Research and analysis

Fire safety: Trigger thresholds (excluding appendices)

Published 22 December 2025

Applies to England

Background

The work reported herein was carried out by a BRE Global Project team under a Contract placed by the Ministry of Housing, Communities and Local Government (MHCLG).  The final stages of this project were completed under a Contract placed by 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.

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 “Fire Safety: Trigger Thresholds”, MHCLG Contract reference CPD/004/121/055. The Technical Policy Team moved from DLUHC to the Building Safety Regulator of the Health and Safety Executive (effective from 1 April 2023). The final stages of this project were completed under HSE contract reference 1.11.4.4444.

Executive summary

The aims of the project were to assess the current provisions related to trigger and threshold levels in Approved Document B (fire safety) and in Regulation 7(4) of the Building Regulations 2010, to ensure that the thresholds used in the guidance are based on relevant and up to date evidence and provide adequate guidance to designers. 

It should be noted that the edition of the Approved Document referred to throughout this project 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 ADB. 

This project had two workstreams, running in parallel one to assess trigger and threshold levels in general, and the other to address the specific question regarding the height above which buildings should not have combustible external walls or cladding systems.  

The BRE Global Project team comprised BRE Global and project partners, Design Fire Consultants, the University of Edinburgh, and Anamil Ltd.

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

The project was broken down into a number of tasks:

• Develop an overarching research methodology.
• Establish a Technical Steering Group.
• Review and collect up to date evidence on the different trigger thresholds used throughout ADB, including regarding their basis/origin and the underpinning concept.
• Review the current trigger thresholds used in ADB considering the available evidence to develop an objective analysis of each trigger with the aim to identify a priority of review.
• Review the approach used in ADB to categorise buildings and compare this approach to potential alternatives.
• Establish and review the evidence base which supports the height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings (as defined in Regulation 7(4)).
• Develop and apply an appropriate modelling methodology to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings.
• Develop and carry out an appropriate experimental methodology to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings.
• Produce final report.

This final report contains the main summary report and supporting appendices A to F containing the full technical details from the project.  This final report provides an overview of the whole project and summarises the work conducted, the analysis and conclusions.

Please note that with the release of this report the appendices have not been reviewed again as these have previously been approved as individual reports.  This final report gathers the content of those reports and these are reproduced in appendices so that the entire project is covered in one document.

Summary of project findings

Workstream A – Approved Document B 

1. A compilation of triggers and thresholds identified within ADB has been completed.  This has involved a page-by-page review to compile a complete list of all triggers and thresholds therein.

2. A literature review has been completed of available historical background information underpinning the current trigger thresholds in ADB.  The review has identified that many triggers and thresholds are based on historic Fire and Rescue Service facilities and practice or as a result of expert judgement, based on ‘committee wisdom’.  However, a limited number of trigger thresholds were based on the findings from historical research projects.  The literature review has shown that the origins and basis of many triggers and thresholds appear to be currently unknown.

3. Interviews with experts have provided a limited amount of new information regarding the original basis/rationale of the specific trigger thresholds.  The general findings were that many trigger thresholds appear to be arbitrary in nature, with no clear scientific or engineering basis, but formed from expert judgement.  It was recognised that the origins of many trigger thresholds appear to be unknown and are based on assumptions of historic Fire and Rescue Service facilities and practice.  The consensus was that general guidance on Fire and Rescue Service access and facilities, relating to modern buildings and use, was outdated (or unknown) and in need of a fundamental review.  There was a consensus that the application of trigger thresholds in ADB could be improved with an increased use of supporting commentary on their intent.

4. A review of alternative UK guidance and examples of international guidance for comparison with ADB has identified similarities and differences in terms of the use of triggers and thresholds between ADB and alternative guidance.  The review has confirmed that all documents utilise triggers and thresholds to inform design.  The review has identified particular themes regarding the variances in guidance methodologies.  These themes also inform fundamental questions to be considered as part of an approach to prioritise particular triggers and thresholds for further review.

5. A Delphi panel analysis has produced a general consensus on further prioritisation and ranking of the specific trigger thresholds identified for analysis, in terms of the need for review in further DLUHC research.

6. A fire risk model, specially developed for this work using a Bayesian Network approach, has been used for an objective analysis of generic trigger threshold metrics and highlighted the following:

• In terms of building height, the highest risk of a fatality is in tall buildings (10 or more floors above ground).
• In terms of total building area damaged, the probability of a fatality or an injury increases significantly according to the total building area damaged by the fire.
• The analysis shows that residential buildings have a higher fire risk (of a fatality or an injury) due to the level of occupancy and because there is a sleeping risk.
• In terms of social deprivation, for residential buildings, the risk to life or injury in the most deprived areas is approximately four times that of the least deprived areas, this is mainly driven by the fire occurrence in most deprived areas.

The model also analysed the fire risk (of a fatality or injury) in buildings with different purpose groups in ADB.  The probability of a fatality or injury, given a fire has occurred, has a trend according to the purpose groups in ADB and can be influenced by:

• building occupation,
• the presence and performance of an alarm system, and
• the condition of the means of escape.

The highest fire risk (of a fatality or injury) is presented in industrial buildings, driven by the high fire frequency, and it is followed by residential (dwellings) driven by the total number of buildings. 

This building use categorisation should be considered for refinement in future developments of ADB, as it would be advantageous to include sub-categories of this use based on risk, considering hazard, processes and occupancy, so that improvements in safety can be relevant and not lead to a general increase in over-design.

1. A literature review of building categorisation within ADB and alternative UK guidance, in addition to examples of international guidance has been completed.  This review has been conducted in conjunction with a survey on seven international approaches to trigger thresholds and building categorisation via experts from the International FORUM of Fire Research Directors.  The literature review and international survey have identified three generic methods of building categorisation in use.  The review concluded that each method is based on comparable principles, with the differences in application based on the intent of the guidance and the relative importance placed on the contributing factors considered.  The advantages and disadvantages of the methodologies have been assessed comparatively, which suggests that the selection of methodology is dependent on the intent of the guidance.  Therefore, there are fundamental questions that require consideration to assess whether a selected methodology is appropriate for purpose.  This comparison confirms that the ADB purpose group methodology is preferred for a document that is intended for use by a wide range of designers and is appropriate for a document that provides guidance rather than prescription.  Based on the themes identified and conclusions reached, the review has identified several fundamental questions and recommendations to be considered by DLUHC for future updates to building categorisation guidance.

The overall findings of this workstream have not identified a need for the ADB purpose group methodology to fundamentally change.  It has assessed that the addition of purpose group definitions and clarity of common building situations is required if it is proposed for use by people who do not have fire safety knowledge. 

Workstream B – Height threshold ban of combustible materials in and on the external walls of buildings

1. A literature review has been undertaken of international fire incidents involving the external walls of tall buildings, existing data from relevant large- and intermediate-scale experiments, and other emerging evidence on the fire performance of external walls in buildings from intermediate-scale fire tests.  The review highlighted key issues and existing data to inform the development of a fire risk model for fire spread on external walls.

