The Commercial Timber Guidebook

COMMERCIAL TIMBER GUIDEBOOK

Industry consensus guidance for delivering robust, safe,

durable and insurable mass timber buildings

November 2024

Published 2024

No part of this publication may be reproduced without the written permission of the publisher or the copyright owner.

Printed in the UK

DISCLAIMER

This publication (the Guidebook) is provided for information purposes only. The views expressed are those of the authors. We do not accept any responsibility for any loss, damage or injury, or consequential loss/es arising from the information contained in the Guidebook. Any user of the Guidebook must satisfy themselves regarding the application to their purpose, of statutory requirements, building (or other) regulations, codes, insurance certification or other obligations or requirements which may arise from time to time. This guidance is given on an “as is” basis with experience and research carried out to identify relevant sources of information concluded in November 2024. To the full extent permitted by applicable law the authors disclaim all or any implied representations or warranties, including (but not limited to) implied warranties of fitness for purpose, accuracy or validity or completeness of information, merchantability, title, quality, and/or non-infringement.

The user of this guidebook assumes full responsibility for any loss resulting from use or inability to use the information, data, or advice presented in this Guidebook or the consequences thereof.

AUTHORS

ILLUSTRATIONS

Elliott Wood Waugh Thistleton Architects OFR Consultants Lignum Risk Partners

Waugh Thistleton Architects

CONTRIBUTORS

BM TRADA Built by Nature Stora Enso

SUPPORTERS

BEAM Built by Nature British Land Derwent London Hines Landsec Lendlease MUSE Related Argent Stanhope

FIRE PRINCIPLES CONSENSUS

OFR Consultants Design Fire Consultants Hoare Lea Semper Fire The University of Edinburgh University College London

EXECUTIVE SUMMARY

PURPOSE OF THE COMMERCIAL TIMBER GUIDEBOOK

The adoption of mass timber in large-scale commercial construction in the UK has been slower than anticipated, despite the increasing interest in mass timber as a sustainable building material.

This hesitancy stems from concerns about durability and moisture, fire safety, and as a result the challenge of obtaining sustainable property insurance. These issues are compounded by a lack of comprehensive guidance specific to mass timber construction. The Commercial Timber Guidebook has been developed to address these challenges. It provides good practice technical guidance to demonstrate how mass timber buildings are designed and constructed to be robust, addressing key concerns about fire safety and durability risks. The Guidebook sits alongside the Mass Timber Insurance Playbook (MTIP). While the MTIP provides a framework for understanding the risks perceived by insurers, the CTG offers the detailed technical information to mitigate these risks, with the aim of improving the insurability of mass timber buildings. This coordinated effort fosters better communication between developers and insurers, promoting a shared understanding of mass timber’s risks and benefits. An expert fire panel was also established to create consistent fire safety principles, which will further improve industry confidence in insuring mass timber structures.

GUIDANCE OVERVIEW

The main part of the Guidebook is divided into Mass Timber Background (chapters 2&3), Technical Guidance (chapters 4&5) and Illustrative Examples (chapter 6), offering context for mass timber and strategies for addressing key challenges. Each chapter starts with an executive summary to allow readers from different backgrounds to quickly understand the essential concepts. The mass timber background and technical guidance is summarised as follows: Insurance The Guidebook provides context of mass timber buildings in the UK from the perspective of the insurance sector. It summarises current available risk guidance documents, and how the construction and insurance sectors have learnt from experience when designing mass timber buildings. Timber Offices Mass timber buildings, especially office spaces, are gaining popularity due to environmental benefits like reduced carbon emissions and improved aesthetics. The Guidebook provides key design considerations for adaptable, sustainable offices using engineered timber, addressing challenges in architectural and structural design. Moisture and durability Timber’s durability depends on how well it is protected from moisture. The Guidebook identifies moisture as a key risk and provides strategies to mitigate this risk throughout the building’s lifecycle, ensuring long-term performance.

Moisture and Durability Principles - The Guidebook outlines principles relating to durability and moisture in mass timber buildings:

1. Specify appropriate materials 2. Risk identification 3. Consider repair and maintenance

6. Moisture management in assembly 7. Remove critical vulnerabilities 8. Durability risk analysis 9. Repair and remediation strategy

4. Keep timber dry 5. Early intervention

By following these principles, designers, contractors, and building operators can demonstrate to insurers that they have taken comprehensive measures to mitigate the risks associated with moisture.

Fire Mass timber introduces unique challenges to fire safety due to its combustibility. The Guidebook outlines fire behaviour in mass timber buildings and offers strategies to manage these risks, acheiving life safety compliance and contributing to property protection.

