Mass Timber & Fire Safety: What The Evidence Shows – CleanTechnica

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Mass timber is scaling for obvious reasons. It is lighter than concrete, faster to assemble than steel, and carries the climate advantage of storing carbon drawn down from the atmosphere. That combination is powerful for developers and policymakers trying to hit cost and emissions targets. The unavoidable question is whether it is safe in a fire. Fire is the most basic stress test of any building material, and investors, insurers, and regulators need to know whether mass timber will stand up as well as steel and concrete when exposed to flames. It does, and arguably better than alternatives in many cases.

This is one of the later articles in my series examining the role of mass timber in Canada’s housing and climate future, and by extension the rest of the world. The first piece laid out Canada’s timber moment, framing cross-laminated timber (CLT) and modular construction as the fastest lever for addressing housing shortages, jobs, and embodied carbon. The second explored how Mark Carney’s housing initiative could industrialize the sector through pre-approved designs, offtake contracts, and regional factories. The third explored the requirement for vertical integration within the industry to maximize efficiencies. The fourth showed how CLT displacement could bend the demand curves for cement and steel, making their decarbonization pathways more realistic. The fifth demonstrated that from harvest to housing, CLT already locks away more carbon than it emits, strengthening its climate case.

The sixth turned to the forestry supply chain, arguing that electrification of harvesting, transport, and processing is essential to maintaining CLT’s carbon advantage. The seventh piece addressed systemic barriers, focusing on high insurance costs and bespoke code approvals, and argued that normalizing mass timber in regulatory and financial frameworks is the key to scaling. The eighth piece, arguably one that should have been much earlier in the series, explored the various technologies in mass timber and its currently dominant form, CLT. The ninth piece assessed the global leaders, opportunities and competition for Canada’s mass timber industry and considers lessons to learn. The tenth piece deals with input regarding labor and financing I received over the course of the series from professionals engaged in the space. The eleventh piece focused more on a speed and labor opportunities that mass timber construction has demonstrated. The twelfth turned to carbon accounting and international standards. The 13th article turned to the subject of end-of-life final resting places for mass timber, something introduced in the previous piece. This piece turns to a question that was raised at various times in comment, the question of fire safety.

The way mass timber behaves in fire is different from what people imagine when they think about a stick-built house fire. Thick timber members form a char layer when exposed to fire. That char insulates the interior of the member, slowing the spread of heat and preserving structural strength in the unburned core. In contrast, steel loses much of its strength once it reaches about 600°C and can deform or buckle quickly unless protected, and concrete, while noncombustible, can spall and expose reinforcing steel. Heavy timber does not collapse suddenly. Its failure mode is gradual, as cross-section is reduced by predictable charring rates of around 0.65 to 0.8 millimeters per minute. Designers can size members with this sacrificial layer in mind, leaving enough intact wood to carry loads for the required fire rating period. Adhesives matter too. Early tests showed that panels made with heat-sensitive glues could delaminate and expose fresh wood, but standards were tightened in 2018 to require heat-resistant adhesives. With those in place, mass timber behaves more like solid timber, charring steadily without layers falling away.

Fire testing has borne out these principles. Standard furnace tests like ASTM E119 in North America or ISO 834 and EN 1365 in Europe measure how long loaded assemblies can withstand a controlled fire curve. Mass timber elements routinely achieve 1 or 2 hour ratings and in some cases much longer. In one widely cited test a 5 ply CLT wall panel under load lasted over three hours before reaching failure criteria. More convincing still are the full-scale compartment burns. In the U.S. and Canada, researchers have set entire furnished timber apartments on fire to see how they behave. The results show that the timber chars as expected, the fire consumes room contents, and the structure remains standing through burn out without collapse. The National Research Council of Canada ran a worst case scenario in 2022 with extra fuel load, no sprinklers, and multiple exposed surfaces. After four hours the fire self-extinguished, leaving a charred but intact two story mass timber office mockup. Similar work in Europe has confirmed that with limits on exposed surface area, rooms will burn out and extinguish rather than sustain ongoing flaming.