2. A simple model of fire risk has been developed, intended to estimate the risk from fire spread up the external wall of a building.  The concept of using existing experimental data as potential input to the simple model, was explored.  Unfortunately, the analysis did not yield any meaningful or reliable measurements/correlations to demonstrate that an intermediate-scale methodology could be representative of large-scale experiments/tests, or another intermediate-scale test method.  It is the opinion of the BRE Global project team that further development work on the fire risk would be required before it could be considered sufficiently robust to enable an objective review of the trigger threshold height to be undertaken.  Such resources are not available within the current project.

3. A desktop investigation into the indicative capabilities of water delivery systems used by Fire and Rescue Services (FRSs) in England has been undertaken.  Published data on how far water delivery systems can ‘throw’ water to various distances is dependent on the available components being used, and there is some reduction in how far water can be ‘thrown’ as it moves from the horizontal plane to the vertical plane and the angles of elevation increase.  Therefore, to complement the desktop study undertaken, an experimental field study was recommended and undertaken to examine the realistic physical limits that water delivery systems used by FRSs in England are able to effectively reach external wall fires.

4. An experimental programme has been completed to provide an indication of how far water delivery systems can deliver water (a range of equipment was selected and considered typical of equipment used by FRSs in England), primarily in terms of height from ground at different angles of elevation.  The experiments provide a simple assessment of realistic physical limits of the maximum height above ground level of effective water delivery to external walls, to support the consideration of external firefighting capabilities to limit external fire spread.  They offer no conclusion on effective suppression but water delivery alone.  Ground monitors generally tend to produce a greater maximum height of effective jet of water from the ground compared to firefighting branches, dependent on factors such as sufficient water supply and pump capacity to deliver water at the required pressure and flow rate, in combination at the point of discharge.  Only experiments using ground monitors consistently demonstrated a maximum height of an effective jet, between 18 m and 24 m above ground.  For this equipment, it demonstrates water delivery was achieved to at least the 18 m height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings as given in Regulation 7(4) of the Building Regulations 2010.  It should be noted that the experiments were conducted assuming idealised conditions in terms of water supply and delivery.  Restrictions regarding access to sites/buildings, access to firefighting water and adverse weather conditions may mean that in some circumstances, the measured water delivery heights in the experiments may not always be practically achieved.

5. A survey of various FRSs in England, to establish which water delivery systems are in use in their respective areas, has identified that equipment used in the experimental field studies was considered to be a fair representation of the range of equipment currently used by FRSs in England.  It is anticipated that water delivery systems used by FRSs would achieve similar throw distances and heights as recorded in the experiments.  It is recognised that actual performance of equipment may vary.  In order to provide similar performance in reality, the water delivery system must be provided with adequate water supply and pump capability to provide sufficient flow and pressure, in addition to adequate FRS access.

6. In terms of the trigger height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings, as given in Regulation 7(4) of the Building Regulations 2010, the relevant evidence obtained from this research has not indicated that this height threshold is inappropriate.  This is primarily based on evidence on realistic physical limits of the maximum height above ground level of effective water delivery to external walls, to support the consideration of external firefighting capabilities to limit external fire spread.  However, further work would be of benefit to generate evidence on effective water suppression of external wall fires, to further inform this trigger height threshold, as the experiments performed in this work consider water delivery only.

1. Acknowledgements

BRE Global authors would like to acknowledge the contributions of project partners, Stephen Murphy and Neal Butterworth, Design Fire Consultants, Paul Hancock, Anamil Ltd, and Grunde Jomaas and Martina Manes, University of Edinburgh.  The various contributions of the Project Partners are detailed in the appendices to this main summary 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 DLUHC officials.  The Technical Steering 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 met six times by video conference.  

1. 11th November 2020

At the first introductory meeting, the proposed research methodology was presented and discussed.

1. 12th January 2021

At the second meeting, the findings of the review and collection of evidence on trigger thresholds were presented and discussed.  

1. 25th February 2021

At the third meeting, the findings of the review of the evidence base of the height threshold used in the ban of combustible materials in and on external walls of buildings, were presented and discussed.  Initial findings on the use of fire statistics for the analysis of trigger thresholds were presented and discussed. 

1. 29th April 2022

At the fourth meeting, the findings from the analysis of trigger thresholds using a probabilistic fire risk model, along with initial findings from the review and collection of evidence on building categorisation in ADB, and alternative approaches, were presented and discussed.  Outline principles for the development of a simple model of fire risk for fire spread on external walls were presented and discussed.   

1. 7th June 2022

At the fifth meeting, the findings from an experimental field study on water delivery were presented and discussed.  The remaining findings from the analysis of trigger thresholds based on expert opinion (i.e. from interviews and a Delphi panel analysis) and a survey of international approaches to building categorisation, were presented and discussed.

1. 13th April 2023

At the sixth meeting, the findings from a simple model of fire risk for fire spread on external walls were presented and discussed.

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

• Department for Levelling Up, Housing and Communities (DLUHC) (formerly the Ministry of Housing, Communities and Local Government (MHCLG))
• BRE Global
• Design Fire Consultants
• University of Edinburgh
• Anamil Ltd
• BRAC Working Group representative
• National Fire Chiefs Council
• London Fire Brigade Fire Engineering Group
• Royal Institute of British Architects
• Health and Safety Executive
• Society of Façade Engineering
• Fire Industry Association

The authors would like to acknowledge the contributions of:

• The Valuation Office Agency who provided supporting data to the probabilistic fire risk model used in the analysis of generic trigger threshold metrics and building categorisation.
• Interviewees who provided expert opinion and the members of the Delphi panel.
• Contributions of respondents from the International FORUM of Fire Research Directors who provided information on approaches to regulation and guidance on trigger thresholds and building categorisation in various countries.
• The National Fire Chiefs Council (NFCC) who provided resources to enable the experimental field study on water delivery and NFCC representatives who were present during the experiments to assess the effectiveness of water delivery and also for co-ordinating the survey.
• Hertfordshire Fire and Rescue Service who organised the supply of facilities, staff and hospitality during the experimental field study on water delivery.

2. Introduction

The work reported herein was carried out by a BRE Global Project team under a Contract placed by the Ministry of Housing, Communities and Local Government (MHCLG).  The final stages of this project were completed under a Contract placed by 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.

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 “Fire Safety: Trigger Thresholds”, MHCLG Contract reference CPD/004/121/055. The Technical Policy Team moved from DLUHC to the Building Safety Regulator of the Health and Safety Executive (effective from 1 April 2023).  The final stages of this project were completed under HSE contract reference 1.11.4.4444.

The aims of the project were to assess the current provisions related to trigger and threshold levels in Approved Document B (ADB) (HM Government, 2019a) (HM Government, 2019b) and in Regulation 7(4) of the Building Regulations 2010 (HM Government, 2010), to ensure that the thresholds used in the guidance are based on relevant and up to date evidence and provide adequate guidance to designers.