Fire Safety Principles - A set of industry consensus principles for fire safety in mass timber buildings:

1. Comprehensive fire strategy 2. Adequate risk to health and safety

3. Assumed redundancies for life safety design 4. Impact on escape from area of fire origin 5. Mitigating internal (vertical) and external fire spread 6. Mitigating internal (horizontal) fire spread 7. Expected performance of structure and compartmentation 8. Survival of burnout with encapsulated mass timber 9. Survival of burnout with exposed mass timber 10. Smouldering Combustion

By following these Fire Safety Principles, designers, contractors, and building operators can significantly reduce the fire risk in mass timber buildings, making them safer and more attractive to insurers.

Illustrative Examples The Guidebook provides four case studies of mass timber office buildings, illustrating how the durability and fire safety principles can be applied to different building heights and types:

1. Full timber offices below 18 meters, up to 4 storeys. 2. Full timber offices between 18 and 30 meters, up to 15 storeys. 3. Hybrid offices between 18 and 30 meters, up to 15 storeys . 4. Hybrid offices over 30 meters or over 15 storeys.

Each case study demonstrates how the durability and fire safety principles can be tailored to different types of buildings, providing real-world applications of the Guidebook’s recommendations.

FUNDERS’ LETTER OF SUPPORT

As a group of developers, we are pleased to lend our collective support to the Commercial Timber Guidebook (CTG), a resource designed to advance the adoption of mass timber in commercial construction. Our commitment to sustainable building practices has driven us to co-fund this guidebook, recognising its role in shaping the future of construction in the UK.

The need for the CTG arises from the growing imperative to reduce the carbon footprint of the built environment and to unlock commercially sustainable insurance cover for commercial mass timber buildings.

This group of developers has a collective potential multibillion development pipeline value, along with the appetite for the adoption of mass timber construction. This presents an opportunity to achieve significant reductions in embodied carbon, supporting the global push towards net-zero emissions. Although the environmental benefits of mass timber are well-recognised, its adoption in the UK has been limited by a lack of familiarity of the inherent risks and their mitigation. This unfamiliarity has created some challenges in obtaining the required coverage, which has slowed the broader use of mass timber on a wider scale. We support the development of technical reports, including this one, to increase the understanding and familiarity of the risks and benefits of timber construction. We look forward to engaging with the industry on how to put this guidance into practice.

Sincerely,

Built by Nature, BEAM, British Land, Derwent London, Hines, Landsec, Lendlease, MUSE, Related Argent, Stanhope

CONTENTS

INTRODUCTION

INSURANCE

TIMBER OFFICES Types of Mass Timber

24 26

3 . 1 3 . 2

Key General Design Aspects

DURABILITY

33 34 40 48 50 55 56 60 64

Moisture and Durability

4 . 1 4 . 2 4 . 3 4 . 4

Risk Mitigation

Water Damage Remediation Guidance

Principles for Durability

FIRE

Compliance, Stakeholder Goals and Fire Safety Objectives Alternative Fire Engineering Approaches and QDR Engineered Timber Thermal Response and fire Safety Implications Principles to Demonstrate Adequacy of Fire Safety Design

5 . 1 5 . 2 5 . 3 5 . 4

EXAMPLE BUILDINGS Example Building - Typology 1 Example Building - Typology 2 Example Building - Typology 3 Example Building - Typology 4

81 82 86 90 94 98

6 . 1 6 . 2 6 . 3 6 . 4 6 . 5

Comparative Examples

APPENDICES Types of Insurance

101

102 104 107 108

7 . 1 7 . 2 7 . 3 7 . 4

Glossary

Other Key Guidance Documents

Core Team

FULL BLEED IMAGE

1. INTRODUCTION

This chapter aims to establish the purpose of Commercial Timber Guidebook and set out how to use it.

Climate change is already affecting every inhabited region across the globe, with human influence contributing to many observed changes in weather and climate extremes. This climate change is caused by human activity, and with approximately 40% of global carbon emissions attributable to the built environment, we have the responsibility to act.

With the UK committed to bring all greenhouse gas emissions to net zero by 2050, mass timber construction will undoubtably play a major role in the reducing of carbon emissions.

Timber is a replenishable and sustainable material and if utilised correctly, mass timber construction offers several sustainable benefits compared with traditional building materials including, lower embodied carbon, carbon sequestration, circular economy benefits, and low waste to name a few. The using mass timber can reduce the embodied carbon emissions in a single building by 20%- 60%.

Despite the above, the increase in mass timber construction in the UK is relatively slow with designers and developers becoming increasingly frustrated with current obstacles to constructing mass timber buildings.

THE GUIDEBOOK KEY THEMES

Demonstrates consensus on fire safety and design principles A panel of fire and design experts collaborated to address perceived inconsistencies in fire safety approaches to reach a consensus on a set of fire safety and design principles for mass timber buildings. For the first time, the Guidebook outlines an agreed upon approach followed by competent fire engineers designing in mass timber. Establishes good practice principles for durability and fire The Guidebook consolidates comprehensive good practice technical guidance on the key risk areas of durability from moisture and of fire. Bridges the knowledge gap The Guidebook will play a critical role in reducing the knowledge gap between the construction industry and the insurance industry regarding the use of mass timber by addressing key concerns around durability and fire.

