confirmation of installation by trained/approved contractors; and
• maintenance requirements.

Premier Guarantee

An insurance-backed structural warranty provider offering a range of 10-year warranties covering:

• new homes;
• build to rent;
• social housing;
• high value (£25m+) schemes;
• commercial buildings;
• completed housing; and
• simple completion.

Developers and builders must ensure that the systems they use onsite are accepted in line with the requirements of Premier’s Technical Manual. Premier Guarantee acceptance is recognition that the system/product can meet Premier’s warranty requirements; this “does not in itself constitute a third-party product approval.”

Local Authority Building Control (LABC) Warranty

LABC Warranty works in partnership with Local Authority Building Control (LABC) to provide structural warranties. For MMC, all systems must be designed and built using certified materials, have quality management systems in place for the manufacture of the system and have been accepted by LABC Warranty Innovations Department prior to an offer of warranty being issued. The technical manual used is also used by Premier Guarantee.

To add to the complexity there can be question marks over whether a standard, certification assurance or warranty applies to a product, system, or whole property. The process can be lengthy and cost intensive, with no guarantee of a successful outcome.

The majority of respondents are most familiar with BOPAS and NHBC Accepts. Confusion exists due to multiple schemes and providers. In August 2020, a memorandum of understanding was signed by warranty providers to introduce a minimum standard for assessing homes built using MMC. While there is commitment for a more unified approach, at the time of writing such a standard is still to reach completion.

Respondents perceive differences between BOPAS and NHBC Accepts (the two most commonly used within volumetric construction) as being potentially problematic – notably where one scheme accepts something which the other would not. For example, NHBC Accepts does not accept systems where timber meets the ground (due to the proximity to moisture), but BOPAS does. This is not to say that one sets a higher bar than the other, but as there is no common minimum standard, it is difficult for manufacturers to make a choice.

Who is right and who is wrong? If one says no, we can go to the other. That is not a quality-led approach. It needs to be centralised

We need one version of the truth – what are the minimum requirements we all need?

Traditional construction also has variation in standards, warranties and product certifications, however greater confusion is perceived in volumetric construction.

12.2.3 Impacts of multiple schemes

Limited sharing of knowledge and data

The existence of multiple schemes limits the sharing of data. This lack of data undermines the concept of the golden thread of information required to understand, manage, and mitigate building safety risk.

The lack of long-term data about volumetric buildings is a concern for the Association of British Insurers (ABI). “Insurers lack confidence in being able to understand these new properties and how they react to these perils, as well as how they are repaired, and how much they will cost to repair, should the worst happen”.

Spreading skills and knowledge thinly

The processes of accreditation and certification require the individuals involved to have specialist skillsets e.g., the ability to scrutinise factory processes and form a judgment on how these translate into onsite performance of materials and systems. The current fragmented market risks spreading specialists too thinly, rather than concentrating knowledge and accelerating learning.

Transparency

There is a lack of transparency around the specifications of component parts of volumetric systems as manufacturers tend to protect their Intellectual Property (IP). Hence any unauthorised product substitution which may occur can lead to confusion around certification and warranties. Unauthorised product substitution – historically rife in the construction sector generally – has been exacerbated by the materials shortages. Another layer of difficulty has been created by the move from CE to UK Conformity Assessed (UKCA) marking following the EU Exit.

Data and knowledge are not always consistently shared in traditional construction as well as in volumetric construction, however there is a stronger need for long-term data about volumetric buildings to help ensure confidence among lenders and insurers.

12.3 Risk mitigation – standards, warranties, accreditation, and certification

12.3.1 A case for one common standard

The fragmentation that exists points to a need for one common standard to provide confidence and mitigate risks. A Publicly Accessible Specification (PAS) for volumetric linked to the regulatory framework would allow clear definition of best practice, standardisation and the same consistently applied checks and balances. This would then become the standard to which any volumetric assurance schemes, approved inspectors and warranty providers would need to comply – in conjunction with Building Regulations. Independent oversight would be required to ensure PAS compliance is being achieved by relevant third-party accreditation and warranty providers.

