Fire Resistant

A common misconception about cross-laminated timber buildings (or wooden buildings in general) is that they could be fire hazard. On the contrary, cross-laminated timber panels are highly resistant to fire. CLT it’s actually safer than steel and concrete. When steel heats, it bends and warps quickly. When CLT is exposed to fire, it chars on the outside at a predictable rate. The interior actually stays just as solid and stable while the exterior chars. Fire safety testing and research prove it.

Solid CLT panels do not ignite as easily as small two-by-fours, and fire can be contained in one room for more than three hours without expanding to other rooms, giving sufficient time for firemen to arrive. Even if panels do burn, charring on the outside protects the interior wood, leaving the panel structurally sound, with no sudden failure. Finishing the panels with wallboard or another material will also improve fire protection.

Fire is the first concern that comes to mind with wood construction. And yet, mass timber is actually safer in a fire than concrete and steel. A thick plank of wood will char on the outside, sealing the wood inside from damage. CLT panels burn slowly and predictably, and form a char-layer. The non-charred internal wood layers retain their strength and dimensional stability to thereby delay the collapse of structural members. In some respect, CLT has an inherent fire resistance better than an unprotected steel structure. Lastly, CLT panels, due to their massiveness and tightness, provide an extremely favorable barrier for gasses and high temperatures during a fire situation, improving compartmentalization. 

CLT assemblies excel in terms of fire protection because, like heavy timber, they char at a rate that is slow and predictable, maintaining their strength and giving occupants more time to leave the building. CLT structures also tend to not to have as many concealed spaces within floor and wall assemblies, which reduces the risk that a fire will spread. The American Wood Council (AWC) conducted a successful ASTM E119 fire resistance test on a CLT wall at NGC Testing Services in Buffalo, NY. The wall, consisting of a 5-ply CLT (approximately 6-7/8 inches thick), was covered on each side with a single layer of 5/8-inch Type X gypsum wallboard. The wall was loaded to the maximum load attainable by the NGC Testing Service equipment. The test specimen lasted 3 hours, 5 minutes, and 57 seconds (03:05:57).

www.awc.org/Code-Officials/2012-IBC-Challenges/NGC-CLT-Report.pdf

See also “performance connections CEE law R60”

In another fire resistance test of a 5-ply cross-laminated timber (CLT) panel wall, the panel was subjected to temperatures exceeding 1,800 Fahrenheit and lasted 3 hours and 6 minutes, far more than the two-hour rating that building codes require.

During fires, exposed mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage. Additionally, when the code requires mass timber to be protected with gypsum wall board, the mass timber can achieve nearly damage-free performance during a contents-fire burnout event.

All fire tests confirmed that mass timber structures meet and exceed the fire resistance requirements in the current codes.

In conclusion, CLT panels excel in fire safety characteristics, proving to be better performing than even metal and concrete. This is because the dense wood heats up much slower during a fire compared to metal or concrete. Also massive timber burns predictably slow. Charring rate is 0.65 mm/min. Therefore the deterioration of structural properties is slow and predictable unlike metal and concrete.