Cross-laminated timber (CLT) is revolutionising construction, but fire regulators remain cautious. We examine the evidence and the regulatory position for tall timber buildings in the UK.. The Timber Revolution Mass timber construction — primarily Cross Laminated Timber (CLT) and Glued Laminated Timber (Glulam) — offers compelling sustainability benefits: Up to 75% reduction in embodied carbon compared to steel/concrete Faster construction times Reduced site disruption Biophilic design benefits Carbon sequestration The UK Regulatory Position The Combustibility Ban Following Grenfell, the government introduced a ban on combustible materials in the external walls of buildings over 18 metres (Regulation 7(2) of the Building Regulations). This effectively prohibits: Exposed timber cladding on tall buildings Timber structural elements within the external wall zone Combustible insulation in external walls Structural Use of Timber The combustibility ban applies specifically to external walls. Internal structural use of timber is not banned, but: Requires fire engineering justification Must demonstrate equivalent safety to non combustible alternatives Building Safety Regulator scrutiny for higher risk buildings Insurance challenges remain significant Fire Performance of Mass Timber Charring Behaviour Mass timber behaves predictably in fire: Char rate: approximately 0.65mm/minute for CLT Char layer insulates the unburned timber beneath Structural capacity can be maintained for extended periods Predictable compared to steel (which can fail suddenly at high temperatures) Encapsulation Strategy The primary fire safety approach for timber buildings: All structural timber encapsulated in non combustible linings (typically 2+ layers of plasterboard) Encapsulation prevents timber contributing to fire load Designed to maintain protection for full fire duration Fall off of linings is the key design concern Exposed Timber Some designs feature intentionally exposed timber: Requires detailed fire engineering analysis Contribution to fire load must be quantified Enhanced suppression systems typically required Ventilation conditions affect burning rate significantly Limited to specific areas (feature walls, ceilings) International Experience Notable Projects Mjøstårnet, Norway (85.4m) — world's tallest timber building HoHo, Vienna (84m) — mixed timber concrete hybrid Dalston Works, London (10 storeys) — UK's tallest CLT building 25 King, Brisbane (52m) — commercial timber tower Lessons Learned 1. Connection design is critical for fire performance 2. Encapsulation detailing requires extreme care 3. Construction phase fire risk is the highest risk period 4. Insurance markets are slowly accepting timber buildings 5. Sprinklers are essential in all tall timber buildings The Path Forward For mass timber to achieve its potential in the UK: Continued fire testing and research Clear regulatory guidance (beyond the current ambiguity) Insurance industry engagement Fire engineering methodology specifically for timber buildings Construction phase fire protection protocols For mass timber fire engineering, contact Magnus Opifex.