13. Digital tools and technologies

13.1 Overview

Digital tools and technologies offer the opportunity to mitigate a wide range of risks in multiple ways. Harnessing digitally underpinned solutions effectively can enhance project management, process monitoring, design, inspection, crane pathways and information sharing – to name only a few. Further research is recommended into digital tools and technologies, and how they can be used in projects across different building heights, types of materials, systems, and construction methods.

13.2 Risks – digital tools and technologies

There are two main risks in relation to digital tools and technologies in volumetric construction:

• incompatibility between systems or incorrect usage; and
• not harnessing digital solutions to enhance project delivery.

These risks are applicable also to traditional construction.

The benefits of Building Information Modelling (BIM) are widely discussed in literature; it is generally accepted that greater process automation within modular construction is very important to its ongoing sustainability and evolution (Olawumi et al., 2022).

It therefore makes sense to investigate the potential uses of other types of digital tools and technologies.

13.3 Opportunities – digital tools and technologies

13.3.1 Integration of BIM with other tools and technologies

A recent review of digital technologies identifies the opportunity to systematically integrate BIM with other tools including Radio-frequency identification (RFID), AI, blockchain and others (Olawumi et al., 2021).

Other examples provided in the literature include:

• use of extended BIM techniques and strain sensors to monitor structural health of modules, notably detecting hidden as well as visible damage that could have been incurred during manufacturing or transit (Valinejadshoubi et al., 2019); and
• 3D laser scanning technologies used to inspect geometric qualities of mechanical, electrical, and plumbing (MEP) modules; scanned data can be compared with the designed module’s BIM model. Although only in experimental phases, early results show greater accuracy than manual inspection processes and could prevent the need for rework of MEP modules onsite ((Olawumi et al., 2021).

13.3.2 Wider application of BIM

Evidence in the literature finds integrating BIM into the manufacturing phase has significantly increased the productivity of manufacturing processes [4]. Increased use of BIM Level 2 with 3D data exchange in the procurement and commissioning of MMC products can mitigate the risk of fragmentation between key stakeholders and help to ‘join up’ the process. Research conducted in 2019 identified a need for improved collaboration and clash detection methods in volumetric construction and recommended that BIM would be the only way to execute these (Duncheva, 2019).

There is less research into the use of BIM across the entire volumetric project lifecycle, which could help to underpin a more integrated process framework. Further research has been recommended into the use of BIM for systems development (Darko et al., 2020).

13.3.3 Radio-frequency identification (RFID)

There is increasing adoption of RFID in the construction sector, and in particular for volumetric construction. Barcodes are another option. Examples cited in the literature include:

• an RFID system to automate production data collection in a factory, which enabled assembly line production to be visualised which in turn helped to optimise the production schedule (Altaf et al., 2018); and
• RFID integration with Personal Digital Assistants (PDA) for quality inspections including production process, materials, moulds, and managerial inspections. The integrated approach minimised information losses (Yin et al., 2009).

13.3.4 Visualisation and optimisation

Game engines have been used as a means of process simulation. Augmented Reality (AR) has the potential to help digitally assure onsite works and has been used by Laing O’ Rourke and the Advanced Manufacturing Research Centre (AMRC).

Research conducted in 2018 reported results of the development of an automated crane planning and optimisation system which used numerical algorithms and the HeviLift software suite to precisely determine feasible crane locations (Taghaddos et al., 2018). In 2020, research demonstrated integration of a 4D lift animation plugin with BIM to simulate and define multi-mobile crane paths (Shahnavaz et al., (2020).

14. Conclusions and recommendations

14.1 Research conclusions

Overview

Modular construction can provide many benefits and has significant potential to improve quality and productivity in construction, reducing time and waste. Advantages can only be fully realised where volumetric construction projects are implemented effectively. Understanding the risks of volumetric construction and how they can be mitigated, is therefore essential to be able to maximise the benefits of this type of construction and encourage its greater adoption.

This research has identified risks that may occur across each stage of the modular construction lifecycle, and cross-cutting risks which underpin every stage. There is not enough evidence to estimate the frequency with which these risks occur, and it should not be construed that this report is stating that all these risks occur on every volumetric construction project.