KEY DEFINITION

Mass timber is the name given to types of engineered timber that can be used as structural building materials. Mass timber buildings can be full timber (entire structural frame comprises timber elements) or hybrid (combination of a steel/concrete frame with mass timber floor slabs).

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WHAT IS THE COMMERCIAL TIMBER GUIDEBOOK?

The aims of the Guidebook are as follows:

– To provide a consensus approach to design and construction of mass timber that assures the insurance industry of intended good practice. – Inform the design, development, and construction community to advance the use of mass timber. – To produce a guide to good practice risk mitigation for anyone engaged in mass timber construction.

The Commercial Timber Guidebook’s (CTG) main purpose is to address the insurance sector’s concerns about the risks associated with adopting mass timber in construction. The CTG represents a consensus of current good practices in the design of mass timber buildings and provides technical guidance for risk mitigation, and remediation strategies in relation to durability due to moisture, and fire. The Guidebook is not, and does not purport to be, a standard for mass timber construction and it does not intend to guarantee, to the user of the guidebook or its associates, the availability of insurance. The authors, contributors and funders recognise the importance of available, affordable and long-term sustainable insurance to the successful development and continued innovation of sustainable construction. The Guidebook is intended as a the beginning of an ongoing and developing dialogue with the insurance industry, its specialist advisers, and consultants in risk.

CHALLENGES OF INSURING MASS TIMBER BUILDINGS

Currently it is proving challenging to obtain sustainable property insurance for mass timber commercial buildings. The insurance sector acknowledges the requirement for an increase in the adoption of mass timber to meet sustainability goals, however there are a number of concerns regarding the safety and resilience of mass timber buildings. They seek assurance that these concerns are fully considered, and design measures are put in place so that risks are satisfactorily mitigated.

REAL ESTATE SECTOR RISK MANAGEMENT GUIDANCE

BRIDGE DOCUMENTS HELP CLOSE THE GAP

INSURANCE SECTOR GUIDANCE TO COMMUNICATE INSURANCE SECTOR’S VIEW ON RISK

2017 - STA:16 Steps to fire safety 2019 - BMTrada: Learning resources, introduction to timber engineering design, moisture and humidity 2020 - STA: Structural timber buildings fire safety in use guidance Volume 6 2022 - STA: Moisture Management Strategy - RDH Building Science: Moisture risk management strategies for mass timber buildings, 2022, Version 2.1 - STA: Technical Note 24 - Moisture protection during construction - Swedish Wood: Moisture-proof CLT construction without a full temporary shelter 2023 - WTA: New Model Building (residential) 2024 - Arup: Fire Safe Design of Mass Timber Buildings - TDUK: Moisture Management During Construction

2023

2022 - RISCAuthority: Insurance Challenges of Massive Timber Construction - FPA: Joint Code of Practice: Fire Prevention on Construction Sites 2024 - Allianz: Emerging Risk Trend Talk 2 - Aviva: Mass Timber - Considerations for Planning & Design (RIBA Stages 0-4)

2024

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INTRODUCTION

HOW CAN THE GUIDEBOOK HELP INSURANCE?

enhances trust and confidence in the fire safety and asset protection of mass timber buildings.

The Guidebook helps to enhance insurer confidence and promotes long-term property safety and durability by demonstrating good practice guidance for mitigation of moisture and fire risks in mass timber buildings. This will be viewed positively by the insurance sector. The Guidebook aims to cover all the stages of a construction project from initial design through to post- occupancy, highlighting key risks and how these can be considered at the design stage, during construction, and during the use of mass timber buildings. The CTG sits alongside the Mass Timber Insurance Playbook (MTIP). The MTIP provides a framework for understanding the perceived risk challenges that hinder developers acquiring an insurance provision for mass timber buildings. The CTG provides the technical guidance to address and mitigate against these durability and fire risks.

A series of fire workshops conducted between August 2023 and May 2024 facilitated iterative discussions among the expert fire panel, leading to a consensus on the 10 fire principles outlined in Section 5.4. The significant coordination and effort invested in this process were aimed at advancing industry standards and building insurer confidence in timber construction. The expert fire consensus panel for the principles comprised of Design Fire Consultants, Hoare Lea, OFR Consultants, Semper, UCL (Jose Torero), and University of Edinburgh (Luke Bisby). Note that the ‘fire consensus’ relates to the 10 fire principles only.

WHO IS THE GUIDEBOOK FOR?

Insurers The CTG serves as an aid to ongoing dialogue with the insurance industry. It is intended to reassure insurers that good mass timber design has properly considered the perceived risks of insurers and includes appropriate mitigation measures. It also covers the remediations (repairs and reinstatements) that could be required in a building’s natural lifetime. The CTG responds to key design challenges and insurance considerations that are outlined in the MITP. Developers and asset owners The CTG can be used by developers and asset owners in discussions with insurers, as set out in the MTIP. There is guidance on in use and maintenance of mass timber buildings which serves to continually mitigate risks during the natural life of a building, together with remediation measures should they be required. Designers The CTG is written primarily for designers with the purpose of addressing specific concerns of the insurance industry in relation to mass timber constructions. The Guidebook provides technical guidance, a framework of design considerations and principles for different building typologies that can be applied by competent consultants to exercise good practice in design and risk mitigation.

