As the world increasingly turns to timber for its sustainability, strength, and natural beauty, questions around timber’s fire performance are becoming increasingly important. Timber’s fire resistance is often misunderstood, as many assume it performs poorly in fire due to the nature of wood. However, timber has natural fire-resistance properties that make it highly predictable and reliable in fire scenarios, especially when engineered into mass timber products like cross laminated timber (CLT) and glue laminated timber (GLT).
How timber responds to fire
When exposed to fire, timber undergoes a process known as charring, where the outer layer of wood burns and forms a protective layer of char. This char layer acts as an insulator, slowing the fire’s progress into the inner layers and protecting the core of the timber. As the outer layer of wood chars, it also releases gases that dilute the oxygen in the surrounding air, further slowing the rate of combustion. This process enables the structural integrity of the timber to be preserved for longer, unlike materials such as steel, which can warp and lose strength at high temperatures.
The fire performance of mass timber: CLT and GLT
Mass timber products like CLT and GLT take timber’s natural fire resistance to a new level, offering substantial benefits in fire performance. CLT, with its layered, cross laminated structure, provides exceptional stability and strength. When exposed to fire, the outer layers of CLT char at a predictable rate, forming an insulating barrier that slows heat transfer to the core, preserving the material’s structural integrity over extended periods. This dependable charring process allows CLT to perform consistently in fire situations, supporting the overall safety and resilience of the building.
Similarly, GLT is engineered from multiple layers of timber bonded together to form beams, columns, and other load-bearing components that contribute to a building’s fire resistance. As the outer layer chars, it shields the inner core from the heat, which maintains the strength and stability of GLT elements even in demanding fire conditions. This self-protective charring behaviour provides a safeguard for the structure, making GLT an effective choice for critical load-bearing applications.
Together, CLT and GLT offer architects and engineers flexible, reliable options for achieving fire-safe designs in various building types.
Above: Sample cut from an unlined XLAM intermediate floor/ceiling, still structurally intact after total building burnout.
Proven performance through full-scale testing
Extensive full-scale testing of XLAM’s CLT panels have been conducted under real-life load conditions. These tests have confirmed the predictable charring and structural performance of CLT over time, which has allowed XLAM to develop complex, stepwise equations describing the fire performance of their panels. These equations form the basis of XLAM’s Fire Span Tables, which are valuable tools for architects and engineers working with CLT in fire-sensitive applications. XLAM’s Fire Design Guide, which includes these span tables, offers detailed information on CLT fire design and is available for download.
Mass timber’s role in sustainable, fire-resilient construction
Combining fire performance with environmental responsibility, CLT and GLT offer an exceptional alternative to conventional building materials. By embracing mass timber’s fire resistance and understanding its unique characteristics, designers can confidently specify timber in diverse applications, from residential to commercial and even high-rise construction. As sustainable, fire-resistant construction materials become more crucial in our built environment, CLT and GLT stand out as forward-thinking solutions for the future of safe, eco-friendly building design.