Research and analysis

Fire safety: Structural fire resistance and fire separating elements (excluding appendices)

Published 22 December 2025

Applies to England

Background

The work reported in this report was carried out by a BRE Global Project team under a Contract placed by the Ministry of Housing, Communities and Local Government (MHCLG) which was novated to the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect MHCLG or HSE policy.

It should be noted that the edition of Approved Document B referred to throughout this project (unless otherwise stated) is the 2019 edition (incorporating 2020 amendments)^{1, 2}. Where reference is made to current provisions within this report it is with reference to this edition of AD B.

This Final report is delivered as part of the Ministry of Housing, Communities and Local Government (MHCLG) (formerly the Department for Levelling Up, Housing and Communities (DLUHC)) project titled “Structural Fire Resistance and Fire Separating Elements”, MHCLG Contract reference CPD/004/120/206. This project was novated from DLUHC to the Building Safety Regulator of the Health and Safety Executive (BSR HSE), HSE contract reference 1.11.4.4436, effective date 1st April 2023.

Executive Summary

The aims of the project were to assess the current provisions in Approved Document B (AD B) regarding structural fire resistance and fire separating elements, by establishing the latest knowledge and reviewing this against the underpinning basis and provisions in AD B, to ensure that AD B provides adequate guidance to meet the minimum requirements under Schedule 1 Part B of the Building Regulations 2010.

It should be noted that the edition of Approved Document B referred to throughout this project (unless otherwise stated) is the 2019 edition (incorporating 2020 amendments). Where reference is made to current provisions within this report it is with reference to this edition of AD B. 

This project had the following objectives:

• Objective A - Undertake a scoping study to provide information on modern buildings, the current state of knowledge, and the basis for the current provisions.
• Objective B - Review the current provisions to provide information to consider whether current policy/guidance is adequate.
• Objective C - Provide evidence for future policy consideration.

Objectives A and B identified issues and problems with the current approach and guidance and identified issues requiring a more detailed consideration as part of Objective C. The outcome of Objectives A and B informed the work required to develop robust evidence to consider future policy options, based on assessment, analysis and experimental research.

The objectives applied to three sub-workstreams:

• Workstream 1 - Structural Fire Resistance
• Workstream 2 - Fire Separating Elements
• Workstream 3 - Resilience.

Workstream 3 is related to both Workstreams 1 and 2, rather than a standalone subject. When reviewing and assessing guidance and requirements related to structural performance in fire and compartmentation, the issue of resilience cannot be separated from performance but is an integral part of the work carried out.

To meet the project objectives, the project was divided into main tasks, as follows:

• Develop a research methodology
• Establish expert Technical Steering Group
• Undertake a scoping study (Objective A)
• Review the current provisions (Objective B)
• Provide evidence for future policy consideration (Objective C)    Produce Final report.

Following completion of Objectives A and B, DLUHC gave consideration to the future research direction and consulted with a Working Group of the Building Regulations Advisory Committee. DLUHC issued a research instruction that gave the direction to the Objective C research.

In summary, the six research sub-tasks taken forward as parts of Objective C were:

• Task C1 AD B Clarification - combustible structures and modular construction
• Task C2 Generate knowledge around mass timber construction
• Task C3 Generate knowledge around modern forms of construction including modular systems
• Task C4 Inform the development of a test methodology (MMC and timber)
• Task C5 Cavity barriers
• Task C6 Car parks

The research was led by BRE Global with contributions from Project Partners Design Fire Consultants, Buro Happold and the University of Edinburgh. 

The work also involved the active participation of a Project Technical Steering Group.

This final report contains the findings of the project for Objectives A, B and C. It comprises a main summary report and supporting appendices. The main summary report provides an overview of the whole project and summarises the work conducted, conclusions and recommendations for future work. The appendices contain the full technical details of the project.

1. Acknowledgements

BRE Global authors would like to acknowledge the contributions of project partners, Neal Butterworth, Weimiao Lu and Adeyanju Teslim-Balogun, Design Fire Consultants, Stuart Martin and Florian Block, Buro Happold and Grunde Jomaas, Stephen Welch and Martina Manes, University of Edinburgh. The various contributions of the Project Partners are detailed in the individual Appendices of this report.

The authors would also like to acknowledge the contributions of the Project Technical Steering Group members. The Technical Steering Group was established at the start of the project to support MHCLG officials. This Group assisted the project where necessary and appropriate, guided the research programme and provided comments and advice on the research methodology, key deliverables and milestones over the course of the project.