WHO PRODUCED THE GUIDEBOOK?

Core team The Guidebook has been produced by a collaboration of leading industry experts from Waugh Thistleton Architects, Elliott Wood, OFR Consultants, and Lignum Risk Partners, combining their collective expertise in engineering, architecture, fire safety, and risk management to provide comprehensive guidance on mass timber construction. The core team has engaged with the insurance markets and the design and structural engineering experts and incorporated their feedback into the Guidebook. Refer to the appendix for more information on the core team. Fire consensus To develop this Guidebook, an expert fire panel consisting of leading academics and consultants was established. These experts, experienced in the development and review of fire safety designs, particularly in commercial mass timber buildings, worked together to reach a consensus on fundamental fire design principles. The aim of this panel was to address the perceived inconsistency in fire design across the industry, and to provide a unified approach that

INTRODUCTION

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MASS TIMBER BACKGROUND

THE GUIDEBOOK STRUCTURE

Insurance This chapter provides context for mass timber buildings from the perspective of the insurance sector along with a summary of current risk guidance documents and where both the construction and insurance sectors have been able to learn from experience when designing mass timber buildings. Timber Offices This chapter provides a general overview of timber office buildings. The chapter discusses the types of engineered timber that are commonly used in mass timber buildings and outlines architectural and structural considerations that influence the design of large office buildings.

The Guidebook is aimed at and will solely focus on commercial (office) buildings where mass timber construction is used, to demonstrate to insurers good practice design and risk mitigation guidance. It is not expected that the whole of the Guidebook will be relevant to all parties due to differences in expertise. The Guidebook is separated into three parts, Mass Timber Background, Technical Guidance and Example Building Typologies.

TECHNICAL GUIDANCE

T IMBER OF F I CES

I NSURANCE

The technical design guidance forms the majority of the guidebook and is written primarily for designers. The scope of the CTG has aimed to cover relevant information for mass timber buildings to address the significant risk areas for mass timber buildings. Durability This chapter focuses on how mass timber’s durability is affected by moisture, outlining the risks associated with water damage and providing strategies for mitigating these risks to create robust, durable buildings.

F I RE

DURABILITY

This includes technical guidance on:

– The properties of timber relating to moisture and durability and specifying appropriate materials. – Highlighting key risk areas and how these can be mitigated in the design phase, construction phase, and in use phase. – Guidance on the remediation process if mass timber is damaged by water. – A set of durability principles that can be adopted by designers to demonstrate good practice relating to identification of risks, risk mitigation measures and remediation strategy.

EXAMP L E BU I LD I NGS

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INTRODUCTION

Fire This chapter focuses on how the use of mass timber impacts fire safety requirements, the additional hazards that a combustible frame introduces and how these can be addressed.

These examples serve as practical models to guide designers, developers, and insurers in applying good practice design and risk mitigation strategies in terms of fire safety and durability. The use of the guidelines provided do not replace the need for design and calculations by responsible and competent designers.

This includes technical guidance on:

– The goals and objectives that may form part of a fire safety design, including the minimum statutory goal of life safety and how fulfilling this goal may contribute to property protection and business continuity goals. – An overview of the burning of wood and its implications on the fire hazards within a building. – A set of proposed principles that can be adopted by designers to demonstrate the adequacy of the fire safety design in a commercial timber building. – Discussion on the anticipated implications of these principles in the fire safety design of a commercial building, and on the potential benefits for property protection and business continuity by addressing the proposed fire principles.

EXAMPLE BUILDINGS

The technical guidance in the Guidebook will allow design teams to establish good practice and appropriate durability and fire safety provisions for mass timber buildings they are designing. This chapter presents an appraisal of four mass timber commercial building examples, highlighting fire safety and durability considerations for each, based on the principles established in the technical guidance in the Guidebook.

The following building typologies are considered:

INTRODUCTION

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FULL BLEED IMAGE

2. INSURANCE

Following the Guidebook will not guarantee the availability of any insurance or guarantee the treatment of any particular insurance claim. Rather, following the Guidebook’s principles and recommendations is intended to improve the project’s risk profile in the eyes of an insurance underwriter, and can help to reassure them that it is a more insurable risk. In turn, this is intended to lead to more sustainable terms and premium rates. Whilst the Guidebook is not intended to provide insurers, their loss adjusters and legal advisers with a benchmark against which any claims can be measured, it is possible that it may be consulted for its consensus of contemporaneous good practice. Mass timber is a relatively new construction technology in a complex, regulation-driven industry. The primary risks associated with it are fire and water damage, which can impact safety and cause costly damage. While the insurance industry is still developing experience and data around these risks, efforts by the construction industry to reduce time and cost in mitigating fire and water damage will aid long-term sustainability. This guidance aims to inform ordinary good practice according to the consensual opinions of the authors of the Guidebook. It promotes risk mitigation strategies for future mass timber construction, reinforcing the essential role of insurance in supporting innovation and sustainability.