The tables that follow summarise the risks identified through this research, which have been explained in detail in the body of the report. Risks are described for each stage of the typical project lifecycle, Risks are categorised into those deemed to be elevated in, or specific only to volumetric construction, compared with traditional construction, and those which are applicable to traditional construction as well as volumetric construction. The detailed evidence in this report enables understanding and management of potential risks in volumetric construction in order that they can be effectively mitigated.

1. Procurement and project management

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Standard contract forms not applicable to MMC Lack of integrated project management and defined processes	Supply chain resilience: risk of insolvency Lack of early engagement of supply chain (including key roles: structural fire engineer, building control)

2. Design

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Shortage of skilled DfMA designers Perceived gaps in design standards and codes	Lack of integration between key individuals could undermine design Insufficient time for design; early design freeze is a critical success factor Potential for conflict between energy efficiency and fire safety design requirements Design defects: risk that defects are ‘built in’

Stages 1: Procurement and project management & 2: Design are critical; any issues here will have a significant ripple effect on all the remaining stages.

3. Manufacture

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Design variants incompatible with mass customisation Damage prior to transit which is undetected	Deviation from original design Unauthorised product substitution Health & safety risks

4. Transportation

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Damage and/or water ingress in transit Lack of process ownership between factory and site; ambiguity relating to responsibilities and accountability

5. Site installation

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Interface between offsite and onsite: misalignment Mismatch in tolerances, typically used in the factory and typically used onsite, creating issues for installation Damage incurred to modules onsite, which may or may not be detected and remedied Limitations of access for inspection/onsite	Skills and knowledge gaps onsite Ineffective crane and lifting operations onsite

6. Occupancy and beyond

Risks deemed to be elevated in, or specific only to volumetric construction	Risks deemed to be applicable to traditional construction as well as volumetric construction
Risk of disproportionate damage in the event of fire or flooding; modules may be difficult to access to replace or repair	Discrepancies between ‘as designed’ and ‘as built’ Limited occupant and building performance evaluation Potential for risk introduced as a result of repair & maintenance

Cross-cutting themes – underpinning the whole lifecycle

• Key issue (applicable to traditional construction as well): Lack of oversight; absence of ‘joined up process’ without clear task allocation, ownership, or accountability. Results in the liability baton being passed around and the risk that issues can be undetected or are identified but it is not clear who has responsibility to take action; may lack ‘cradle to grave’ quality assurance process
• Gaps in relation to volumetric construction in the regulatory framework and in design guidance
• Standard fire testing and product testing not fit for purpose for volumetric construction
• Gaps in knowledge still being researched: behaviour/reaction of different materials in volumetric construction in the event of fire
• Serious fires in modular buildings likely to result in more serious consequences in a modular building if choice made to use combustible elements in the voids and cavities through which fire and smoke can travel quickly, unless risk has been mitigated via effective design/install
• Gaps in knowledge still being researched: behaviour/long-term structural integrity of modular construction; high rise steel volumetric construction; progressive collapse
• Absence of one clear common standard for volumetric construction
• Skills and knowledge gaps and shortages
• Supply chain fragmentation and resilience; lack of integration and collaboration
• Confusion in relation to multiple warranties, accreditation, and certification

While the scope of this research was risks in volumetric construction, it was stated at the outset and is reiterated here in the conclusions – that many of these risks are also present in traditional construction. The Department for Levelling Up, Housing & Communities (DLUHC) and industry may wish to consider where research recommendations may also apply to traditional as well as volumetric construction.

Key messages

1. Within volumetric construction it is common that there will be multiple layers and levels of authority and approval; the process is not joined up and it is not clear who is responsible throughout the project. Ownership of the whole process from start to finish is often missing – there may be multiple owners in the design team, in the factory, in logistics and onsite. This can be true even within vertically integrated organisations. Liability and accountability are therefore “sliced and diced” – problems, associated risk and responsibility for resolution are passed from organisation to organisation, and/or person to person.

2. Volumetric construction relies on collaboration and an integrated approach to project management. Information sharing and collaboration can be undermined by the absence of clear workflows, critical expert input at the outset, and overarching programme ownership. Key roles that can be absent from the start of the project (or at pre-project planning stage) are those of the structural fire engineer, design lead and building control.

