Mass timber construction is the sustainability revolution architecture has been waiting for. But can timber buildings really be safe from fire? The science says yes — if you get the engineering right.. The Mass Timber Revolution Mass timber — primarily Cross Laminated Timber (CLT), Glued Laminated Timber (Glulam), and Laminated Veneer Lumber (LVL) — is transforming construction. Its benefits are compelling: Carbon sequestration — timber stores carbon rather than emitting it during manufacture Speed of construction — prefabricated panels installed 50% faster than concrete frames Weight reduction — mass timber is 1/5 the weight of concrete, reducing foundation costs Aesthetic appeal — exposed timber interiors command premium values Sustainability credentials — aligns with net zero carbon commitments But What About Fire? The instinctive reaction to tall timber buildings is concern about fire. This reaction is understandable but not supported by fire science. Mass timber behaves very differently from light frame timber construction. "Mass timber doesn't burn like a matchstick. It chars at a predictable rate, forming a protective layer that insulates the unburned timber beneath. A CLT panel in fire behaves more like a log in a fireplace than a twig." — Professor of Timber Engineering Fire Behaviour of Mass Timber Charring Rate Mass timber chars at a predictable rate (approximately 0.65mm per minute for CLT). This means: A 120mm CLT panel can maintain structural integrity for over 90 minutes in fire The char layer acts as insulation, protecting the unburned timber core Structural capacity can be calculated by subtracting the char depth from the member size Self Extinguishment Research has demonstrated that mass timber can self extinguish after the fire load has been consumed: In compartments with limited fuel load, mass timber fires can reach a peak and then decay The char layer inhibits continued combustion once external fuel is consumed This behaviour is dependent on timber thickness, room geometry, and ventilation Delamination Risk The key concern with CLT in fire is delamination: Adhesive between CLT layers may fail in heat, causing charred layers to fall off This exposes fresh timber to fire, re accelerating the fire Polyurethane adhesives are more susceptible than melamine or resorcinol adhesives ADB 2025 requires CLT adhesives to be tested to demonstrate delamination resistance UK Regulatory Position Approved Document B — 2025 The current ADB position on mass timber: Under 11m : Mass timber construction permitted with standard fire safety measures 11 18m : Mass timber permitted but with additional fire safety requirements (sprinklers mandatory) Over 18m : Mass timber permitted only with full fire engineering justification (BS 7974) Exposed timber : Limited to buildings under 18m; over 18m requires encapsulation The Encapsulation Debate Encapsulation (covering timber with plasterboard) eliminates the fire risk but also eliminates the aesthetic benefit: Full encapsulation : 2 layers of fire rated plasterboard (Type F, 15mm), provides 60 minutes protection Partial encapsulation : Ceilings encapsulated, walls exposed (compromise approach) No encapsulation : Exposed timber with fire engineering justification (currently limited to <18m) Fire Engineering for Mass Timber Design Fire Scenarios Fire engineering for mass timber must consider: Contribution of timber to fuel load — the structure itself is a fuel source Char depth calculations — structural capacity reduction over fire duration Delamination scenario — behaviour if adhesive fails and layers separate Self extinguishment — can the fire decay without external intervention? Post fire structural integrity — residual capacity after charring Sprinkler Integration Sprinklers are critical for mass timber buildings over 11m: Control fire growth before significant charring occurs Limit total heat exposure to structural timber Provide redundancy against compartmentation failure Enable exposed timber by controlling fire size Compartmentation Strategy Mass timber buildings require careful compartmentation design: Floor to floor compartmentation using CLT floors with protective ceiling layers Wall compartmentation using CLT or plasterboard lined CLT Service penetration fire stopping tested specifically for CLT substrates Junction details (wall to floor, wall to wall) tested for fire resistance International Precedents Mass timber construction is well established internationally: Mjøstårnet, Norway — 85.4m, 18 storeys, world's tallest timber building HoHo, Vienna — 84m, 24 storeys, mixed timber concrete hybrid Brock Commons, Vancouver — 53m, 18 storeys, student accommodation 25 King, Brisbane — 52m, 10 storeys, commercial office All achieved fire safety approval through performance based fire engineering. Magnus Opifex Mass Timber Fire Services Fire strategy for mass timber buildings — BS 7974 performance based design CFD modelling — timber contribution fire scenarios St