The Group has met eight times by video conference. Meetings were held on 4^th November 2020, 11^th June 2021, 18^th March 2022, 21^th October 2022, 12^th April 2023, 26^th January 2024, 25^th September 2024 and 27^th February 2025:

• At the first introductory meeting, the proposed research approach was presented and discussed. As this was a wide-ranging and complex research project, the primary project objectives, the considered scope of the project, scope of buildings were also discussed.
• At the second meeting, the findings of the scoping study (Objective A) and the review of current provisions (Objective B) were presented and discussed.
• At the third meeting, the DLUHC research direction summary and the methodologies for each of the six research (Objective C) tasks were presented and discussed.
• At the fourth meeting, the Task C1 AD B clarification findings were presented and discussed. The Task C2 and Task C3 experimental programme updated matrix and the findings from the initial control experiment (precast concrete roof panels and masonry walls) were presented and discussed.
• At the fifth meeting, the Task C6 car park findings were presented and discussed. Task C2 and Task C3 experimental programme report update following receipt of the BSR HSE and Steering Group members’ comments and findings from the second experiment involving a modular steel frame system with hot rolled corner posts and a concrete floor slab were presented and discussed.
• At the sixth meeting, the update made to the Task C6 car parks report, following receipt of the BSR HSE and Steering Group members’ comments, was verbally outlined. Task C2 and Task C3 experimental programme report update and findings from the third experiment involving a cross laminated timber (CLT) compartment were presented and discussed.
• At the seventh meeting, an update on the Task C2 and Task C3 experimental programme was presented and discussed, focussing on the findings of the fourth experiment involving a panelised light steel frame system. A presentation on Task C2 smouldering combustion issues and implications was presented and discussed. Task C5 cavity barriers which had recently started was outlined.
• At the eighth and final meeting, an update on the Task C2 and Task C3 experimental programme was presented and discussed, focussing on the findings of the fifth experiment involving a timber frame compartment and the Task C4 draft test methodology. A presentation on Task C5 cavity barriers was presented and discussed.

Individuals from the following organisations were members of the Project Technical Steering Group: 

• Building Safety Regulator, Health and Safety Executive
• Ministry of Housing, Communities and Local Government (formerly Department for Levelling Up, Housing and Communities)
• The Office for Zero Emission Vehicles
• BRE Global
• Building Regulations Advisory Group (BRAC) Working Group representative
• Association for Specialist Fire Protection
• B/525/-/32 (BSI committee)
• British Constructional Steelwork Association
• British Parking Association
• Buro Happold
• Design Fire Consultants
• Fire Industry Association (MMC Group)
• Institution of Fire Engineers
• Institution of Structural Engineers
• Local Authority Building Control
• National Fire Chiefs Council
• NHBC
• RISCAuthority
• Royal Institute of British Architects
• Steel Construction Institute
• Structural Timber Association
• The Concrete Centre
• University of Edinburgh.

BRE Global authors would like to acknowledge the Industrial Collaboration Group for providing programme assistance and expertise in defining and facilitating (through the provision of experimental samples) the experimental programme.

BRE Global authors would like to acknowledge the contributions to the experimental programme of the following:

• The BRE Global experimental team.
• The Concrete Centre for arranging the provision of materials for the construction of the control fire compartment for the first experiment.
• Supplier 1 for supply, delivery and removal of the module for the second experiment.
• The industry group for the design, supply and construction of the CLT compartment for the third experiment, coordinated through the Structural Timber Association.
• Supplier 2 for supply, delivery and removal of the panellised steel system for the fourth experiment.
• The industry group for the design, supply and construction of the timber frame compartment for the fifth experiment, coordinated through the Structural Timber Association.

The authors would also like to thank the participants of the Car Parks Focus Group for their insights and valuable contributions to the open discussion at the virtual Focus Group meeting held on 29^th July 2022.

This Car Parks Focus Group comprised representatives from: DLUHC, the Project team (University of Edinburgh and BRE Global) and invited participants from the Office for Zero Emission Vehicles (OZEV), the British Parking Association, the National Fire Chiefs Council (NFCC) and consultants from OFR Consultants and Arup with expertise in relation to fires in car parks.

The authors would like to thank the participants of the Smouldering Combustion Focus Group for providing additional input to the project on the Fire and Rescue Service experience of post-fire smouldering combustion in timber structures at the virtual Focus Group meeting held on 3^rd July 2024.

This Smouldering Combustion Focus Group comprised members from: BSR HSE, BRE Global, the National Fire Chiefs Council (NFCC) and the Structural Timber Association. BRE Global presented a summary of the CLT experiment post-fire findings and the Focus Group members were asked to contribute to an open discussion.

2. Introduction

The work reported in this report was carried out by a BRE Global Project team under a Contract placed by the Ministry of Housing, Communities and Local Government (MHCLG) which was novated to the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect MHCLG or HSE policy.

It should be noted that the edition of Approved Document B referred to throughout this project (unless otherwise stated) is the 2019 edition (incorporating 2020 amendments) (HM Government. Approved Document B (fire safety) Volume 1: Dwellings, 2019 edition incorporating 2020 amendments (for use in England)),(HM Government. Approved Document B (fire safety) Volume 2: Buildings other than dwellings, 2019 edition incorporating 2020 amendments (for use in England)). Where reference is made to current provisions within this report it is with reference to this edition of AD B.  

