May 20, 2025
Wood Burns
At first glance, the word “resilience” may seem optimistic when discussing fire. However, extensive real-world testing across Europe, Canada, and Australia consistently shows that large-section mass timber elements retain their structural capacity during fire exposure. This performance is rooted in the predictable way wood reacts to heat.
Fire Testing
A clear example comes from a full-scale fire test conducted on June 10, 2022, at the Ottawa Fire and Explosives Testing Facility.
The Canadian Wood Council, supported by fire researchers from the National Research Council, tested a two-storey mass timber office building with an open floor plan. The final test was intentionally designed as a “worst-case scenario,” conducted without sprinkler activation or firefighter intervention.
The interior was furnished with wood cribbing representing approximately 120% of a typical modern office fuel load. Doors and windows were left open to allow unrestricted air intake, and an aggressive ignition source was used to ensure rapid flame spread to the ceiling.
Ten minutes after ignition, the fire reached peak intensity before beginning a gradual decay.
By 25 minutes, only residual contents continued to burn at low intensity. The fire ultimately lasted four hours and ten minutes and self-extinguished once the fuel load was exhausted. Despite the severity of the exposure, the structure remained stable and was safe to enter after the test.
Char Saves the Day
The building’s survival is explained by the predictable charring behavior of large timber members. When exposed to fire, timber forms a surface char layer that is both slow-burning and highly insulating. This char layer dramatically reduces heat transfer to the interior of the member, slowing the rate of material loss and preserving load-bearing capacity.
By accounting for char depth in design, mass timber elements can be sized to maintain structural integrity through severe fire exposure.
The images above demonstrate that while fire presents a serious threat to life and property, mass timber behaves in a manner comparable to non-combustible systems. It is the contents that burn, while the structure retains its ability to carry loads. In multi-storey buildings, this behavior supports fire resilience scenarios where evacuation may be delayed and shelter-in-place becomes necessary.
Sprinklers Work
Modern building codes require automatic sprinkler systems in multi-unit residential buildings four storeys and higher, along with interconnected smoke detection and fire alarm systems. As a result, true worst-case fire scenarios are statistically rare.
Fire testing by engineering firm Arup in Europe demonstrated that low-pressure water mist systems effectively suppressed fires in mass timber buildings, limiting damage to minor surface discoloration of ceiling panels and resulting in minimal water damage.
Conclusion
Fire will always remain a risk, but modern mass timber design significantly improves life safety and structural resilience. While mass timber is difficult to ignite, its charring behavior protects load-bearing capacity when exposed to fire. When combined with active fire protection systems, fires are typically contained to the area of origin.