It should be noted that the edition of the Approved Document referred to throughout this project is the HM Government, 2019a/b edition.  Where reference is made to current provisions within this report it is with reference to this edition of ADB. 

This project had two workstreams, running in parallel, one to assess trigger and threshold levels in general, and the other to address the specific question regarding the height above which buildings should not have combustible external walls or cladding systems.  The workstreams had the following objectives.

2.1 Workstream A – Approved Document B

A1. Review and collect up to date evidence on the different trigger thresholds used throughout ADB, including regarding their basis/origin and the underpinning concept.

A2. Review the current trigger thresholds used in ADB considering the available evidence to develop an objective analysis of each trigger with the aim to identify a priority of review.

A3. Review the approach used in ADB to categorise buildings and compare this approach to potential alternatives.

2.2 Workstream B – Height threshold ban of combustible materials in and on the external walls of buildings

B1. Establish and review the evidence base which supports the height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings (as defined in Regulation 7(4)).

B2. Develop and apply an appropriate modelling methodology to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings.

B3. Develop and carry out an appropriate experimental methodology to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings.

The scope of buildings covered by this project were those specifically included in Approved Document B. 

Uncommon building situations (e.g. airports and shopping centres) and those with alternative guidance (e.g. hospitals and schools) are outside of the scope for this project.  Fire safety engineering principles have been applied to underpin this project.

To meet the project objectives, the project was broken down into tasks, as follows:

• Develop an overarching research methodology.
• Establish a Technical Steering Group.
• Review and collect up to date evidence on the different trigger thresholds used throughout ADB, including regarding their basis/origin and the underpinning concept (Objective A1).
• Review the current trigger thresholds used in ADB considering the available evidence to develop an objective analysis of each trigger with the aim to identify a priority of review (Objective A2).
• Review the approach used in ADB to categorise buildings and compare this approach to potential alternatives (Objective A3).
• Establish and review the evidence base which supports the height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings (as defined in Regulation 7(4)) (Objective B1).
• Develop and apply an appropriate modelling methodology to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings (Objective B2).
• Develop and carry out an appropriate experimental methodology to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings (Objective B3).
• Produce final report.

This final report contains a main summary report and supporting appendices A to F containing the full technical details from the project.  This final report provides an overview of the whole project and summarises the work conducted, the analysis and conclusions.

3. Methodology

The methodology that has been implemented for workstreams A and B of this project was as follows.

3.1  Workstream A – Approved Document B

Objective A1 

The methodology that has been implemented for the review and collection of evidence on trigger thresholds was as follows:

• Compiled a list of all the trigger thresholds in Approved Document B, Volumes 1 and 2, 2019 edition incorporating 2020 amendments.  This involved a page-by-page review to compile a complete list of all triggers and thresholds therein, linked to the different defined Purpose Groups and differentiation of recommended fire safety provisions within ADB.
• Undertook a literature review of available historical background information underpinning the current trigger thresholds in ADB to understand how and why these triggers have been included within the Statutory Guidance.  Sources of information included: *Journal papers, e.g. Fire Safety Journal, Fire and Materials, Fire Technology, Fire Safety Science, Journal of Fire Protection Engineering.
  • Conference papers, e.g. Proceedings of IAFSS symposia, Interflam, NFPA and SFPE conferences.
  • Proceedings of one-off conferences and workshops.
  • Books and other publications, e.g. those available from BRE Bookshop and research reports produced for MHCLG.
  • Relevant historic sources of information known to inform the development of the Building Regulations and Statutory Guidance within England and Wales (e.g. Post-War Building Studies).

Additionally, a scoping web search was carried out using a range of relevant keywords, with the aim to identify fire safety trigger thresholds both in the UK and worldwide (recognising that terminology may vary).

• Reviewed alternative UK guidance provided by BS 9999 (BSI, 2017a) and BS 9991 (BSI, 2015) and examples of international guidance from Australia, New Zealand, Sweden and the USA for comparison with ADB to identify similarities and differences in terms of the use of triggers and thresholds.
• Reviewed the 98 responses submitted to the MHCLG Call for Evidence on ADB that related to stakeholder perceptions on the use of triggers and thresholds.
• Sought expert judgement to fill gaps in knowledge.  The Technical Steering Group members were also asked for their views on the origins and intentions of the different trigger thresholds.  This was continued into Objective A2 of this project.

3.1.2 Objective A2 

The methodology carried out for the analysis of trigger thresholds was as follows:

• Undertook a literature review of relevant academic and scientific studies on the application of probabilistic fire risk assessment, and the role of fire statistics, relating to the analysis of trigger thresholds.  Sources of information included:
  • Journal papers, e.g. Fire Safety Journal, Fire and Materials, Fire Technology, Fire Safety Science, Journal of Fire Protection Engineering.
  • Conference papers, e.g. Proceedings of IAFSS symposia, Interflam, NFPA and SFPE conferences.
  • Proceedings of one-off conferences and workshops.
  • Books and other publications.
• Developed and applied a bespoke probabilistic fire risk model, informed by fire statistics data (Incident Reporting System (IRS)) from England.  A Bayesian-Networks approach was used in the development of the model to carry out an objective analysis of the fire risk of generic trigger threshold metrics such as building height, depth, area, etc.  The model was also used to examine the fire risk associated with the issue of social deprivation.
• Developed a focused and prioritised list of 25 specific trigger thresholds to enable a subjective analysis using expert judgement and opinion.  The trigger thresholds for the focused and prioritised list were chosen in terms of importance, based on criteria such as their influence on design, perceived reliance by designers, the need for clarity and modernisation, and other considerations.
• Carried out a subjective analysis, based on expert judgement and opinion, namely:
  • Held a series of interviews with experienced experts who were expected to have knowledge of the historical background of fire safety guidance in ADB.  The intent of the interviews was to seek a consensus on the original basis/rationale of the focused list of trigger thresholds and opinion on the need for further DLUHC research.
  • Carried out a Delphi panel analysis, involving a wider group of experts than for the interviews, to produce a consensus on further prioritisation and ranking of the focused and prioritised list of trigger thresholds, in terms of the need for review in further DLUHC research.  A Delphi panel approach is a long-established technique that is used to build a consensus, and organised in such a way as to ensure all participants are able to contribute without being overwhelmed by the more dominant or ‘eminent’ personalities.

3.1.3 Objective A3

The methodology that has been implemented for the review of building categorisation was as follows:

• Reviewed building categorisation within ADB and alternative UK guidance provided by BS 9999 and BS 9991, in addition to examples of international guidance from Australia, New Zealand, Sweden and the USA for comparison with ADB to identify similarities and differences in terms of the building classification.

• Collated and reviewed information received via the International FORUM of Fire Research Directors on international approaches of seven countries to fire safety regulation and/or guidance on trigger thresholds and building classification.  The intent of the survey with international experts is to identify similarities and differences in international approaches compared to ADB, to provide relevant and up to date evidence.

