Explore the fire engineering challenges and solutions for tall mass timber buildings, including CLT charring behaviour, encapsulation strategies, and regulatory pathways.. The Rise of Mass Timber Construction Mass timber construction, particularly using Cross Laminated Timber (CLT), is experiencing rapid growth driven by sustainability goals, carbon reduction targets, and architectural innovation. Buildings up to 18 storeys have been constructed in timber internationally, and the UK is seeing increasing numbers of CLT projects. Fire Safety Concerns Despite timber's predictable structural behaviour in fire (charring at known rates), several concerns exist for tall timber buildings: Combustible Structure Unlike steel and concrete, the structure itself contributes fuel to a fire Extended fire duration possible if timber becomes involved Questions about self extinction after contents fire burnout Regulatory barriers — combustible materials ban above 11m in England CLT Fire Behaviour Charring CLT chars at approximately 0.65 mm/min under standard fire exposure. The char layer protects underlying timber, similar to how it functions in heavy timber construction. However, CLT's layered construction introduces unique behaviour: Delamination When the adhesive between CLT layers fails (typically above 300°C), charred lamellae can fall off, exposing fresh timber to fire. This can: Increase the heat release rate Prevent self extinction Extend the fire duration beyond contents fire burnout Polyurethane (PUR) adhesives generally show better fire performance than melamine based adhesives in resisting delamination. Encapsulation Strategy The most common fire safety strategy for tall CLT buildings is encapsulation — covering all timber surfaces with non combustible protection: Full Encapsulation All CLT surfaces covered with fire rated plasterboard Typically 2 layers of 15mm Type F plasterboard (60 minute protection) Prevents timber contribution to the fire Building effectively behaves like a non combustible structure Partial Encapsulation Some timber surfaces left exposed for aesthetic purposes Requires detailed fire engineering analysis Exposed timber quantity limited to control additional fire load Not currently accepted for buildings above 11m in England Regulatory Pathway in England The Combustible Materials Ban Regulation 7(2) of the Building Regulations 2010 (as amended) bans combustible materials in external walls and specified attachments of buildings over 11m containing residential accommodation. This effectively prevents exposed timber facades but does not apply to internal structure. CLT with Full Encapsulation Fully encapsulated CLT structures can comply with ADB because: The structural timber is protected from fire involvement The building performs comparably to non combustible construction Fire resistance can be demonstrated through testing and calculation Performance Based Fire Engineering BS 7974 provides the framework for demonstrating that CLT buildings meet functional requirements through engineering analysis rather than prescriptive compliance. Compartment Fire Testing Recent research has advanced understanding of CLT fire behaviour: Natural fire tests — monitoring CLT performance in compartment fires beyond standard exposure Self extinction studies — determining conditions for timber to stop burning after fire decay Delamination research — adhesive performance and char fall off behaviour Tall building evacuation — extended evacuation times for timber buildings Design Recommendations Based on current evidence and best practice: 1. Full encapsulation for all CLT buildings above 11m 2. PUR adhesives for CLT where fire performance is critical 3. Sprinkler protection throughout (required by ADB above 11m) 4. Enhanced compartmentation details at CLT panel junctions 5. Service penetration management — maintaining encapsulation integrity 6. Construction phase fire safety — CLT is most vulnerable during construction before encapsulation Our Mass Timber Fire Engineering Magnus Opifex SEVEN LTD has provided fire engineering services for multiple mass timber projects across the UK. We combine deep understanding of timber fire behaviour with practical construction experience to deliver safe, buildable, and economically viable fire safety solutions for CLT and glulam structures. Magnus Opifex SEVEN LTD — UK's Leading Fire Safety & Fire Engineering Consultancy 🌐 magnus opifex.co.uk 📞 +44 7486 691724 ✉️ office@magnus opifex.co.uk Founders: Nicoleta Vasile, Baroness of Brattleby — CEO, Lawyer and Barrister, Legal & Administrative Director Alina — Technical Director & Expert Fire Engineer (BEng) Head Office: Ealing Cross, 85 Uxbridge Road, London W5 5BW Magnus Opifex SEVEN LTD delivers engineering led fire engineering, fire risk assessments, CFD modelling, and building safety consultancy across the United Kingdom and internationally. With over 20 years of combined experience and a UK portfolio spanning healthcare, resid