3. Vulnerability within the supply chain can arise from multiple factors including the use of standard contract forms which are not designed for volumetric construction and a perception among clients of higher upfront costs. The risk of insolvency in the supply chain can have knock-on effects for the continuity of partly completed volumetric construction programmes.

4. Current Approved Documents were developed for traditional forms of construction, and do not explicitly focus on volumetric construction or other types of MMC. There are mixed views among respondents, with one perspective suggesting a need for new Approved Document(s) bespoke to volumetric (and MMC generally), but the majority view favours tightening wording of existing regulatory documentation, and additional details/clarifications added with signposting to new technical guidance specifically developed for all forms of MMC.

5. It is critical to ensure design and manufacturing/construction teams are integrated from an early stage in the process; a ‘golden thread’ which runs from design through to manufacture through to installation. Early design freeze is a critical success factor, underpinned by relevant expertise in Design for Manufacture and Assembly (DfMA). A key risk is that poor design will affect module quality; essentially meaning that defects are ‘built in’ and replicated across all units.

6. With appropriate quality control processes in the factory environment, defects in manufacturing should be less likely to occur. However, this is not guaranteed - for example if quality control is deficient, or in the case of unauthorised material substitution, therefore deviating from the original design.

7. Product and component testing for volumetric construction is subject to gaps, notably where suitable tests do not exist (in respect of new/innovative products), or the testing conditions do not consider the modular context. Products deemed compliant may not have been subject to the right test conditions.

8. There are also gaps in relation to industry standards in volumetric construction; most construction sector standards were developed with traditional construction in mind and do not explicitly address volumetric construction (or do but are out of date). It can be challenging to understand and navigate the confused space of multiple standards, warranties, accreditations, and certification relevant for modular construction.

9. If any defects are present either before modules leave the factory or as a result of damage which occurs in transit, which are not identified and remedied, there is the further risk that issues are replicated across all modules manufactured. Defects would then be either identified at site, whereby modules would need to be re-manufactured (incurring delays and additional costs), or not identified and assembled – leading to risks to the building’s structural integrity, performance, and safety.

10. The risk of damage in transit is most likely to occur if modules are not correctly stored or made stable during transportation. As stated above, if damage is not identified upon arrival onsite and are installed, this creates longer-term risks for the structural integrity, performance, and safety of the building.

11. A highly critical risk is the point of interface between offsite and onsite. There is less control once modules leave the more controlled factory environment; elements onsite such as weather events cannot be controlled. Typical site tolerances differ from typical manufacturing tolerances which may result in misalignment, with the potential for unintended gaps to be created between modules. There is strong reliance on having an onsite installation team with relevant skills and knowledge of volumetric construction (and of the particular system being installed)– e.g., sequencing, crane operations - to achieve the precision required. If errors are introduced at site installation stage, this can undermine the intended design and result in quality, performance, and/or safety issues if not detected and rectified. Remediation of errors can incur additional costs and create programme delays.

12. It is essential that fire performance and the requirements particular to modular construction are well understood from the outset and factored into the design. Risks can be introduced if there is an inadequate fire strategy, in the event of lack of continuity of structural fire engineering expertise throughout the project and where standard fire testing does not work effectively for volumetric products and systems. There is no standard fire test methodology which is directly applicable for volumetric construction; testing that takes place is likely to be sub-optimal.

13. While there is insufficient evidence to suggest whether a fire is more or less likely in a modular building compared with a traditionally constructed building, the event of a serious fire is likely to result in more serious consequences in a modular building if the choice has been made to use combustible elements in the voids and cavities through which fire and smoke can travel quickly – unless these risks are mitigated via appropriate design, manufacture, and installation. Further research is critical to have a clearer understanding of how materials (and combinations of materials/components) behave in modular buildings in the event of a fire.

14. There is a potential risk that occupants, or builders without the relevant skills and understanding of the volumetric building, could make changes via repair and maintenance that compromise the integrity of the building. However, post-occupation phases of volumetric buildings and repair and maintenance issues do not appear to be major areas of research, and so it is not possible to understand the full extent of this potential risk.