• Application of a bespoke probabilistic fire risk model that was developed in Objective A2.  The model is based on a Bayesian-Networks approach, informed by fire statistics data (Incident Reporting System (IRS)) from England.  The model was used to carry out an objective analysis of the fire risk associated with building categorisation, as defined in ADB.

3.2  Workstream B – Height threshold ban of combustible materials in and on the external walls of buildings

3.2.1 Objective B1

The methodology that has been implemented for the review and collection of evidence which supports the height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings was as follows:

• Evidence on developments in modern firefighting equipment and operational practice has been collated and reviewed, with a specific focus on the height thresholds associated with the requirements for the external walls of buildings.

• A desktop investigation has been undertaken into the indicative capabilities of water delivery systems used by Fire and Rescue Services (FRSs), as well as the typical and maximum potential heights to which they are able to deliver.  Types and numbers of water delivery systems that Fire and Rescue Services use in England have been identified.

• The evidence base related to international fire incidents involving the external walls of tall buildings has been reviewed.  The review of international fire incidents involving combustible exterior wall assemblies which was part of the previous MHCLG project ‘Fire performance of cladding materials research’ CCZZ17A36, conducted by BRE Global and published in 2020 (BRE Global, 2020), has been revisited and updated, where relevant, in light of recent fire incidents.

• The DCLG series of large-scale BS 8414 tests (BSI, 2017b), conducted by BRE Global, to establish how different types of Aluminium Composite Material (ACM) panels in combination with different types of insulation behave in a fire, has been reviewed.  An additional large-scale BS 8414 test conducted by the Fire Protection Association on a High Pressure Laminate (HPL) rainscreen cladding system has also been reviewed.  The review considers how the existing large-scale experimental data could be utilised in the development and application of an experimental methodology to assess the height threshold ban of combustible materials.

• The MHCLG Project ‘Fire Performance of Cladding Materials Research’ CCZZ17A36 has also been reviewed with the intention to build upon this previous research work to assess the height threshold ban of combustible materials, utilising the intermediate-scale experimental methodology that has been developed to consider issues such as contribution to fire growth, incident heat flux, cavity fire spread and total heat release.

• Other emerging evidence on the fire performance of external walls in buildings has been reviewed from the recent work by Bonner et al published in 2020 (Bonner et al, 2020) on a statistical approach to understand the fire performance of building facades using standard test data.

This Objective B1 review of the evidence base was used to inform work carried out in objectives B2 and B3 of this project.

3.2.2 Objective B2

The modelling methodology that has been implemented to enable supporting evidence to inform the review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings, was as follows: 

• Development of a simple model of relative fire risk for fire spread on external walls. The model is intended to estimate the risk from fire spread up the external wall of a building.  The risk metrics chosen as measure of the risk are the number of storeys damaged by fire and the total area damaged by the fire.

• Review of existing experimental data to inform whether an intermediate-scale experimental methodology could be representative of a large-scale experiment/test or an alternative intermediate-scale test method, to justify the potential use of existing data as input to the simple model of fire risk.

3.2.3 Objective B3

At the start of the project, it was proposed to carry out a limited and targeted experimental programme (a maximum of five new experiments) in Objective B3, to provide additional intermediate-scale results incorporating other aspects of external wall construction not considered in the previous work (e.g. presence of external panels in conjunction with different types of insulation in the cavity, presence of cavity barriers, etc.)  The results were intended to provide additional experimental data as input to the model of fire risk developed in Objective B2. 

Upon review and discussion with DLUHC on the benefits of carrying out these additional experiments, it was felt that although useful information would be gained, the limited number and specific nature of those experiments would not provide a significant additional benefit in the development of the fire risk model beyond the use of the existing experimental data.

Therefore, proposed resource for these fire experiments was reallocated to a series of field study experiments to examine the maximum height that water delivery systems (used by Fire and Rescue Services in England) are able to effectively reach external walls from ground level, to complement the desktop study on this issue carried out in Objective B1. 

Therefore, the experimental methodology that has been implemented to enable the objective review of the trigger height threshold used in the ban of combustible materials in and on the external walls of buildings was as follows:

• Undertook an experimental field study to examine the realistic physical limits, primarily in terms of the maximum height from ground level, that water delivery systems used by FRSs in England are able to effectively reach external wall fires.
• Undertook a survey of various FRSs in England to establish which water delivery systems are in use in their respective areas, specifically in terms of the pump type and capabilities, and details of ground monitors/firefighting branches used.  The survey results were used to add context to the results of the experimental field study, to provide indicative heights from ground level that different water delivery systems used by FRSs in England can achieve.

4. Summary of project findings

A summary of the findings for each objective of workstreams A and B, using the methodologies described in section 3 are, as follows.

4.1  Workstream A – Approved Document B

4.1.1 Objective A1

Details of the review and collection of evidence on trigger thresholds are given in Appendix A.

A compilation of triggers and thresholds identified within Approved Document B 2019, incorporating 2020 Amendments, has been completed.  

The review identified 225 sections/clauses in ADB Volume 1 and 226 sections/clauses within ADB Volume 2 that contained a trigger or threshold.  There were 62 sections/clauses in Volume 1 which contained a trigger or threshold that was not repeated in Volume 2.  Therefore, within both volumes of ADB, 288 individual section/clauses containing a trigger or threshold were identified (62 in Volume 1 and 226 in Volume 2).  There were 28 different metrics associated with the trigger thresholds identified, with the most frequent being “height” (approximately 30% of all those identified).

A literature review has been completed of available historical background information underpinning the current trigger thresholds in ADB.  The review has identified that many triggers and thresholds are based on historic Fire and Rescue Service facilities and practice (particularly with regard to building height) or as a result of expert judgement, based on “committee wisdom”.  However, a limited number of trigger thresholds were based on the findings from historical research projects.  The literature review has shown that the origins and basis of many triggers and thresholds appear to be currently unknown.    

Consultation with Technical Steering Group members on evidence regarding the origins of the specific triggers and thresholds, identified similar findings to those of the literature review.  

A review has been completed of alternative UK guidance and examples of international guidance from Australia, New Zealand, Sweden and the USA for comparison with ADB.  The review has identified similarities and differences in terms of the use of triggers and thresholds between ADB and alternative guidance.  The review confirmed that all documents utilise triggers and thresholds to inform design.  The triggers and thresholds on which any particular fire protection measure is recommended can vary by both metric and value but, generally, a different metric is in addition rather than alternative to the metrics in ADB guidance.  The review has identified particular themes regarding the variances in guidance methodologies.  This could be considered as part of a review of the future application of triggers and thresholds in ADB.  These themes also inform fundamental questions to be considered as part of an approach to prioritise particular triggers and thresholds for further review.

A review has been completed of the 98 responses submitted to the MHCLG Call for Evidence on ADB that related to stakeholder perceptions on the use of triggers and thresholds.  The review has identified common themes on views on the application of triggers and thresholds in ADB.   The most common general theme was that holistic risk assessment was required as a basis for the future application of triggers and thresholds within ADB.

4.1.2 Objective A2

Details of the analysis of trigger thresholds are given in Appendix B.