FOCUS

This chapter discusses the relatively recent adoption of mass timber in UK construction and the associated risks, particularly fire and water. It also summarises the insurance industry’s current position on the use of mass timber in the UK.

KEY THEMES

The insurance industry currently has insufficient access to loss data and technical development dialogues. This restricts the availability of and impacts the cost of insurance for mass timber buildings in the UK.

Following the Guidebook’s principles and recommendations is intended to improve the project’s risk profile in the eyes of an insurance underwriter and help to assure them that it is a more insurable risk.

The insurance industry wishes to support the UK construction sector in the use of mass timber and encourage continuing dialogue to mutual benefit.

CONCLUSIONS

The insurance industry encourages more collaboration between contractors, designers and insurers to share knowledge, data and experience.

The insurance industry welcomes all initiatives that help to increase adoption of ordinary good practice in the use of mass timber.

INSURANCE

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CHALLENGES OF INSURING MASS TIMBER BUILDINGS

The UK insurance sector’s brokers and insurers are generally supportive of the use of mass timber in the UK. They recognise that increased use of mass timber across the world is an imperative and an enabler to support the construction industry to meet – and to continue to meet - its required net zero commitments. They also recognise the need to meet their own net zero commitments and agree that deploying underwriting capital into mass timber supports their and the construction sectors’ needs. With that comes an appreciation that the use of mass timber will increase. The biggest challenge the insurance sector faces is their own nascent knowledge and the small amount of data available to them compared with more well-known building materials. The comparative lack of data makes it harder for insurers to assess, quantify and underwrite the risk, hence the higher premium rates and less commercially sustainable terms that they currently require. They therefore would welcome more collaboration between contractors, developers, insurers, brokers and risk advisers to share new learning possibilities, acquired knowledge and even to create an industry wide knowledge database for the benefit of all those involved in the use of mass timber. The insurance industry’s view of the use of mass timber in commercial buildings has evolved over the last two years from slightly anti/neutral to neutral/ positive. There is more work to be done by all parties but it would appear that there is now a desire to support greater adoption of the use of mass timber in the UK from the insurance sector. Everything that the construction and property investment community can do to contribute to the reduction in time and cost of remediating reinstating and mitigating the losses, deriving mainly from fire and water perils, will contribute to the long-term sustainability of insurance products and services to support the continued innovation in the mass timber sector. Insurance is essential in the continued innovation and development of the future built environment.

Insurance is an economic necessity, without which innovation, development and progress in everything that supports society would be hampered by the inability of business or government to take the risk of doing anything new. Investors and developers wanting to promote and incorporate mass timber construction often find that the insurance sector is unable to offer insurance that meets their commercial requirements, partly due to insufficient statistical data and partly due to the use of timber being a comparatively new risk in the UK insurance market. The insurance industry does acknowledge the requirement for an increase in the adoption of mass timber if environmental targets and net zero ambitions are to be met, however there are a number of concerns regarding the safety and resilience of mass timber buildings. They seek assurance that these concerns are fully considered, and design measures are put in place so that risks are satisfactorily mitigated. Within the long history of construction, mass timber is a relatively young technology. In the UK, the 21st century building industry and insurance sector remains generally unfamiliar with the dynamics and properties of modern mass timber as an increasingly dominating feature of the modern built environment, which is a much more complex and regulation-driven environment than when timber was last the dominant building resource. There are two primary risks deriving from mass timber construction; fire risk and water damage risk. There are two principal hazards deriving from fire and water risks: safety to human life and the costs resulting from the loss of amenity of a property damaged by fire or water and the reinstatement or remedial cost of the property itself. The insurance industry is required to bear the majority of the burden of these risks and in the early 21st century the insurers do not yet have the reliable long term statistical evidence of the ultimate cost of loss and damage from this source, nor do they have an experienced resource for the remedial works necessary to put the property back into its pre loss condition.

RISK AND RISK GUIDANCE DOCUMENTS

Driven by the necessity to rapidly decarbonise the built environment, over the past few years a sector of the

