The international Building Code (IBC) requires all buildings to perform to the same level of fire safety, regardless of the materials used in construction. Repeated fire resistance testing has demonstrated how mass timber structures meet or exceed IBC requirements.
The fire performance of mass timber is fundamentally different from dimension lumber. The inherent fire resistance of mass timber is due to the charring that occurs on the exterior of mass timber components in a fire event. This charring creates a protective barrier for the inner structure of mass timber and commonly self-extinguishes.
Successful ASTM E119 fire resistance testing is one reason the ICC recognized mass timber construction (Cross Laminated Timber) in the 2015 IBC. At that time, two-hour resistancewas required by code, while actual testing demonstrated resistanceof 3 hours and 6 minutes. This exceptional performance is essential for occupant and first responder safety and compares favorably with any other material.
More recent 2017 testing conducted at the federal government’s ATF research laboratory, the world’s largest research facility for fire investigations, has added substantially to the body of science on mass timber. Five tests were performed on full-scale, multi-story mass timber apartment buildings at the federal lab. The results have confirmed the earlier findings. Importantly, several tests were conducted that assumed the unlikely event of a sprinkler system failure, while others confirmed the effectiveness of sprinkler systems in mass timber fire events:
- A mass timber structure fully protected by two layers of 5/8-inch type X gypsum wallboard. The fire burned itself out after 3 hours with no significant charring on the protected mass timber surfaces.
- A mass timber structure with 30% of the CLT ceiling exposed. Test concluded after 4 hours, with apartment contents burned out and CLT self-extinguished due to char that protected the underlying mass timber.
- A mass timber structure with 2 CLT walls fully exposed. Apartment furnishings and contents consumed after 4 hours and CLT self-extinguished after formation of protective char surface.
- A fully exposed mass timber structure with sprinkler system. A single sprinkler quickly contained the fire.
- A fully exposed mass timber structure with sprinkler system. Fire allowed to grow to flashover – 23 minutes – before being quickly extinguished by sprinkler.
Fire resistance testing for any material under the IBC must be rigorous and clearly demonstrate compliance with the code. Experience and testing indicate that mass timber technologies provide ample safety margins for building occupants and first responders in any fire event.