This final report is delivered as part of the Ministry of Housing, Communities and Local Government (MHCLG) (formerly the Department for Levelling Up, Housing and Communities (DLUHC)) project titled “Structural Fire Resistance and Fire Separating Elements”, MHCLG Contract reference CPD/004/120/206. This project was novated from DLUHC to the Building Safety Regulator of the Health and Safety Executive (BSR HSE), HSE contract reference 1.11.4.4436, effective date 1^st April 2023.

The aims of the project were to assess the current provisions in Approved Document B (AD B) regarding structural fire resistance and fire separating elements, by establishing the latest knowledge and reviewing this against the underpinning basis and provisions in AD B, to ensure that AD B provides adequate guidance to meet the minimum requirements under Schedule 1 Part B of the Building Regulations 2010. This project had the following objectives:

• Objective A - Undertake a scoping study to provide information on modern buildings, the current state of knowledge, and the basis for the current provisions.
• Objective B - Review the current provisions to provide information to consider whether current policy/guidance is adequate.
• Objective C - Provide evidence for future policy consideration.

Objectives A and B identified issues and problems with the current approach and guidance and identified issues requiring a more detailed consideration as part of Objective C. The outcome of Objectives A and B informed the work required to develop robust evidence to consider future policy options, based on assessment, analysis and experimental research.

The objectives applied to three sub-workstreams:

• Workstream 1 - Structural Fire Resistance
• Workstream 2 - Fire Separating Elements
• Workstream 3 - Resilience.

Workstream 3 is related to both Workstreams 1 and 2, rather than a standalone subject. When reviewing and assessing guidance and requirements related to structural performance in fire and compartmentation, the issue of resilience cannot be separated from performance but is an integral part of the work carried out.

To meet the project objectives, the project was divided into main Tasks, as follows:

• Develop a research methodology
• Establish expert Technical Steering Group
• Undertake a scoping study (Objective A)
• Review the current provisions (Objective B)
• Provide evidence for future policy consideration (Objective C)
• Produce Final report.

Following completion of Objectives A and B, DLUHC gave consideration to the future research direction and consulted with a working group of the Building Regulations Advisory Committee. DLUHC issued a research instruction that gave the direction to the Objective C research.

In summary, the six research sub-tasks taken forward as parts of Objective C were:

• Task C1 AD B Clarification - combustible structures and modular construction
• Task C2 Generate knowledge around mass timber construction
• Task C3 Generate knowledge around modern forms of construction including modular systems
• Task C4 Inform the development of a test methodology (MMC and timber)
• Task C5 Cavity barriers
• Task C6 Car parks.

This Final report contains the findings of the project for Objectives A, B and C. It comprises a main summary report and supporting appendices. The Main summary report provides an overview of the whole project and summarises the work conducted, conclusions and recommendations for future work. The appendices contain the full technical details of the project. 

To assist the reader, Table 1 (below) relates the supporting appendices to the related objectives and sub-tasks. 

Table 1 – Appendices and related objectives and sub-tasks

Objective and sub-task	Title	Appendix	Appendix title
A	Undertake a scoping study	A	Scoping study
B	Review the current provisions	B	Summary of scoping study and review of current provisions
C1	AD B Clarification - Combustible structures and modular construction	C	AD B clarification
C2	Generate knowledge around mass timber construction	D1	Experimental programme and development of a test methodology
C2	Generate knowledge around mass timber construction	D2	Smouldering combustion issues and implications
C3	Generate knowledge around modern forms of construction including modular systems	D1	Experimental programme and development of a test methodology
C4	Inform the development of a test methodology (MMC and timber)	D1	Experimental programme and development of a test methodology
C5	Cavity barriers	E	Cavity barriers
C6	Car parks	F	Car parks

3. Project Summary

3.1 Scoping study (Objective A)

A scoping study on the review and collection of evidence on Structural Fire Resistance, Fire Separating Elements and Compartmentation was undertaken. 

The work in this area was reviewed based on the outputs from a previous research project completed in 2015, BD 2887 dealing with Compartment Sizes, Resistance to Fire and Fire Safety³. 

The methodology undertaken for Objective A was as follows:

• 136+ responses submitted to the MHCLG Call for Evidence on AD B that related to “Compartmentation”, “Other B3 issues” and Car parks and Fire tests considered under “Other issues” were reviewed (and areas have been identified for focus in Objective B);

• The background to the current guidance in relation to the specification and assessment of performance in fire in relation to structural fire resistance were presented;

• Alternative approaches to the specification of performance in Approved Document B (BS 9999⁴ , BS 9991⁵ , the Travelling fires approach, Structural Eurocodes) were identified (for subsequent review in Objective B);

• Issues relating to Modern Methods of Construction were identified (for subsequent review in Objective B);

• The relevant Fire Statistics (for England) were identified (for further investigation in Objective B);

• The test standards related to fire resistance and compartmentation (including extended application documents) were identified (for a subsequent review of current test methods and classification criteria in Objective B);

• From the above, additional references were identified (for subsequent review in Objective B).