A literature review has been completed of relevant academic and scientific studies on the application of probabilistic fire risk assessment and the role of fire statistics, relating to the analysis of trigger thresholds.  The application of probabilistic fire risk assessment has been identified as a useful approach for evaluation of fire risk.  The risk can vary according to aspects such as building type, societal and personal characteristics, in addition to the fire safety measures in place.  The review identified that fire statistics can provide information representative of real building fire incidents that can be successfully adopted as input variables to probabilistic fire risk assessments.  The review supports the technical approach used in this work, to enable an objective analysis of fire risk associated with generic trigger threshold metrics.

A fire risk model, specially developed for this work using a Bayesian Network approach and based on fire statistics data, has been used for an objective analysis of generic trigger threshold metrics.  In terms of building height, the highest risk of a fatality is in tall buildings (10 or more floors above ground).  This is influenced by the number fatalities and fire occurrence recorded in the IRS on these buildings and the posterior probability of fatalities in a fire.  In general, residential buildings have a higher risk than non-residential buildings due to the level of occupancy and because there is a sleeping risk.  In terms of building depth, the probability of a fatality and an injury, given there is a fire, is steady with respect to increasing depth.  The risk is mainly influenced by the fire occurrence; therefore, the highest risk is in buildings with one floor below ground.  Further analysis could be done according to the floor of origin of the fire, as this risk is in buildings with one floor below ground but does not mean the floor of origin of the fire is necessarily in the floor below ground.  In terms of social deprivation, for residential buildings, the highest level of risk to life or injury is in the most deprived areas, with the lowest risk to life or injury in the least deprived areas.  The risk to life or injury in the most deprived area is approximately four times that of the least deprived area; this is mainly driven by the fire occurrence in the most deprived areas.  In terms of total building area damaged, the probability of a fatality or an injury increases significantly according to the total building area damaged by the fire. 

Interviews with experts provided a limited amount of new information regarding the original basis/rationale of the specific trigger thresholds.  The general findings were that many trigger thresholds appear to be arbitrary in nature, with no clear scientific or engineering basis, but formed from expert judgement based on ‘committee wisdom’.  It was recognised that the origins of many trigger thresholds appear to be unknown and are based on assumptions of historic Fire and Rescue Service facilities and practice.  The consensus was that general guidance on Fire and Rescue Service access and facilities, relating to modern buildings and use, was outdated (or unknown) and in need of a fundamental review.  There was a consensus that the application of trigger thresholds in ADB could be improved with an increased use of supporting commentary on their intent.  

A Delphi panel analysis has produced a general consensus on further prioritisation and ranking of the 25 specific trigger thresholds identified for analysis, in terms of the need for review in further DLUHC research. The trigger threshold ranked with the highest priority for further research related to the building height/evacuation/firefighting interaction in tall buildings (i.e. 30 m and 45 m building height thresholds).  

4.1.3 Objective A3

Details of the review and collection of evidence on building categorisation is given in Appendix C.

A literature review has been carried out of building categorisation within ADB and alternative UK guidance, in addition to examples of international guidance.  This review has been conducted in conjunction with a survey on seven international approaches to trigger thresholds and building categorisation via experts from the International FORUM of Fire Research Directors.  

The literature review and international survey has identified three generic methods of building categorisation in use, these are:

1. A set of predetermined use categories, with or without subdivisions, where guidance applies equally to the defined use category or sub-category.  ADB is an example of this type of categorisation.
2. A set of predetermined use categories, with or without subdivisions, where guidance applies equally to the defined use category or sub-category; adjusted by additional categorisation factors (e.g. height, construction type, or hazard) that apply across the applicable use categories.  NFPA provides an example of this type of categorisation.
3. A set of predetermined characteristics from which a building profile can be determined.  The characteristics focus on the occupants and the fire hazard and combine to determine the applicable guidance, with the building ‘use’ being an input rather than a defining attribute.  BS 9999 provides an example of this type of categorisation.

Based on the intended use of the sample documents that follow each method of categorisation, each method has been developed to meet different objectives as follows:

• Method (i) (e.g. ADB) – Intended to provide practical guidance on common building situations about how to meet the requirements set out in the relevant regulations.
• Method (ii) (e.g. NFPA) – Intended as a prescriptive document with mandatory requirements.  The increased sophistication of categorisation allows for guidance that is risk proportionate to use.
• Method (iii) (e.g. BS 9999) – A risk-based approach, intended to be sufficiently flexible to suit any particular building.  Typically described as an advanced approach in comparison to Method (i).

This review concluded that each method is based on comparable principles, with the differences in application based on the intent of the guidance and the relative importance placed on the contributing factors considered.

The advantages and disadvantages of the methodologies have been assessed comparatively, which suggests that the selection of methodology is dependent on the intent of the guidance.  Therefore, there are fundamental questions that require consideration to assess whether a selected methodology is appropriate for purpose.

This comparison confirms that the ADB purpose group methodology, Method (i), is preferred for a document that is intended for use by a wide range of designers and is appropriate for a document that provides guidance rather than prescription.

Based on the themes identified and conclusions reached, the review has identified several fundamental questions and recommendations to be considered by DLUHC for future updates to building categorisation guidance.

The international survey identified that the historical background to the development of international trigger thresholds is similar to that identified in Appendices A and B.  The most common international trigger thresholds are the height and floor area of a building, building or hazard classification/use and the number of occupants in the building.  These align with common trigger thresholds used in ADB.  

The international survey showed that the most common factors that determine the building categorisation system internationally are the building height/number of floors and the building purpose/use, followed by the fire load.   The international survey identified other factors, including consideration of the number of occupants and the occupants familiarity within a building.

A fire risk model, specially developed for this work using a Bayesian Networks approach and based on fire statistics data, calculated the posterior probability of fatalities and injuries in buildings of the different purpose groups in ADB, with the following conclusions:

• The probability of a fatality or injury, given a fire has occurred, has a trend according to the purpose groups in ADB and can be influenced by several conditions during a fire.  Building occupation, the presence and performance of an alarm system, and the condition of the means of escape have an impact on the posterior probability of fatalities and injuries in buildings during a fire.

• The highest probability of a fatality, given a fire has occurred, is in residential buildings, both institutional and other, and dwellings.  Amongst non-residential buildings, the highest posterior probability of fatalities is in office buildings and the lowest in storage buildings.  These findings appear to be linked to the probability of these buildings being occupied at the time of the incident, and the presence and (if present) successful operation of alarm system.

• The highest probability of an injury, given a fire has occurred, is in residential buildings, both institutional and other, and dwellings.  Amongst non-residential buildings, the highest posterior probability of injuries is in shops and commercial buildings, although office buildings present a similar value; the lowest posterior probability is in storage buildings.  These findings appear to be linked to the probability of these buildings being occupied at the time of the incident and the presence and (if present) successful operation of alarm system.  The high probability of an injury in shops and commercial buildings could be due to the relatively high probability of these buildings not having means of escape provisions in good condition.

