Mass timber construction is reaching new heights in the UK. We examine the fire engineering challenges, regulatory barriers, and solutions for CLT and glulam structures.. Timber's Renaissance in Construction Mass timber construction — principally Cross Laminated Timber (CLT) and Glued Laminated Timber (glulam) — represents one of the most significant shifts in UK construction practice in decades. Driven by sustainability imperatives, carbon reduction targets, and the speed advantages of prefabrication, timber buildings are growing taller and more complex. However, the post Grenfell ban on combustible materials in external walls of buildings over 18m has created a significant regulatory challenge for tall timber buildings. The structural material itself is combustible, and demonstrating fire safety equivalence to non combustible alternatives requires sophisticated fire engineering. The Combustibility Paradox Timber presents a paradox in fire safety terms: Positive Characteristics Predictable charring rate — CLT chars at approximately 0.65mm per minute Structural integrity during fire — charred outer layer insulates the structural core No spalling — unlike concrete, timber doesn't explosively fragment in fire Calculable performance — Eurocode 5 provides detailed design methods for fire resistance Challenging Characteristics Combustible — timber is fuel, contributing to fire load Delamination risk — CLT adhesive failure can cause charred layers to fall away, exposing uncharred timber and accelerating burning Concealed cavities — junctions and service zones can create hidden fire paths Post fire smouldering — deep seated smouldering in thick timber elements can reignite hours after apparent extinguishment Fire in exposed timber — architectural desire to leave timber exposed conflicts with encapsulation strategies Regulatory Position in the UK The 18m Ban (Regulation 7) Applies to external wall materials only — does not prevent timber structural frames External cladding and insulation must be A1/A2 rated CLT structural walls set back from the external face with non combustible cladding Building Safety Act Implications Higher risk buildings (over 18m) require Gateway approval BSR will scrutinise fire safety case for timber buildings Enhanced evidence requirements for fire performance Insurance Challenges Many insurers unwilling to cover tall timber buildings Those that do may impose significant premium loadings Construction phase insurance particularly difficult to obtain Sprinkler protection typically a non negotiable requirement Fire Engineering Strategies for Mass Timber Encapsulation The most common approach — covering all timber surfaces with layers of non combustible board: Typically 2 layers of Type F plasterboard (25mm + 15mm minimum) Prevents timber contribution to fire load during the design fire Allows conventional fire safety design approaches Preserves timber's structural and sustainability benefits while neutralising fire risk Partial Encapsulation Leaving some timber surfaces exposed for aesthetic reasons: Requires detailed fire engineering analysis Additional timber contribution to fire load must be accounted for Enhanced sprinkler protection typically required Limited to lower risk occupancies in practice Char Based Design Designing timber members with sacrificial charring allowance: Oversized members that maintain structural capacity after charring Eurocode 5 reduced cross section method Typically requires 30 90 minutes of charring capacity depending on fire resistance requirement Connection Design: The Critical Detail Connections between timber elements are where fire performance is won or lost: Steel connectors — must be recessed and protected from fire exposure Adhesive bonds — adhesive type affects delamination risk (polyurethane vs melamine) Panel to panel joints — must maintain compartmentation through full fire duration Floor to wall junctions — critical for preventing fire spread between floors Service penetrations — all penetrations must be fire stopped with tested systems compatible with timber substrates Lessons from International Projects The UK can learn from international mass timber projects: Mjøstårnet, Norway (85m) — world's tallest timber building, extensive use of sprinklers and encapsulation HoHo Wien, Austria (84m) — timber concrete hybrid with full encapsulation Brock Commons, Canada (53m) — CLT and glulam with complete encapsulation, built in 70 days Dalston Works, London (33m) — UK's tallest CLT building at time of completion Common themes: sprinkler protection throughout, systematic encapsulation, enhanced construction phase fire safety, and detailed fire engineering analysis. For mass timber fire engineering and regulatory support, contact Magnus Opifex.