Details of the Objective A scoping study can be found in Appendix A.

3.2 Review of current provisions (Objective B)

The methodology undertaken for the Objective B review of current provisions was as follows. 

Information relating to the areas identified during the Objective A scoping study was reviewed in Objective B against the underpinning basis and provisions in AD B, to determine whether AD B provides adequate guidance to meet the minimum requirements under Schedule 1 Part B of the Building Regulations 2010.

Based on the initial scoping study and review, a number of specific topics were identified as the primary focus of the current research project. The specific areas where further evidence was considered to be required were:

• Modern Methods of Construction
• Mass timber construction
• Timber frame construction. The original intention was for the project to focus on mass timber. However, at the request of BSR HSE and following discussions with the timber frame industry, additional work was undertaken to investigate the performance of timber frame construction.
• Cavity barriers
• Car parks
• Alternative approaches to defining the performance required
• Deemed to Satisfy solution
• Maximum compartment sizes

A summary of the Objective A scoping study and details of the Objective B review of current provisions can be found in Appendix B.

3.3 Provision of evidence for future policy consideration (Objective C)

Following completion of Objectives A and B, DLUHC gave consideration to the future research direction and consulted with a Working Group of the Building Regulations Advisory Committee (BRAC). DLUHC subsequently issued a research instruction that gave the direction to the Objective C research.

In summary, the six research tasks taken forward as parts of Objective C were: 

• AD B Clarification - combustible structures and modular construction
• Generate knowledge around mass timber construction
• Generate knowledge around modern forms of construction including modular systems
• Inform the development of a test methodology (MMC and timber)
• Cavity barriers
• Car parks

Research on maximum compartment sizes in relation to large single-storey buildings and on Deemed to Satisfy solutions were not taken forward to the Objective C research stage of this project.

3.3.1 AD B Clarification - Combustible structures and modular construction (Objective C1)

Details of Objective C1 AD B Clarification (Combustible structures and modular construction) can be found in Appendix C.

In order to provide clarification in relation to combustible structures and modular construction (and other uncommon building types), the authors of Appendix C1 summarised the findings from the Objective A and B studies in relation to the current recommended guidance in terms of exposure to a specified period in a standard fire test. The main findings of Objectives A and B in relation to AD B clarification are:

• The regulatory requirement for fire resistance is functional but the functional intent is not specifically identified in the current guidance.
• AD B should be explicit as to the type of structures covered by the guidance and make clear reference to other guidance for alternative routes for compliance or determine appropriate performance requirements.

Appendix C focuses specifically on the Intention sections which, to be effective, must:

• Be sufficiently clear to ensure that guidance can be interpreted correctly where there is the potential for ambiguity or insufficient detail available.
• Be sufficiently clear to ensure that alternative methods (to the recommendations of the guidance) meet the requirements.
• Ensure (current) implicit assumptions within AD B regarding its scope in relation to “common building situations” are explicitly stated.

Appendix C provides proposed clarification to the Intention sections of AD B in relation to requirement B3 (HM Government. Approved Document B (fire safety) Volume 1: Dwellings, 2019 edition incorporating 2020 amendments (for use in England)). The intention of the proposed clarification is to make the Intention sections applicable not only to “common building situations” but to all buildings.

The overall intention is to identify implicit assumptions within AD B and to make them explicit.

3.3.2 Experimental programme (Objectives C2 and C3)

Details of the Objective C2 experimental programme can be found in Appendices D1 and D2.

The approach to developing knowledge of the behaviour in fire of mass timber structures and other modern forms of construction including modular (both volumetric and panellised) systems is intrinsically linked to the development of a methodology for test and assessment that takes into account aspects of system behaviour that cannot be considered in standard fire testing. Therefore, Research Tasks C2, C3 and C4 were considered together. 

The experimental methodology that formed the basis of the experimental programme undertaken is summarised in section 3.3.5 (below).

A programme of experimental fires was conducted, as summarised in Table 2.  

The first fire experiment was a control sample intended to be representative of “traditional” forms of construction. 

The experimental programme covered a range of different systems representative from “traditional” forms of construction to volumetric steel framed modules, panellised steel frame systems, mass timber systems and timber frame systems. Fire resistance periods of 60, 90 and 120 minutes were covered. The details of the construction and the findings from the experiments can be found in Appendix D1.