• The highest fire risk to life is presented in industrial buildings, driven by the high fire frequency, and it is followed by residential (dwellings) driven by the total number of buildings.  The lowest risk of a fatality is presented in office buildings due to the low fire frequency and total number of buildings.  It should be noted that there is a high level of uncertainty of fatalities due to the low number of fatalities recorded in the statistics.

As industrial buildings have the highest risk to life or of an injury, this building use categorisation should be considered for refinement in future developments of ADB, as it would be advantageous to include subcategories of this use based on risk, considering hazard, processes and occupancy, so that improvements in safety can be relevant and not lead to a general increase in over-design. 

The overall findings of this workstream have not identified a need for the ADB purpose group methodology to fundamentally change.  It has assessed that the addition of purpose group definitions and clarity of common building situations is required if it is proposed for use by people that do not have fire safety knowledge.

4.2  Workstream B – Height threshold ban of combustible materials in and on the external walls of buildings

4.2.1 Objective B1

Details of the review of the evidence base of the height threshold used in the ban of combustible materials in and on external walls is given in Appendix D. 

Developments in modern firefighting equipment and operational practice have been reviewed, with a specific focus on the height thresholds associated with the requirements for the external walls of buildings.  This has identified that Fire and Rescue Authorities assess risks across a spectrum of operational incident types, including those associated with fires in tall buildings, as part of their integrated risk management planning process.  The operational response of Fire and Rescue Services (FRSs) is underpinned by National Operational Guidance (NOG) on the hazards, risks and control measures associated with dealing with fires in tall buildings, which takes account of the trigger thresholds outlined in ADB.  

A desktop investigation into the indicative capabilities of water delivery systems used by FRSs in England has been undertaken.  Water delivery systems can ‘throw’ water to various distances within a range up to a maximum of 100 m (when aerial appliances as well as high volume pumps are used); this is an optimum figure, and actual distances are dependent on the available components being used.  There is some reduction in how far water can be ‘thrown’ from a water delivery system as it moves from the horizontal plane to the vertical plane and the angles of elevation increase.  Therefore, to complement the desktop study undertaken, an experimental field study was recommended to examine the realistic physical limits that water delivery systems used by FRSs in England are able to effectively reach external wall fires, as a part of Objective B3 of this work.

International fire incidents involving the external walls of tall buildings have been reviewed, revisited and updated, where relevant.  The key initiating fire scenarios identified from this review have been considered in the fire risk model developed in Objective B2, to represent how fires involving external walls initiate and develop in reality.

The MHCLG series of large-scale BS 8414 tests to establish how different types of Aluminium Composite Material (ACM) panels in combination with different types of insulation behave in a fire has been reviewed, in addition to a review of a large-scale BS 8414 test on a High Pressure Laminate (HPL) rainscreen cladding system.  This has identified existing large-scale experimental data (in terms of surface spread of flame, temperature measurement and visual observations) that could be utilised in the development and application of a fire risk model to assess the height threshold ban of combustible materials on external walls in Objective B2.

A series of intermediate-scale experiments conducted in the MHCLG project ‘Fire Performance of Cladding Materials Research’ has been reviewed.  The intention was to build upon this previous research in the development and application of a fire risk model to assess the height threshold ban of combustible materials on external walls in Objective B2.  The intention was to utilise this intermediate-scale experimental methodology to consider issues such as fire growth, incident heat flux, cavity fire spread and total heat release of external panels of façade systems due to fire. 

Data from relevant large- and intermediate-scale experiments were identified to seek potential correlations between these experiments in the development of the fire risk model in Objective B2.

Other emerging evidence on the fire performance of external walls in buildings from intermediate-scale fire tests has been reviewed, and the use of such evidence has highlighted key issues to inform the development of the fire risk model in Objective B2. 

4.2.2 Objective B2

Details of the simple model of fire risk for fire spread on external walls, as part of the objective review of the height threshold used in the ban of combustible materials in and on external walls, is given in Appendix E. 

A simple model has been developed, intended to estimate the risk from fire spread up the external wall of a building and attempts to combine this model with experimental determination of the fire properties of cladding assemblies. 

The simple model incorporates the following:

• building geometry
• the compartment fire(s)
• the external cladding
• a cavity between the main structure of the building, and the external cladding
• cavity barriers in cavities within the cladding system
• window failure
• material properties of all building elements, where relevant
• external flaming
• FRS firefighting (affected by extent of access to the perimeter, and “reach” of different appliances)
• stochastic factors, where relevant
• risk metrics (a) number of storeys affected (b) total area of fire damage.

This project has shown that a “simple” model is in fact complex.  Due to the unknown robustness of the unvalidated model outputs, they are not considered suitable to enable an objective review of the trigger threshold height used in the ban of combustible materials in and on the external walls of buildings to take place.

The concept of using existing experimental data as potential input to the simple model, was explored. Unfortunately, the analysis did not yield any meaningful or reliable measurements/correlations to demonstrate that an intermediate-scale methodology could be representative of large-scale experiments/tests, or another intermediate-scale test method.  Attempts to utilise existing data as relevant input to the simple model were therefore abandoned.  Further work could be carried out to develop an intermediate-scale fire test similar to the BRE Global (or other) experimental method/test for complete façade systems (i.e. rainscreen, cavity, insulation, cavity barrier, etc.) that could be representative of a large-scale test.  This is likely to require further development of a method with more extensive instrumentation and to facilitate more robust visual observations of flame propagation, etc. than current methods.  This would also likely involve further experiments/tests/CFD modelling at large-scale (e.g. BS 8414 scale), also with more extensive instrumentation/visual observations than existing methods, to facilitate potential correlations to demonstrate a relationship between different scales.  

It is the opinion of the BRE Global Project team that further development work would be required before the fire risk modelling approach could be considered sufficiently robust to enable an objective review of the trigger threshold height to be undertaken.  Such resources were not available within this project.  Even with further development of a “simple” model, it is possible that the various nuances of different cladding systems may preclude the development of simple guidance suitable for use in the review of ADB.

4.2.3 Objective B3

Details of the water delivery system experiments and survey, as part of the objective review of the height threshold used in the ban of combustible materials in and on external walls is given in Appendix F. 

A series of 104 experiments has been completed to provide an indication of how far water delivery systems can deliver water (a range of equipment was selected and considered typical of equipment used by FRSs in England), primarily in terms of height from ground level at different angles of elevation.  The experiments provide a simple assessment of realistic physical limits of the maximum height above ground level of effective water delivery to external walls, to support the consideration of external firefighting capabilities to limit external fire spread.  They offer no conclusion on effective suppression but water delivery alone.  Assessment of effective water delivery was based on subjective measurement, determined by expert judgment, on the maximum height above ground where a well-structured jet of water impinges on the external elevation of a building.

The experiments make the fundamental simplifying assumption of effective FRS intervention being provided to control external fire spread, in terms of response time, suitable and sufficient water supplies, access to elevations of a building, etc.  If any of these simplifying assumptions is not achievable in reality, then the findings from these experiments may not be valid.