15. There is also a need for further, on-going research into materials behaviour/long-term structural integrity of volumetric construction, connections to underpin robust stability for high rise steel volumetric construction and the risk of progressive collapse. Structural engineering research in modular buildings appears to be increasing, but there are still gaps in knowledge and an incomplete understanding of these critical elements.

16. Long-term, comprehensive data about thermal and fire performance, structural robustness, repair & maintenance (ease/costs), interaction of different materials and components and behaviour in the event of water or fire damage for modular buildings is not readily accessible. There is a need for greater transparency and accessible data to build up a long-term view of the longevity of a modular building asset.

14.2 Recommendations for consideration

As stated in the previous section, these research recommendations relate to volumetric construction and investigation of risks in traditional construction were outside of the research scope. Nonetheless, the Department for Levelling Up, Housing & Communities (DLUHC) and industry may wish to consider where these recommendations may apply to traditional as well as volumetric construction.

Recommendations have been informed by the evidence analysed for this research, notably including examples of best practice shared by organisations interviewed for this research and those responding to the online consultation. Recommendations are listed in order of priority.

Key recommendation for government, industry, and industry bodies to consider:

1. Adopt a systems approach to ensure integrated programme management with clearly defined processes

Given the reliance on integration and collaboration within the modular construction supply chain, and the vital importance of the early stages of the project (errors at the outset would trigger a series of knock-on effects likely to intensify risks at each subsequent stage), a systems approach to project management is recommended.

This would take into consideration that all elements of the project are mutually dependent, but that each stage has its own particular requirements to be carefully managed. It also builds on recommendations from the Hackitt report, which identified the need to give consideration given to how components interact with one another to achieve complete performance when designing buildings. Continuity and consistency must be central to programme and process management, underpinned by a ‘cradle to grave’ approach to quality assurance.

• This could be achieved via Standard Operating Procedures (SOP) and/or an ISO Quality Management Standard (QMS) specific to volumetric construction. While it is not possible (or desirable) for all organisations to follow exactly the same process, a set of common steps and processes to be achieved, as a minimum, would be instrumental in defining roles, responsibilities, and programme/process owners. In particular, this should include provision for having the right expertise in design, building control and structural fire engineering from the outset, ideally with continuity of the same people/organisations throughout project delivery.

• Any handover points in the process should be clearly identified, with the process for quality checking and compliance measures set out – on-going checks should eliminate the risk of unauthorised product substitution and deviation from the original design. It should be clear which roles have ownership for signing off each element of the process before moving to the next stage. There should be a significant focus on checking the modules before they leave the factory, as they are packaged for transit and upon arrival at site – including checking for movement of elements fitted in the factory, moisture content/water ingress or any other damage and storage of modules. Any remediation and who takes responsibility for this, should be defined where possible.

• A risk register to identify all critical objectives of the project, impacts of identified risk factors and mitigations developed on a case-by-case basis should be maintained and updated throughout the project. This should include mitigations for product substitution to provide reassurance that any materials/products/elements which need to be substituted due to supply chain issues, have undergone rigorous safety and fire performance testing appropriate for the modular context, and would not have any negative impact on the overall design. It should also include plans for transportation, thinking about module material, size, weight, distance, and route to be travelled, as well as protection against exposure to cold, heat and moisture.

• Module storage processes need to consider:

  • length of time for optimum storage;
  • how packages should be stored outside onsite, including any need to raise them off the ground (even when still in external packaging);
  • whether/how external packaging should be opened while stacked in storage;
  • ventilation requirements of the storage facility;
  • moisture control planning and frequency of testing for moisture content; and
  • processes for inspection (upon arrival onsite and while stored onsite).

• The process should also ensure there has been development of tolerance and fire strategies.

• An effective fire strategy in volumetric construction needs to consider as a minimum:

  • structural design of the building;
  • which building elements are structural and which are not;
  • which structural elements are loadbearing and which non-loadbearing to apply the correct fire resistance criteria;
  • how fire resistance is determined;
  • installation of cavity barriers taking into consideration building materials, likely extent of building movement over time and assembly sequencing; and
  • locations for cavity barriers/fire stopping (likely to be visualised and communicated using software such as BIM).