Table 2 − Experimental programme

Ref.	System Description	Applied height	Relevant sector(s)	Recommended structural fire resistance requirement  (minutes)	AD A Consequence Class	Floor loaded (Yes/No)
1	Traditional (precast concrete roof panels and masonry walls) Control experiment	Under 18 m	Residential, student accommodation	60	2a or 2b	No
2	Volumetric with SHS corner posts and concrete slab	8-50 storeys	Residential	120	3	No
3	Panellised Cross Laminated Timber (CLT) walls and floors	Under 18 m	Residential, hotels, student accommodation	60	1, 2a and up to 2b	Yes (1.14 kN/m2)
4	Panellised light steel frame with concrete slab	Above 18 m	Residential, hotels, student accommodation	90	Up to 2b	Yes (1.5 kN/m2)
5	Timber frame	Under 18 m	Residential, hotels, student accommodation	60	1, 2a and up to 2b	Yes (1.2 kN/m2)

3.3.3 Generate knowledge around timber construction (Objective C2)

Details of the Objective C2 experimental programme can be found in Appendix D1.

As part of the overall experimental programme, two large-scale fire experiments were conducted on compartments constructed from Cross Laminated Timber (CLT) (Experiment 3) and timber frame (Experiment 5) to consider issues around the potential contribution of the structure itself to fire growth and development.

3.3.3.1 Experiment 3 Panellised Cross Laminated Timber compartment

The third fire experiment carried out as part of the current research project consisted of a Cross Laminated Timber (CLT) compartment constructed from wall and roof panels. The details of the construction and the findings from the experiment can be found in Appendix D1. 

Based on the results from the fire experiment involving the CLT compartment with a design fire resistance of 60 minutes, the following conclusions can be drawn:

• The compartment survived complete burn out of the moveable fire load while maintaining overall stability. There were no signs of any integrity failure during the course of the crib fire and no evidence of any external flaming away from the ventilation opening.
• There was no evidence of any insulation failure during the fire exposure from the moveable fire load, based on the criterion adopted in standard fire testing.

However, some hours after completion of the fire experiment, localised areas of smouldering combustion were present, eventually leading to reignition of the CLT in localised areas. Despite persistent attempts to deal with this, the smouldering continued over a period of several days with damage extending to three of the wall panels and spreading through to the roof panel.

The post-experimental smouldering issue is considered in a separate Appendix, based on observations and data collected for an extensive period following the end of the fire experiment.

Details of Objective C2 Smouldering combustion issues and implications can be found in Appendix D2.

3.3.3.2 Experiment 5 Timber frame compartment

Following discussions with BSR HSE, suppliers and key stakeholders, the fifth large-scale fire experiment was undertaken on a timber frame compartment formed from timber frame wall panels and engineered floor joists with a design fire resistance of 60 minutes. The details of the construction and the findings from the experiment can be found in Appendix D1. 

Based on the results from the fifth large-scale fire experiment, the following conclusions can be drawn:

• The compartment survived complete burn out of the moveable fire load while maintaining overall stability. There were no signs of any integrity failure during the course of the crib fire and no evidence of any external flaming away from the ventilation opening.
• There was no evidence of any insulation failure during the fire exposure from the moveable fire load, based on the criterion adopted in standard fire testing.

3.3.4 Generate knowledge around modular forms of construction including modular systems (Objective C3)

The experimental programme included a modular compartment with a design fire resistance of 120 minutes (Experiment 2) and a panellised system with a design fire resistance of 90 minutes (Experiment 4). The detailed observations can be found in Appendix D1, but in each case the following conclusions can be drawn:

3.3.4.1 Experiment 2 Modular steel frame system

Based on the results from the second fire experiment of a modular system with a design fire resistance of 120 minutes, the following conclusions can be drawn:

• The compartment survived complete burn out of all combustible material while maintaining overall stability. There were no signs of any integrity failure during the course of the fire and no evidence of external flaming away from the ventilation opening.
• There was no evidence of any insulation failure during the fire exposure based on the criterion adopted in standard fire testing.

3.3.4.2 Experiment 4 Panellised light steel frame system

Based on the results from the fourth fire experiment of a panellised steel system with a design fire resistance of 90 minutes, the following conclusions can be drawn:

• The compartment survived complete burn out of all combustible material while maintaining overall stability. There were no signs of any integrity failure during the course of the fire and no evidence of external flaming away from the ventilation opening.
• There was no evidence of any insulation failure during the fire exposure based on the criterion adopted in standard fire testing.

3.3.5 Inform the development of a test methodology (MMC and timber) (Objective C4)

Details of the draft test methodology can be found in Appendix D1.

The experimental methodology that forms the basis of the experimental programme undertaken is based on that set out in Loss Prevention Standard LPS 1501 Fire test and performance requirements for innovative methods of building construction⁶ and subsequently modified to form Annex B of the BRE Product Standard BPS 7014 Standard for Modular Systems for Dwellings (withdrawn)⁷ . The BRE Global Project team believes this Annex represents the most rational and logical approach to the assessment of modern forms of construction currently available.

The primary purpose of the experimental programme was twofold:

1. To assess those aspects of system behaviour that cannot be evaluated using a standard fire test procedure predicated on isolated elements and with no cooling phase.
2. To provide a means of assessment that is consistent and reproducible for all systems where, for whatever reason, reliance on the results from standard fire testing is insufficient to demonstrate compliance with the mandatory requirements of the Building Regulations in relation to performance in fire.