Visual observations of the experiments have shown that the structure of a jet of water discharged from a firefighting branch or monitor, tended to become less stable and more diffuse, with the increased tendency for loss (or fall out) of water from the jet to ground, with:

• Increasing horizontal distance from the monitor/branch
• Increasing angle of elevation of water jet
• Wind effects.

Ground monitors (which generally operate at higher pressures and flow rates than firefighting branches) were found to throw water at greater horizontal distance compared with the firefighting branches.  Of the equipment tested, the maximum throw measured was approximately 35 m for the monitors compared to approximately 20 m for the branches operating at their recommended flowing pressure.  The height of an effective jet above ground tends to increase with increasing angle of elevation of jet, with the maximum height being achieved at an angle of elevation of 50 to 60 degrees for the ground monitors, and at an angle of 60 degrees for the firefighting branches.  Note that 60 degrees was the highest angle of elevation examined due to limits in the specification on the use of equipment and realistic limits of operational practice.

Ground monitors generally tend to produce a greater maximum height of effective jet of water from the ground compared to firefighting branches.  The maximum height of an effective jet varied between 18 m and 24 m above ground for the ground monitors compared to a height of 14 m to 20 m for the branches operating at their recommended flowing pressure.  The maximum height of an effective jet above ground required the monitors to be located between 15 m and 22 m from the building elevation, and for the branches, between 8 m and 11 m from the building elevation.  For all firefighting branches studied, the maximum height of an effective jet of water above ground tends to decrease as the flowing pressure at the branch decreases below the recommended operating flowing pressure.  Firefighting branches and ground monitors which operate at a higher flowing pressure were found to increase the maximum height of an effective jet of water from ground.

The maximum height of an effective jet of water above ground is dependent on factors such as sufficient water supply and pump capacity to deliver water at the required pressure and flow rate, in combination at the point of discharge.  Only experiments using ground monitors consistently demonstrated a maximum height of an effective jet, between 18 m and 24 m above ground.  For this equipment, it demonstrates water delivery was achieved to at least the 18 m height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings as given in Regulation 7(4) of the Building Regulations 2010.  Ground monitors operating at a minimum of 7 bar/1500 l/min, or 10 bar/790 l/min are expected to produce a maximum height of effective jet to at least 18 m.  This performance could only be achieved in the experiments with monitors located between 15 m and 22 m from the building elevation.  A firefighting branch operating at a minimum of 7 bar/700 l/min is expected to produce a maximum height of effective jet to at least 18 m.  This performance could only be achieved in the experiments with the branch located 11 m from the building elevation.  It should be noted that the experiments were conducted assuming idealised conditions in terms of water supply and delivery.  Restrictions regarding access to sites/buildings, access to firefighting water and adverse weather conditions may mean that in some circumstances, the measured water delivery heights in the experiments may not always be practically achieved.

The survey responses identified that the technical specification and performance characteristics of the different types of equipment available on fire appliances and used in water delivery systems by 14 (out of

45) FRSs in England, were the same or similar to the technical specification and performance characteristics of the items of equipment used in the experimental field study.  

More specifically, all of the responding 14 FRSs were identified to:

• have fire appliances with a pumping capacity capable of delivering at least the minimum flowing pressure (7 bar) and flow rate (700 l/min for a firefighting branch and 790 l/min for a ground monitor), used in the water delivery experiments, to achieve a height of effective jet of water to at least 18 m.
• have access to, and use, at least one ground monitor on fire appliances, capable of a performance to at least the minimum flowing pressure (7 bar) and flow rate (790 l/min) used in the water delivery experiments, to achieve a height of effective jet of water to at least 18 m.

Five of the responding 14 FRSs were identified to:

• have access to, and use, at least one firefighting branch on fire appliances, capable of a performance to at least the minimum flowing pressure (7 bar) and flow rate (700 l/min) used in the water delivery experiments, to achieve a height of effective jet of water to at least 18 m.

The equipment used in the experimental field studies was considered to be a fair representation of the range of equipment currently used by FRSs in England.  It is anticipated that water delivery systems used by FRSs would achieve similar throw distances and heights as recorded in the experiments.  It is recognised that actual performance of equipment may vary.  In order to provide similar performance in reality, the water delivery system must be provided with adequate water supply and pump capability to provide sufficient flow and pressure, in addition to adequate FRS access.

5. Conclusions

A summary of the general conclusions of workstreams A and B is as follows.

5.1 Workstream A – Approved Document B

1. A compilation of triggers and thresholds identified within ADB has been completed.  This has involved a page-by-page review of ADB to compile a complete list of all triggers and thresholds therein.

2. A literature review has been completed of available historical background information underpinning the current trigger thresholds in ADB.  The literature review has identified that many triggers and thresholds are based on historic Fire and Rescue Service facilities and practice or as a result of expert judgement, based on ‘committee wisdom’.  However, a limited number of trigger thresholds are based on the findings of historical research projects.  The literature review has shown that the origins and basis of many triggers and thresholds appear to be currently unknown.

3. Interviews with experts have provided a limited amount of new information regarding the original basis/rationale of the specific trigger thresholds.  The general findings were that many trigger thresholds appear to be arbitrary in nature, with no clear scientific or engineering basis, but formed from expert judgement.  It was recognised that the origins of many trigger thresholds appear to be unknown and are based on assumptions of historic Fire and Rescue Service facilities and practice.  The consensus was that general guidance on Fire and Rescue Service access and facilities, related to modern buildings and use, was outdated (or unknown) and in need of a fundamental review.  There was a consensus that the application of trigger thresholds in ADB could be improved with an increased use of supporting commentary on their intent.

4. A review of alternative UK guidance and examples of international guidance for comparison with ADB has identified similarities and differences in terms of the use of triggers and thresholds between ADB and alternative guidance.  The review has confirmed that all documents utilise triggers and thresholds to inform design.  The review has identified particular themes regarding the variances in guidance methodologies.  These themes also inform fundamental questions to be considered as part of an approach to prioritise particular triggers and thresholds for further review.

5. A Delphi panel analysis has produced a general consensus on prioritisation and ranking of the 25 specific trigger thresholds identified for analysis, in terms of the need for review in further DLUHC research.

6. A fire risk model, specially developed for this work using a Bayesian Network approach, has been used for an objective analysis of generic trigger threshold metrics and highlighted the following:

• In terms of building height, the highest risk of a fatality is in tall buildings (10 or more floors above ground).

• In terms of total building area damaged, the probability of a fatality or an injury increases significantly according to the total building area damaged by the fire.

• The analysis shows that residential buildings have a higher fire risk (of a fatality or an injury) due to the level of occupancy and because there is a sleeping risk.

• In terms of social deprivation, for residential buildings, the risk to life or injury in the most deprived areas is approximately four times that of the least deprived areas; this is mainly driven by the fire occurrence in most deprived areas.