Real world data is still sparse because mass timber buildings are relatively new, but what exists is encouraging. A handful of compartment fires in occupied timber buildings have been suppressed by sprinklers with minimal structural damage. The more concerning incidents have occurred during construction, when sprinklers and fire separations are not yet in place. Several large timber buildings, including one glulam framed lab in the UK, were total losses during construction fires. These underline that the riskiest window is before a building is complete, and that hot work protocols, site fire watches, and temporary protections are essential. Once complete and sprinklered, the track record is good, and no completed code compliant mass timber high rise has suffered catastrophic fire failure.

Design strategies are straightforward. Sprinklers are the most effective line of defense, suppressing fires before timber ever contributes significantly. Encapsulation with gypsum board provides additional time, delaying charring for an hour or more. Fire rated detailing protects connections, seals joints, and ensures penetrations do not become weak points. Limits on exposed timber surface in larger compartments prevent the structure from becoming the primary fuel. Together these strategies create multiple layers of safety, equivalent in intent to the fireproofing sprays or concrete encasement routinely applied to steel.

Building codes have already absorbed this evidence. The 2021 International Building Code introduced new construction types IV-A, IV-B, and IV-C specifically for tall timber, allowing buildings up to 18 stories with varying degrees of required encapsulation and allowable exposure. Canada followed with its Encapsulated Mass Timber Construction pathway in the 2020 National Building Code, permitting 12 stories, and is moving to 18 in the 2025 edition. Eurocode 5 includes calculation methods for fire design, and several European countries allow timber high rises through performance based approvals. Australia and New Zealand permit mid rise timber under their national codes. These changes were only possible because full scale tests demonstrated that timber buildings could burn out without collapse and that charring behavior was predictable.

Insurance has lagged behind the codes. Underwriters remember images of timber construction fires and have little actuarial data on completed timber buildings. Builder’s risk insurance premiums for timber projects can be four to ten times higher than for concrete, erasing cost advantages and making projects uneconomic. In operation, insurers worry about total loss in a fire or costly repairs from water damage. That conservatism is beginning to soften as more data emerges. Industry groups have published playbooks to educate insurers, and some carriers are offering better terms when developers implement extra measures during construction. The path to parity is clear. More data on real fires and demonstration burns, better construction phase controls, and transparent engagement with insurers will eventually bring premiums for timber in line with those for steel and concrete.

The cost tradeoffs are manageable. Adding layers of drywall or a water mist system increases construction cost slightly, but can reduce insurance premiums and smooth approvals, often saving money over the life of the project. Engaging a fire engineer and the authority having jurisdiction early can prevent delays and redesigns later. In some cases, the marginal investment in fire protection pays for itself in avoided financing costs and faster project delivery.

Research is still needed. How much exposed timber can be safely allowed in different occupancies remains under study. Connection performance, especially in innovative seismic designs, requires more fire testing. Repair of charred timber and assessment of residual capacity after a fire is another open question with big implications for insurers and owners. Fire service tactics for timber buildings are also being refined. These uncertainties are narrowing as projects scale and more tests are run.

Canada in particular has an opportunity to lead. By harmonizing with U.S. provisions, funding more full scale tests, and requiring construction phase fire plans, it can give regulators, insurers, and investors confidence. Creating a national database of fire incidents and test results would reduce uncertainty faster. With a large forestry sector and a growing mass timber industry, Canada can pair economic development with safety leadership.

For project teams the practical guidance is simple. Bring a fire engineer in early. Decide on how much wood you want to expose and plan the encapsulation accordingly. Talk with building officials and insurers before finalizing designs. Implement strong fire controls during construction. Have a plan for assessing and repairing any fire damage if it occurs. These steps reduce risk and build confidence, which pays back in smoother projects and lower financing hurdles.

The conclusion is that mass timber can meet the same fire safety outcomes as steel and concrete. Its behavior in fire is different but well understood. Testing has shown that with the right design choices, mass timber buildings can survive severe fires without collapse. Codes have absorbed those lessons, and insurers are gradually catching up. The remaining work is execution discipline and sharing of data. As that continues, mass timber will become a normal, accepted part of the construction landscape, delivering carbon and cost benefits without compromising safety.