The performance criteria adopted are those of the standard fire test system (loadbearing capacity (stability), integrity and insulation) and the fire design is related to an equivalent severity to the level of fire resistance appropriate to the specific system under consideration. In this way, performance can be assessed and understood by a range of construction professionals in a language with which they are already familiar. It also means that the individual experiment can be tailored to the specific application in terms of the required minimum recommended period of fire resistance from Table B4 of AD B. The largescale methodology also allows assessment of other non-standard performance characteristics such as the potential for smouldering combustion, connection details and the interaction between wall and ceiling details which would generally not be determined during a standard fire resistance test.

For practical purposes, the maximum module size that can be accommodated is approximately 6m wide by 4m long. To aid comparative assessment and promote reliability and repeatability a number of parameters will be fixed. As far as possible, the ventilation conditions will be standardised, and the primary variable will be the fire load density. The other variables are the compartment geometry and ventilation.

The choice of the appropriate fire load density was based on the intended end use application provided by the manufacturer which will be related to both the recommended minimum fire resistance period and the required consequence class. The manufacturer/supplier is the only party who can provide information on the intended end use application, based on the markets in which they operate.

The concept of time equivalence as set out in the fire part of the Eurocode for Actions⁸ was used to relate severity to an equivalent period of exposure to the standard fire test. In this way, performance can be related to a metric familiar to many construction professionals. Part of what the BRE Global Project team is trying to assess is whether there is anything specific to the system (such as sensitivity to the rate of heating) that may make reliance on standard fire test results potentially inappropriate.

Where appropriate, the incorporation of active suppression systems can be taken into account through a reduction in the design fire load density within the overall methodology. Sprinkler systems were not incorporated in any of the experiments used to develop the test methodology.

3.3.6 Cavity barriers (Objective C5)

Details of Objective C5 cavity barriers can be found in Appendix E.

The methodology for Objective C5 cavity barriers was as follows.

The current means of test and assessment for open and closed state fire cavity barriers does not necessarily reflect the end use application. Evidence from standard fire testing was investigated. Alternative approaches to standard fire testing were also reviewed, such as the Association for Specialist Fire Protection (ASFP) Technical Guidance Document TGD 19 Fire resistance test for ‘open-state’ cavity barriers used in the external envelope or fabric of buildings⁹ which will form the basis of a new harmonised test standard and research undertaken in this area was looked at to try and identify the areas where additional information was required.

It was hoped that the experimental work covered under Tasks C2, C3 and C4 above would also provide an opportunity to assess the performance of cavity barriers when used in conjunction with modern forms of construction. Cavity barriers were included in the final timber frame experiment within party wall and external wall details. The experimental methodology developed during this project makes allowance for the incorporation of cavity barriers, if required. 

For modular systems, cavity barriers are often used between modules and fixed to substrates that may differ significantly from those used to determine their performance as linear gap seals in a standard fire test configuration. This leads to some confusion as to the difference between a cavity barrier and a linear gap seal. The impact of movements between modules on the performance of cavity barriers including differential thermal expansion can be evaluated as part of the assessment of performance of realistic structural configurations. This approach will also enable an assessment of service penetrations under realistic conditions of installation and assessed against a realistic thermal exposure.

The justification and background for the recommendations in Approved Document B relating to cavity barriers and cavity closures for masonry walls constructed in accordance with Diagram 5.3 of AD B Volume 1 and Diagram 9.2 of AD B Volume 2, were considered, in particular, the relaxation in the provisions for cavity barriers alongside the stated performance that “do not necessarily achieve the performance specified in paragraph 5.20 (AD B Volume 1)”, i.e. do not necessarily achieve 30 minutes integrity and 15 minutes insulation in a fire resistance test. The recommendations were assessed in relation to uninsulated cavities and insulated cavities including partial fill and, where possible, evidence was drawn from real fire incidents.

3.3.7 Car parks (Objective C6)

Details of Objective C6 car parks can be found in Appendix F.

A Focus Group including representatives from BSR HSE, the BRE Global Project Team and invited participants from the Office for Zero Emission Vehicles (OZEV), the British Parking Association (BPA), the National Fire Chiefs Council (NFCC) and consultants from OFR and Arup with expertise in relation to fires in car parks was set up. A virtual meeting was held on 29th June 2022 to define research priorities in relation to car parks.

The principal issue identified in the meeting was the justification for the specification in AD B of a recommended period of fire resistance of 15 minutes for open sided car parks up to 30m high. This issue has been investigated in relation to the original research used in support of the change and, in the light of recent high profile fire incidents and recent research into the changing nature of vehicles and car park design.