(Arup (2021), Fire Safety Compliance for MOD Single Living Accommodation Modular Buildings Modular Fire Safety Standard Version).

• The process should ensure there is an understanding of how to demonstrate and achieve required levels of fire resistance, including the requirement for and placement of cavity barriers and fire stopping, and consider how different elements of design may impact other areas – for example any knock-on effects for fire performance as a result of using insulation materials. This will become critical in the next few years when the Future Homes Standard is introduced.

• Quality controls and inspections should include inbound and outbound quality checks to ensure any modules with defects are not transported to site and installed. Also, the process needs to factor in air/temperature monitoring, mould prevention and moisture content testing, to prevent unintended damage to materials or modules.

This process should be underpinned by clarity within contracts via a bespoke suite of contracts developed specifically for volumetric and other forms of MMC. It should also link to a common Standard for volumetric construction (see point 3 below). Where possible, digital tools and technologies should be used to enhance quality and accuracy.

Recommendations for government

2. Develop one common Standard for volumetric construction

A Publicly Available Specification (PAS) should be developed for volumetric construction, to create one common standard (split into a range of different sub-categories e.g., fire, structural, thermal performance, maintenance and repairability, and covering different materials). A PAS could also be extended to other categories of MMC. The systems approach (as described in point 1 above) should also be incorporated into the PAS. The authors note that this is already in early stages of development.

3. Consider amendments to Approved Documents, and develop additional technical guidance specific to volumetric construction

Any revisions to Approved Documents should consider where additional clauses may be needed in respect of volumetric construction and MMC generally; notably to clarify where existing content is not applicable to volumetric construction, with signposting to alternative documents such as technical guidance and/or FAQs.

Technical guidance documents need to define, as a minimum:

• processes regarding the interfaces: module to module, risk of water ingress, cavities between modules;
• testing requirement: fire performance of different elements, testing of complete units/systems rather than elements in isolation and the use of fire tests appropriate for the modular context (also see point 4 below);
• maintenance of digital records showing an audit trail (design, manufacture, transportation, installation); and
• critical details required for photographic evidence e.g., fire collars, fire barriers, vapour membranes, structural connectors between modules, connection base (also see point 6 below).

4. Develop a standard fire test methodology and British Standard for fire testing

A standard fire test methodology would define minimum criteria for fire testing applicable to a modular context. A standalone British Standard for fire testing in volumetric is required to ensure clarity of the requirements that need to be met and provide greater reassurance to insurers. Whole systems and combinations of products/components need to be tested, rather than just individual units/elements. It is important that tests are application specific and include module connections and all fixings in scope.

5. Ensure an adequate product testing regime for volumetric construction

New tests may need to be developed for effective product testing for volumetric construction, ensuring test conditions take the modular context into account. Product installation details need to explicitly state the conditions and environment for the use of the product – for example:

• insulation materials which can/cannot be combined with the product;
• environments in which the product can/cannot be used and any conditions e.g., specific coatings to be applied, use of specific fixings etc.; and
• building heights suitable/not suitable for the product.

Actions in relation to product testing should link into relevant findings from the Morrell Review of the Construction Products Testing Regime.

6. Undertake further research in relation to structural integrity

Further research to better understand the structural robustness of modular buildings and inter-module connections is recommended, in particular for light gauge steel frame buildings and high-rise modular buildings. This should focus on different types of connections and building design used in a range of different circumstances – such as building type, environment, different weather conditions, and building height. Research should also assess long-term structural robustness of different types of connections used by monitoring buildings over a period of time.

7. Collect long-term data to build comprehensive knowledge and understanding of modular buildings

Mode of construction data should be captured as part of the existing suite of housing statistics. All opportunities should be taken to capture and analyse data about modular buildings post completion and during occupancy. This could be mandated where Government funding has supported volumetric and other MMC developments. Data should be captured in a uniform format for ease of collation and analysis.

