Cross-laminated timber towers are rising across Europe. With the UK government cautiously opening the door to tall timber, fire engineers must answer the hardest question: is it safe?. The Timber Revolution Cross laminated timber (CLT) and other mass timber products are transforming construction. With embodied carbon fractions of concrete and steel, mass timber is the darling of sustainable construction. Buildings like Mjøstårnet in Norway (85.4m) and Sara Kulturhus in Sweden (75m) demonstrate that tall timber is structurally viable. But in the UK, the shadow of Grenfell looms large over any discussion of combustible construction materials. The ban on combustible materials above 18m (Regulation 7) currently excludes mass timber from the external wall — but the structure itself remains a contentious topic. How Mass Timber Behaves in Fire Mass timber's fire behaviour is fundamentally different from light timber framing: Charring rate: CLT chars at approximately 0.65mm per minute The char layer acts as insulation, protecting the unburned timber beneath A 200mm CLT panel can provide 90+ minutes of fire resistance through charring alone Delamination risk: When adhesive layers fail, charred laminations fall away, exposing fresh timber This 'delamination' can accelerate the burn rate significantly Adhesive performance at high temperature is the critical variable Contributed fire load: Unlike concrete or steel, the structure itself is combustible In an unprotected CLT room, the structure can contribute to the fire's intensity This affects firefighting operations and structural behaviour Self extinguishment: In well designed compartments, mass timber fires can self extinguish once contents are consumed But this depends on the ratio of exposed timber surface area to ventilation The UK Regulatory Position The UK takes a cautious approach: Regulation 7 bans combustible materials (including timber) in external walls above 18m Approved Document B does not specifically address mass timber structures BS EN 1995 (Eurocode 5) provides structural fire design methods for timber The BSR has no formal position on mass timber high rise but will scrutinise Gateway 2 applications closely In practice, mass timber buildings above 18m in England require: 1. Full encapsulation of all timber surfaces in non combustible board (typically 2 layers of plasterboard) 2. Fire engineering analysis demonstrating structural adequacy 3. Specific consideration of construction phase fire risk (before encapsulation) 4. Enhanced detection and suppression provision Design Strategies for Safe Tall Timber 1. Full encapsulation : All CLT surfaces protected by non combustible board — no exposed timber in fire critical areas 2. Sprinkler protection : Automatic suppression in every compartment to control fires before structural involvement 3. Connection design : Timber to timber connections must be protected from fire exposure 4. Construction phase protection : Fire rated temporary encapsulation during construction, when the timber is most vulnerable 5. Adhesive specification : Use only adhesives tested to maintain bond integrity at elevated temperatures 6. Fire testing : Full scale fire tests of proposed assemblies, not just component tests 7. Monitoring : Consider embedded sensors to detect internal timber temperatures The Future Mass timber will be part of the UK's construction future — the sustainability imperative demands it. But fire safety cannot be compromised for carbon savings. The fire engineering community must develop robust, evidence based guidance that enables safe tall timber construction. Magnus Opifex provides fire engineering consultancy for mass timber projects. Contact us for a pre application review.