Appendix F includes a critical review of the underpinning research behind the 15 minutes recommendation which, for open sided car parks, was based on an assumption that the number of cars involved varies between 0 and 3. Recent fires such as the Echo Arena fire in Liverpool¹⁰ and the fire at Luton Airport¹¹ have shown that, under certain circumstances, fires in open sided car parks can involve multiple vehicles on multiple floors simultaneously. Reports following these fires have suggested that multi-storey car parks designed for a fire resistance rating of 15 minutes may not satisfy the functional requirement of the Building Regulations.

Recent research has established that the rate of heat release for modern vehicles is greater than that typically used for fire engineering design purposes. This is in part due to a greater use of plastics and increased vehicle size rather than any specific differences between electric vehicles and vehicles relying on internal combustion. There is also evidence that parking spaces have reduced in size with a resulting decrease in separation distances between cars and an associated increase in incident heat flux from one car to another should ignition take place.

In terms of an international perspective, England has the lowest recommended level of fire resistance for open sided car parks with the least restrictive definition of what constitutes an open sided car park. Based on the review undertaken, the research evidence underpinning the current recommendation for open sided car parks is akin to a fire in the open with no restrictions on ventilation and large heat losses to the external environment while a number of recent fire incidents are more representative of a fire within an internal compartment or a closed car park.

The review concluded that the current recommendations with respect to the fire resistance of open sided car parks up to 30 m can no longer be justified. Further work is required to look at the sensitivity of fire growth based on a variation in the relationship between the open area and the overall geometry of the car park including the ratio between open area and floor area and the minimum distance between any area within the car park where a fire might be initiated and the closest opening. 

The current situation where 15 minutes fire resistance is used to design open sided car parks up to 30 m in height should be revisited. Based on the review conducted of the current guidance related to the fire resistance of open sided car parks and based on published information and real fire incidents, it is the BRE Project Team’s understanding that the following tasks are required:

1. A study to relate assumed heat release rates to an equivalent period of fire severity. In this way more recent data on fire growth can be used to provide input to structural calculations (see bullet point 2 below) as design in England is generally not based on structural fire engineering from first principles but on prescribed periods of fire resistance. -2. A parametric study to consider the response of typical structural members (generally composite beams and steel columns) to a variation in fire severity related to either specific design fire scenarios or specified periods of exposure to the standard fire curve. The study should be informed by those responsible for the design of car parks to ensure the structural elements are representative of current practice.
2. Consideration should be given to the use of automatic sprinkler systems as a potential compensatory measure for existing structures.

4. Summary of conclusions and potential further work

A summary of the general conclusions and potential further work from the Research Tasks covered under Objective C are as follows. Full detailed conclusions and potential further work are included in the Appendices of this report.

4.1 Research Task C1 AD B Clarification – Combustible structures and modular construction

The principal output from this part of the project was clarification of AD B guidance to identify implicit assumptions and make these explicit to allow the intent section to be applicable not only to “common building situations” but to all buildings.

It is recommended that BSR HSE review the findings from Research Task C1 to see if any specific changes to the guidance are required to achieve greater clarity in relation to “uncommon building systems”.

4.2 Research Task C2 Generate knowledge around mass timber construction

The original intention was for the project to focus on mass timber. However, at the request of BSR HSE and following discussions with the timber frame industry, additional work was undertaken to investigate the performance of timber frame construction. The results from the large-scale fire experiments involving Cross Laminated Timber and timber frame have demonstrated that:

• It is possible to design a fire protection system to ensure that the construction can survive burn out of the moveable fire load for the specific design fire resistance period without any impact on the fire dynamics within the compartment.
• Even where the fire protection has demonstrated the ability of the compartment to survive burn out, this does not guarantee that there will not be reignition at a later period. The issue of smouldering combustion and potential reignition, such as occurred following the CLT experiment, cannot be accounted for in standard fire resistance tests. In the absence of an analytical approach, such issues can only be evaluated through a large-scale test methodology such as that provided under Research Task C4.

4.3 Research Task C3 Generate knowledge around modern forms of construction including modular systems

The results from the large-scale fire experiments involving volumetric and panellised steel systems have demonstrated that:

• The construction can survive burn out of the moveable fire load for the specific design fire resistance.
• The behaviour of the specific systems provided by the suppliers can be conservatively predicted based on the results from standard fire resistance tests.
• However, the experiments carried out did not include some features that are now part of the proposed test methodology.

4.4 Research Task C4 Inform the development of a test methodology (MMC and timber)

The results from Research Tasks C2 and C3 have demonstrated that:

• There is a benefit in a large-scale experimental approach to the assessment of performance in fire where for whatever reason, the results from standard fire tests may be inappropriate or insufficient.
• The methodology developed is robust enough to meet the requirements of a standard test (i.e. it is reliable, repeatable and reproduceable).

It is recommended that BSR HSE reviews the draft test methodology provided which may form the basis of a future standardised test method capable of providing additional assurance that modern systems of construction can meet the functional requirements of the Building Regulations in terms of structural fire resistance.