The model also analysed the fire risk (of a fatality or injury) in buildings with different purpose groups in ADB.  The probability of a fatality or injury, given a fire has occurred, has a trend according to the purpose groups in ADB and can be influenced by building occupation, the presence and performance of an alarm system, and the condition of the means of escape.  

The highest fire risk (of a fatality or injury) is presented in industrial buildings, driven by the high fire frequency, and it is followed by residential (dwellings) driven by the total number of buildings.  This building use categorisation should be considered for refinement in future developments of ADB, as it would be advantageous to include sub-categories of this use based on risk, considering hazard, processes and occupancy, so that improvements in safety can be relevant and not lead to a general increase in over-design.

1. A literature review of building categorisation within ADB and alternative UK guidance, in addition to examples of international guidance has been completed.  This review was conducted in conjunction with a survey on seven international approaches to trigger thresholds and building categorisation via experts from the International FORUM of Fire Research Directors.  The literature review and international survey has identified three generic methods of building categorisation in use.  The review concluded that each method is based on comparable principles, with the differences in application based on the intent of the guidance and the relative importance placed on the contributing factors considered.  The advantages and disadvantages of the methodologies have been assessed comparatively, which suggests that the selection of methodology is dependent on the intent of the guidance.  Therefore, there are fundamental questions that require consideration to assess whether a selected methodology is appropriate for purpose.  This comparison confirms that the ADB purpose group methodology is preferred for a document that is intended for use by a wide range of designers and is appropriate for a document that provides guidance rather than prescription.  Based on the themes identified and conclusions reached, the review has identified several fundamental questions and recommendations to be considered by DLUHC for future updates to building categorisation guidance.

The overall findings of this workstream have not identified a need for the ADB purpose group methodology to fundamentally change.  It has assessed that the addition of purpose group definitions and clarity of common building situations is required if it is proposed for use by people that do not have fire safety knowledge. 

5.2 Workstream B – Height threshold ban of combustible materials in and on the external walls of buildings

1. A literature review has been undertaken of international fire incidents involving the external walls of tall buildings, existing data from relevant large- and intermediate-scale experiments, and other emerging evidence on the fire performance of external walls in buildings from intermediate-scale fire tests.  The review highlighted key issues and existing data to inform the development of a fire risk model for fire spread on external walls.

2. A simple model of fire risk has been developed, intended to estimate the risk from fire spread up the external wall of a building.  The concept of using existing experimental data as potential input to the simple model, was explored.  Unfortunately, the analysis did not yield any meaningful or reliable measurements/correlations to demonstrate that an intermediate-scale methodology could be representative of large-scale experiments/tests, or another intermediate-scale test method.  It is the opinion of the BRE Global Project team that further development work on the fire risk would be required before it can be considered sufficiently robust to enable an objective review of the trigger threshold height to be undertaken.  Such resources were not available within the current project.

3. A desktop investigation into the indicative capabilities of water delivery systems used by FRSs in England has been undertaken.  Published data on how far water delivery systems can ‘throw’ water to various distances is dependent on the available components being used, and there is some reduction in how far water can be ‘thrown’ as it moves from the horizontal plane to the vertical plane and the angles of elevation increase.  Therefore, to complement the desktop study undertaken, an experimental field study was recommended to examine the realistic physical limits that water delivery systems used by FRSs in England are able to effectively reach external wall fires.

4. An experimental programme has been completed to provide an indication of how far water delivery systems can deliver water (a range of equipment was selected and considered typical of equipment used by FRSs in England), primarily in terms of height from ground at different angles of elevation.  The experiments provide a simple assessment of realistic physical limits of the maximum height above ground level of effective water delivery to external walls, to support the consideration of external firefighting capabilities to limit external fire spread.  They offer no conclusion on effective suppression but water delivery alone.  Ground monitors generally tend to produce a greater maximum height of effective jet of water from the ground compared to firefighting branches, dependent on factors such as sufficient water supply and pump capacity to deliver water at the required pressure and flow rate, in combination at the point of discharge.  Only experiments using ground monitors consistently demonstrated a maximum height of an effective jet, between 18 m and 24 m above ground.  For this equipment, it demonstrates water delivery was achieved to at least the 18 m height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings as given in Regulation 7(4) of the Building Regulations 2010.  It should be noted that the experiments were conducted assuming idealised conditions in terms of water supply and delivery.  Restrictions regarding access to sites/buildings, access to firefighting water and adverse weather conditions may mean that, in some circumstances, the measured water delivery heights in the experiments may not always be practically achieved.

5. A survey of various FRSs in England, to establish which water delivery systems are in use in their respective areas, has identified that equipment used in the experimental field studies was considered to be a fair representation of the range of equipment currently used by FRSs in England.  It is anticipated that water delivery systems used by FRSs would achieve similar throw distances and heights as recorded in the experiments.  It is recognised that actual performance of equipment may vary.  In order to provide similar performance in reality, the water delivery system must be provided with adequate water supply and pump capability to provide sufficient flow and pressure, in addition to adequate FRS access.

6. In terms of the trigger height threshold for the in-effect ban of combustible materials in and on the external walls of relevant buildings, as given in Regulation 7(4) of the Building Regulations 2010, the relevant evidence obtained from this research has not indicated that this height threshold is inappropriate.  This is primarily based on evidence on realistic physical limits of the maximum height above ground level of effective water delivery to external walls, to support the consideration of external firefighting capabilities to limit external fire spread.  However, further work would be of benefit to generate evidence on effective water suppression of external wall fires, to further inform this trigger height threshold, as the experiments performed in this work considered water delivery only.  The effect of water suppression could consider fires on the surface of external walls, fires in external walls cavities, potentially shielded by construction, rainscreen cladding, etc. and fires generated from a compartment of fire origin fronting onto the external wall.

6. References

Bonner M, Wegrzynski W, Bartlomiej K, Papis K B and Rein G (2020).  KRESNIK: A top-down, statistical approach to understand the fire performance of building facades using standard test data, Building and Environment, Volume 169, February 2020. 

BRE Global (2020).  Fire Performance of Cladding Materials Research, CCZZ17A36 Final Research Report. Available at: https://www.gov.uk/government/publications/fire-performance-of-cladding-materialsresearch (Accessed 3 February 2021).

BSI (2015).  BS 9991.  Fire safety in the design, management and use of residential buildings – Code of practice, British Standards Institution.

BSI (2017a).  BS 9999.  Fire safety in the design, management and use of buildings – Code of practice, British Standards Institution.

BSI (2017b).  BS 8414-1:2015 + A1: 2017 Fire performance of external cladding systems, Part 1: Test method for non-load bearing external cladding systems applied to the masonry face of a building, British Standards Institution.

HM Government (2010).  Statutory Instruments, 2010 No. 2214 Building and Buildings, England and Wales, The Building Regulations 2010 (as amended).

HM Government (2019a).  Approved Document B (fire safety) Volume 1: Dwellings, 2019 edition incorporating 2020 amendments (for use in England).

HM Government (2019b).  Approved Document B (fire safety) Volume 2: Buildings other than dwellings, 2019 edition incorporating 2020 amendments (for use in England).