Mass timber is transforming UK construction, but fire safety remains contentious. This technical guide examines CLT and glulam fire behaviour, engineering approaches, and the evolving regulatory landscape.. The Mass Timber Revolution Cross Laminated Timber (CLT) and glued laminated timber (glulam) are increasingly used for multi storey buildings in the UK. Buildings of 8 10 storeys in mass timber are now being constructed, with proposals for even taller structures. Why Mass Timber? Significant carbon sequestration (embodied carbon benefits) Rapid construction (prefabricated elements) Reduced weight compared to concrete/steel (foundation savings) Architectural appeal and biophilic design benefits Government support for timber in construction decarbonisation Fire Behaviour of Mass Timber Char Rate Analysis Mass timber burns predictably: Char rate : Approximately 0.65mm/min for CLT (per Eurocode 5) Char layer : Insulates the remaining cross section Residual section : Retains full structural capacity below char layer Zero strength layer : Additional 7mm beyond char to account for heated but uncharred timber Design Methods 1. Reduced cross section method (Eurocode 5): Calculate residual section after charring 2. Reduced properties method : Apply strength reduction factors to full section 3. Advanced calculation : FE analysis with temperature dependent material properties Auto Extinction Debate Research shows CLT panels can self extinguish after burnout of moveable fire load: Dependent on encapsulation failure and ventilation conditions Panel thickness and number of laminations affect behaviour Adhesive type critical — some adhesives cause delamination, exposing fresh timber Ongoing research at multiple UK universities UK Regulatory Position Current Requirements Ban on combustible materials 18m : Regulation 7(2) of Building Regulations Exemption for structural timber : CLT/glulam structural elements ARE permitted above 18m Non structural elements : Must be non combustible above 18m (insulation, cladding) Approved Document B : Provides no specific guidance for mass timber BRE/DLUHC Research Programme Full scale fire tests of CLT compartments Post fire structural assessment methodology Auto extinction conditions and reliability Results informing future ADB revisions Fire Engineering Approach For mass timber buildings, fire engineering must address: Contribution of structural timber to fire load Delamination risk and cascading fire development Compartment burnout and structural integrity Fire spread through concealed timber construction Residual structural capacity post fire Encapsulation Strategy Full Encapsulation All exposed CLT surfaces covered with non combustible boards Typically 2 layers of 15mm fire rated plasterboard (60 minutes) Timber does not contribute to fire in fully encapsulated design Loss of architectural timber aesthetic Simplified fire engineering analysis Partial Encapsulation Some timber surfaces exposed (typically ceilings or feature walls) Exposed surfaces contribute to fire load and must be calculated Fire engineering required to demonstrate acceptable fire behaviour Time equivalence calculations for increased fire load Sprinkler protection typically required Hybrid Approach Exposed timber in low risk areas (lobbies, common spaces with sprinklers) Full encapsulation in high risk areas (dwellings, sleeping risk) Fire engineered approach balancing aesthetics and safety Construction Phase Fire Safety Mass timber buildings are most vulnerable during construction: No encapsulation in place No sprinkler system operational Construction waste increasing fire load Hot works creating ignition risk Security against arson essential Construction phase fire strategy mandatory Insurance requirements (many insurers reluctant without robust fire plan) Magnus Opifex SEVEN LTD — UK's Leading Fire Safety & Fire Engineering Consultancy 🌐 magnus opifex.co.uk 📞 +44 (0) 20 3488 1926 ✉️ info@magnusopifex.co.uk Founded by Daniel Sheridan, Magnus Opifex SEVEN LTD delivers award winning fire engineering, fire risk assessments, and building safety consultancy across the United Kingdom and internationally.