4.5 Research Task C5 Cavity barriers

Based on the research undertaken as part of this project and a review of related data from previous research projects, the following conclusions can be drawn:

• The current guidance in AD B in relation to the deemed to satisfy provisions for cavity barriers in a stud wall or partition or provided around openings should be reviewed.
• The findings from Research Task C5 in relation to the correct specification and installation of cavity barriers should be widely disseminated. The impact of gaps and discontinuities should be communicated across the industry.
• The development of a specific harmonised European standard for cavity barriers is a welcome development and will enable the use of open state cavity barriers where required. These products are capable of inhibiting fire and smoke spread within cavities even where gaps and discontinuities may be present.
• It is recommended that consideration be given to mandatory checks of life-safety critical aspects of building construction such as the correct installation of cavity barriers, particularly in high risk buildings.

4.6 Research Task C6 Car parks

The review undertaken as part of Objective B of this project established that the underpinning basis for 15 minutes fire resistance for open sided car parks up to 30 m high is no longer valid for modern car park designs.

The work undertaken in Research Task C6 found that the HRR (Heat Release Rate) is similar for ICEV (Internal Combustion Engine Vehicles) and EV (Electric Vehicles) although the heat release rate in the early stages may be faster for the latter. Of more concern, in terms of vehicle design and car park design, is the size and amount of plastic materials present in modern cars (including plastic fuel tanks) and the reduced size of spaces between cars. These issues together result in an increase in the total heat release for an individual vehicle and a reduced time to achieve peak HRR leading to faster spread between cars and an increased probability of multiple cars being involved in a fire incident. 

The assumption of a localised fire with a limited number of vehicles involved at any one time can no longer be maintained. This was the basis for the reduction in the recommended fire resistance rating in the guidance based on outdated research. 

The research concluded that in international terms England has one of the lowest requirements for fire resistance in open car parks, combined with the least restrictive constraints on what constitutes an “open sided” car park. 

It is recommended that the current periods in the AD B guidance to the Building Regulations for England for a fire resistance of 15 minutes in open sided car parks up to 30 m in height should be revisited.

Based on the review conducted of the current guidance related to the fire resistance of open sided car parks and based on published information and real fire incidents, it is recommended that the following activities are undertaken:

• A study to relate assumed heat release rates to an equivalent period of fire severity. In this way more recent data on fire growth can be used to provide input data to structural calculations as design in England is generally not based on design from first principles but on prescribed periods of fire resistance from the guidance.
• A parametric study is required to consider the response of typical structural members (generally composite beams and steel columns) to a variation in fire severity related to either specific design fire scenarios or specified periods of exposure to the standard fire curve. This study should be informed by those involved in the design of car parks to ensure the structural elements are representative of current practice.
• Consideration should be given to the use of automatic sprinkler systems as a potential compensatory measure for existing structures.

5. References

1. HM Government. Approved Document B (fire safety) Volume 1: Dwellings, 2019 edition incorporating 2020 amendments (for use in England).
2. HM Government. Approved Document B (fire safety) Volume 2: Buildings other than dwellings, 2019 edition incorporating 2020 amendments (for use in England).
3. BRE Global (various authors). BD 2887 Compartment sizes, resistance to fire and fire safety project. Seven workstreams, Final workstream reports for Department for Communities and Local Government, published 2019. Available from https://www.gov.uk/government/publications/compartment-size-resistance-to-fire-and-fire-safetyresearch. Last accessed 5th March 2025.
4. British Standards Institution, BS 9999:2017 Fire safety in the design, management and use of buildings – Code of practice, BSI, London, 2017
5. British Standards Institution, BS 9991:2015 Fire safety in the design, management and use of residential buildings, BSI, London, 2015.
6. Loss Prevention Standard LPS 1501 Issue 1.1 Fire test and performance requirements for innovative methods of building construction, BRE Global Limited 2014.
7. BRE Product Standard BPS 7014 Standard for Modular Systems for Dwellings Issue 1.0 Annex B Fire test and performance requirements for whole modules, BRE Global Limited 2021 (withdrawn).
8. Association for Specialist Fire Protection, Technical Guidance Document TGD 019 Fire resistance test for ‘open state’ cavity barriers used in the external envelope or fabric of buildings, ASFP, 2017.
9. British Standards Institution, BS EN 1991-1-2, Eurocode 1: Actions on structures – Part 1-2: General actions – Actions on structures exposed to fire, BSI, London, 2002.
10. Merseyside Fire & Rescue Service, Kings Dock car park fire - Protection Report, April 2018. Available from: kings-dock-car-park-fire-protection.pdf merseyfire.gov.uk. Last accessed 5th March 2025.
11. Bedfordshire Fire & Rescue Service, Significant Incident Report: London Luton Airport Terminal Car Park 2, 10/10/2023, Terminal Car Park 2, Easy Way, Luton, Bedfordshire, GB-040100-2023, published 9th October 2024. Available from: Significant Incident Report LLA Car Park 2 fire.pdf. Last accessed 5th March 2025.