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I NSURANCE

industry has prioritised learning and swiftly enhanced its knowledge and expertise in mass timber, and also its understanding about the potential design, construction and use pitfalls. This growing expertise in the construction industry is evidenced by collectively produced guidance: documents such as 16 Steps To Fire Safety (2017) 1 , Structural timber buildings fire safety in use guidance, Volume 6 (2020) 2 , Moisture management strategy (2022) 3 , all by the STA, and the Mass Timber Insurance Playbook (2023) 4 . Signposts to key documents are given in Section 7.3. This Guidebook is the latest collective industry guidance document representing the consensus of the mass timber buildings design experts. Some companies have also published individual company guidance documents such as Arup’s Fire Safe Design of Mass Timber Buildings (2024) 5 , which is a useful document sitting along this Guidebook and allows designers to make more informed decisions based on risk and a useful summary of relevant research and the ‘New Model Building’ (2023) 6 design guide for residential buildings by WTA. The insurance industry itself has also recently produced documents such as The Insurance Challenges of Massive Timber Construction (2022) 7 and Joint Code of Practice: Fire Prevention on Construction Sites (2022) 8 , both authored by the RISCAuthority / FPA, as well as guidance documents drafted by individual companies such as Allianz’s Emerging Risk Trend Talk 2 (2024) 9 . Signposts to key documents are given in Section 7.3. This Guidebook is the latest collective industry guidance document representing the consensus of the mass timber buildings design experts in what constitutes ordinary good practice.

Of the comparatively few loss events specifically associated with mass timber construction, even fewer have been publicised, with only one notable loss involving an impact to the mass timber structure. In 2014 the University of Nottingham’s GSK Laboratory was destroyed during construction from a fire caused by an electrical fault. Guidance documents that cover the risks exposed by this event include the STA’s 16 Steps to fire safety Promoting good practice on construction sites (2017) and the Fire Protection Association’s Fire Prevention on Construction Sites (2023). Relevant risk-mitigation measures, during construction, in this case include prioritising the completion of fire compartments including fire doors and fire stopping and installing sprinklers if and where required. Although moisture-related losses are more frequent and make up the bulk of insurance payouts, these tend not to be publicised, and we are unable to list these here. In two recent high-profile loss events involving moisture and fire respectively, the mass timber structure played neither a role in, nor was it impacted by, the loss. The first example is at Sky Central in London, from 2014 onwards. The building suffered moisture ingress during construction which resulted in damage to the timber cassettes set in the glulam roof (not the glulam itself). The STA’s Moisture management strategy, Process guidance for structural timber buildings (2022) covers this risk. The second example is the Wood Innovation Design Centre in Prince George, Canada, which suffered a loss in 2024. The building’s timber cladding was damaged by a fire caused by a blast following a ruptured gas line in an adjacent building. The extent of the subsequent fire was limited by the building’s non-flammable insulation-filled external wall void and its sprinkler system. In fact, the damage was mitigated by the flexible glulam structure which allowed the building to bend and absorb the shock of the blast, preventing potentially even greater damage.

LEARNING FROM EXPERIENCE

The sector has learnt from the relatively small number of loss events, many of which the Guidebook’s authors have been able to closely study. The Guidebook’s content reflects this built-up knowledge.

INSURANCE

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REFERENCES

1 https://www.structuraltimber.co.uk/wp-content/uploads/2024/01/16stepstofiresafety-3.pdf 2 https://www.structuraltimber.co.uk/wp- content/uploads/2023/12/structuraltimberbuildingsfiresafetyinuseguidancevolu me6-masstimberstructures.pdf (Accessed September 2024) 3 https://www.structuraltimber.co.uk/wp-content/uploads/2022/09/STA-Moisture-Management-Strategy-v1-July-2022.pdf (Accessed September 2024) 4 https://asbp.org.uk/project/mass-timber-insurance-playbook (Accessed September 2024) 5 https://www.arup.com/insights/fire-safe-design-of-mass-timber-buildings/ (Accessed September 2024) 6 https://issuu.com/waughthistleton/docs/nmb_guide_book-1.1 (Accessed September 2024) 7 https://www.thefpa.co.uk/news/insurance-challenges-of-mass-timber-construction (Accessed September 2024) 8 https://www.thefpa.co.uk/resource-download/785 (Accessed September 2024) 9 https://commercial.allianz.com/news-and-insights/reports/emerging-risk-mass-timber.html (Accessed September 2024)

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INSURANCE

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3. TIMBER OFFICES

Despite the difficulties outlined in the introductory chapter, during the past two decades, the timber construction sector has seen a steady increase in the number of commercial buildings being built around the world.

Timber offices have been proven to generate significant environmental and financial benefits throughout their life cycle and have become symbols of environmental awareness in architecture.

Beyond the environmental benefits of timber, mass timber buildings are valued for their attractive aesthetics and the biophilic benefits they provide. These have been shown to enhance creativity, productivity, and employee satisfaction— particularly important in the post-pandemic workplace. While timber buildings are perceived as more expensive than traditional concrete or steel structures, a recent study found the cost difference can be minimal. When factors like faster construction, lighter materials, and high prefabrication are considered, timber could become the more cost-effective choice.

FOCUS

This chapter outlines key architectural and structural factors that influence the design of mass timber office buildings, such as the choice of structural system, materials, and the organisation of core and service areas. Careful consideration of these factors can influence the resilience and robustness of the completed building.

KEY THEMES

– Types of engineered timber – Key general design aspects

CONCLUSIONS

This chapter offers guidance on mass timber office buildings and the general design considerations that need to be considered. A building that is designed with the aim of maximising adaptability and space efficiency should consider the following design aspects.