8. Develop a digital database of buildings

Building on the Hackitt report and recommendation for the golden thread, maintaining digital records of construction is increasingly important for the sector as a whole. For volumetric construction this provides an opportunity to give confidence and reassurance to insurers and lenders in respect of the “hidden” details i.e., limited scope for inspections once modules are connected. A database of homes/buildings constructed – regardless of the mode of construction – would create greater transparency and provide vital information for homeowners, building/facilities managers, and other relevant parties.

A database of this nature would take time to establish and may need to be limited to certain building types (e.g., those deemed to be higher risk). Such a database could collate high level data, for example:   - mode of construction; - materials used; - product tests undertaken and results; - evidence of accreditation, warranties, certification; - photographic evidence (digitally dated/time stamped) of manufacture and installation of each module, including evidence of critical areas such as module connections, utility connections, fire stopping and cavity barriers; and - fire testing undertaken and results. It should be considered whether this can be limited to the type of construction, material(s), fixings, insulation materials, test undertaken and under what conditions, date and results in a way that does not compromise Intellectual Property.

Consideration should be given to the creation of a ‘Homeowners’ Manual’ (or similar wording), to provide each occupant with relevant data about their own home and help facilitate repair & maintenance. However, the investment required to produce such manuals on an on-going basis needs to be carefully considered, as it is not clear from existing evidence whether homeowners would actually make use of these – further research would be required to establish this.

Recommendations for industry and industry bodies

9. Collect long-term data to build comprehensive knowledge and understanding of modular buildings

As per the recommendation for government, industry should take all opportunities to capture and analyse data about modular buildings post completion and during occupancy.

10. Look for ways to educate clients and improve skills

There is a need to develop a clearer understanding of the training and qualifications available for volumetric construction; this would identify any gaps which need to be addressed through new qualifications, training courses and Continuing Professional Development (CPD). A collaborative approach to developing client knowledge and understanding of volumetric construction is encouraged, to help educate clients on the optimal processes for commissioning and managing volumetric programmes.

11. Consider how to harness digital tools and technologies to help mitigate risks

Further research is recommended for industry to better understand the use and likely benefits or wider applications of different digital tools and technologies for volumetric construction. This could include as a minimum:

• use of Building Information Modelling (BIM) and integration with other technologies;
• optimisation and visualisation tools (could also support training as well as programme delivery and process management); and
• Radio-frequency identification (RFID).

Appendix 1: Steering Group

The authors would like to express their gratitude to the members of the Steering Group who provided their valuable input as this study was being researched:

Allison Whittington, Head of Housing – Zurich Municipal
Andrew Shephard, Managing Director – TopHat
Anthony Burd, Associate Director – BSI Group
Chris Hall, External Affairs Director – Siderise Insulation Ltd
Dave Green, Senior Fire Engineer – National Fire Chiefs Council
Graeme Culliton, Managing Director UK – BoKlok UK
Hew Edgar, Associate Director – Chartered Institute of Building
Jack Goulding, Director – Independent BIM Consultancy
Jamie Parr, Founder and Director – Better Delivery
Jeff Maxted, Director – BLP Insurance
John Askew, Regulatory Specialist – Local Authority Building Control
Jonathan Purvis, Policy Advisor – Association of British Insurers
Lindsay Richards, Senior Lecturer Quantity Surveying and Construction – University of Wales Trinity St David
Mark Farmer, Founding Director and CEO – Cast Consultancy
Matthew Jupp, Principal – UK Finance
Mike Ormesher, Director – Ottersbrook Consulting Ltd
Nigel Ostime, Partner – Hawkins/Brown
Paul McGivern, MMC Advisor – Homes England
Paul Valentine, Director – Structensor Consulting Ltd
Professor Pete Walker, University Climate Action Chair – University of Bath
Professor Tony Thorpe, Professor of Construction – Loughborough University
Professor William Swan, Director of Energy House Laboratories – University of Salford
Richard Lankshear, Innovation Manager – NHBC
Stewart Dalgarno, Director of Innovation and Sustainability – Stewart Milne Group
Stuart Blackie, Principal Risk Management Surveyor – Ecclesiastical Insurance Group
(Formerly Technical Team Leader – Zurich Risk Engineering and in this role for the duration of the fieldwork)
Trevor Clements, Head of Business Development – Hertfordshire Building Control

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