– Building form – Structural system – Materials

– Structural grid – Service distribution – Core design

TIMBER OFFICES

3 . 1

TYPES OF MASS TIMBER

CLT

Mass timber is the name given to types of engineered timber that can be used as structural building materials. Mass timber components are made from small planks or layers of timber bonded together to increase their strength and form load-bearing components such as panels, beams and columns. This Guidebook focuses on three most commonly used types of mass timber. Cross laminated timber Cross laminated timber (CLT) is manufactured by combining multiple layers, or lamella, of timber with abutting layers stacked such that the grain orientation alternates 90°. CLT is commonly used for floor structures and walls. Glulam Glulam, also referred to as GLT, is similar to CLT in that layers of timber are combined; however glulam differs from CLT in that all layers have the same grain orientation. Glulam is commonly used in applications where a large span is required, such as roof joists, beams, and columns. Laminated veneer lumber Laminated veneer lumber (LVL) is again constructed similarly to the aforementioned types of engineered timber, being made up of multiple layers. However, LVL is manufactured from considerably thinner layers frequently under 3mm in thickness. LVL is commonly used for similar applications as Glulam. Types of adhesive Typically melamine or polyurethane based adhesives are used in the manufacture of engineered timber products, which may lose adhesion at different temperatures. Non heat-resistant adhesives are common for low risk buildings where fire will not affect the bond line. Heat-resistant adhesives maintain bond integrity during fire and are best suited for higher-risk buildings where auto-extinction is necessary.

Glulam

LVL

Figure: The three main types of mass timber

TIMBER OFFICES

3 . 2

KEY GENERAL DESIGN ASPECTS

This section outlines architectural and structural considerations that influence the design of large office buildings (eg choice of structural system, materials, core and service distribution design), with the aim of maximising: Adaptability: the ability to change functional and spatial arrangements to suit changing circumstances, thereby increasing the financial resilience of an asset. Space efficiency: maximising usable space, a parameter that is particularly important in buildings seeking to increase their financial value through the quantity and quality of space they are able to offer.

– The opportunity for repetition and modularity: design approaches that both optimise the use of building components.

STRUCTURAL SYSTEM

Mass timber buildings can be formed by one-dimensional (post and beam or post and slab type), two-dimensional (platform type) structural elements, or by three-dimensional components (volumetric modular) which are composed of off-site, pre-fabricated walls and floors, plus a series of combinations of these three basic configurations. Although contemporary multi-storey timber construction presents a variety of forms and spatial configurations to meet different contextual, regulatory and design requirements, a common thread seems to be the choice of structural system which maximises the adaptability of internal spaces to meet tenants needs. Like steel and concrete, large timber office buildings are typically frame structures which are defined as post and beam structural systems. These one-dimensional, column- based structures allow for adaptable, open-plan layouts that are easy to organise and reconfigure, with the only permanent obstacles being the load-bearing structure, cores (with their vertical and horizontal circulation and service distribution routes) and possibly some functional areas (wet and high load areas). Furthermore, in post and beam structures, the building envelope has no structural function and can therefore be dismantled and replaced as required.

These key design aspects are then used to inform the examples provided within Section 6 example buildings.

BUILDING FORM

With regards to massing, prismatic, regular building shapes are widely used because they are both simple and suited to maximise space efficiency, structural stability and adaptability.

Prismatically shaped office buildings offer:

– Uniformity and repetition of internal spaces (structural bays) simplifies the organisation of open plan work- spaces. – Maximised natural light and ventilation for work-areas. – Uniform escape distances to lifts/cores, especially if these are positioned centrally. – Simple form that mitigate complexities in a building’s design and assembly, lowering the potential for errors and delays and all0wing for compatibility with existing off-the shelf products and construction techniques (eg non-structural prefabricated, façade materials or internal separation walls etc).

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POST & BEAM With prefab facades POST AND BEAM with Prefabricated Façades With prefab facades POST & BEAM

POST & SLAB With prefab facades POST AND SLAB with Prefabricated Façades With prefab facades POST & SLAB POST & SLAB With prefab facades

POST & BEAM With prefab facades POST & BEAM With prefab facades

POST & SLAB With prefab facades

HYBR I D with Steel Primary Structure

HYBRID HYBR I D with RC Primary Structure

HYBRID

Steel primary structure HYBRID Steel primary structure HYBRID

RC primary structure

Figure: Mass timber building systems

HYBRID

35 PITFIELD STRE

TIMBER OFFICES Steel primary structure HYBRID

35 PITFIELD STREET

Steel primary structure HYBRID

2 7

RC primary structure

MATER I AL S

This Guidebook assumes a 9x6 metre grid for the office buildings in the Illustrative Examples in Section 6. This grid has been chosen for material optimisation and embodied carbon reduction, making it an efficient and sustainable choice. It is worth noting in this context that the 2023 update of the British Council for Offices (BCO) Key Design Criteria, has acknowledged the benefits of using 6-7m structural grids, balancing carbon reduction with the desire for efficient, column-free spaces. This approach prioritises sustainability alongside spatial efficiency. Structural grid sizes and layouts must be planned alongside services distribution from the early stages of the project. This coordination is vital for achieving a holistic design approach in mass timber construction.

The structural design commonly used in mass timber office buildings fall into two categories: full timber and hybrid (concrete/timber or steel/timber). In full timber structures the entire superstructure including the building’s core/s are exclusively made of engineered timber; materials other than timber are used only in the substructure and in the ground floor slab. In hybrid structures large part of the superstructure is made of materials other than timber, such as steel, reinforced concrete, or a mix of both. This type of structure generally makes use of concrete or steel both for columns, beams and core/s, with the use of timber confined to floor plates (made of CLT, composite RC/CLT or RC/GLT) and separating walls. The hybridisation of timber with other materials can enhance its structural, acoustic and vibrational performance, particularly in larger structures with longer spans.

CORE DESIGN

A core is a vertical distribution space for the circulation of building users and services, containing stairs, lifts, services risers, bathrooms or WC’s. In typical post and beam type mass timber structures, the core often also fulfils a structural function, providing lateral stability to the entire structure. The location, number, size and material (CLT or LVL, concrete or steel framed) of these cores is primarily determined by a building’s use, height, occupancy and the consequent fire escape distances. Cores can be classified into two main categories: in- grid and site-adaptive. In-grid cores are aligned with the building grid and occupy one or more structural bays (central, atrium and peripheral cores). In contrast, side-adaptive cores function outside of the structural grid, enabling a regularly shaped building to adapt to the irregularities of a real-world site. This is achieved by connecting or infilling the irregular shape between regular forms. Recent research has highlighted the prevalence of central core configurations in timber office buildings: a strategy that maximises both structural robustness and space efficiency. This allows a large number of users to be accommodated along a building’s perimeter, increasing

STRUCTURAL GRIDS

Property agents, building owners and investors expect office spaces to have a good spatial efficiency and large, adaptable column free spaces that can easily be reconfigured. These expectations emphasise the importance of selecting an optimal structural grid. For mass timber buildings, it is crucial to design the most efficient span for the structural timber frame from the beginning of the project to meet these requirements effectively. Trying to force timber into structural grids originally conceived for traditional building materials such as steel and concrete can lead to material inefficiencies. The structural grid dimensions (together with building height and structural material choice) influence the number and size of every system component. Considerations such as standard product sizes, transport, site logistics, construction speed, and material efficiency should be integrated with the overall spacial design.

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access to natural light and views. In addition, central cores facilitate fire evacuation procedures and logistics.

early appointment of a services engineer is essential. It is beneficial if they have a clear understanding of ‘plug and play’ systems and can provide detailed construction information rather than performance specifications. Most mass timber office buildings use cores for vertical distribution while the soffit of service and distribution corridors surrounding the core accommodate their horizontal distribution without compromising the clear heights of workspaces. These distribution corridors present a relatively short span (approximately. 3 to 4.5 m grid depth), and consequently a shallow structural beam depth (potentially even a localised post and slab arrangement) that creates a deeper service zone containing primary service distributions such as ventilation ductwork and electrical cabling. Experience shows that this ceiling zone requires a free depth of approximately. 500 mm and that horizontal service runs each serve a zone of a maximum of six bays each without relying on additional vertical risers. This services distribution strategy facilitates future adaptation as office-bays can easily be upgraded or downgraded should heating or ventilation requirements change, furthermore, it is compatible with all office typologies. Service Bay Service Bay T y p ical Bay

By centralising the core, designers consolidate load- bearing elements and streamline vertical circulation; this approach also ensures optimum use of materials and overall structural stability and safety. This optimisation is not just about structural and spatial efficiency, but also about a more adaptable environment capable of accommodating a wide/changing range of functions.

SERVICES DISTRIBUTION

Proper coordination of mechanical, electrical and plumbing (MEP) services with the building structure is essential, particularly in mass timber office buildings. Avoiding MEP routing holes improves the structural performance of building components and ensures flexibility for future service modifications. Service design is influenced by building occupancy, layout and bay depth and height targets. For this reason, many early design decisions deeply influence the MEP strategy. For example, the choice between natural and mechanical ventilation has a significant impact on massing height, core layout roof and façade design. This is why the Service Bay T y p ical Bay

T y p ical Bay

Service Bay

T y p ical Bay Typical Bay

T y p ical Bay

Core with service risers Core with service risers

Ser

Service channel Service channel

Dow

Service Bay Service Bay

Service distribution Service distribution

Core with service risers

Downstand beam Downstand beam

Service channel

Service Bay Service Bay

T y p ical Bay Typical Bay

Core with service risers

Service distribution

Figure: Services distribution in different core arrangements

Service channel

Downstand beam

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