The 2021 IBC includes provisions for mass timber construction up to 18 stories and 270 feet. Cross-laminated timber (CLT) can be used as part of an exterior wall assembly in most construction types. However, in Type IV projects (including A, B, C and HT), exterior walls greater than 40 feet may be required to pass the NFPA 285 Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components fire test. Without the results of a successful test, a team seeking approval for a project that includes CLT exterior walls over 40 feet, whether load or non-load bearing, could be required to invest in their own testing—adding significant expense and uncertainty to the permitting process.

To remove this uncertainty, a project supported by a 2021 U.S. Forest Service (USFS) Wood Innovation Grant has demonstrated that a CLT-based exterior wall assembly can pass the NFPA 285 fire test. The project included the design of two CLT-based exterior wall assemblies that are appropriate for mid-rise and high-rise construction, and successful fire testing of these assemblies to the NFPA standard.

This paper details the requirements for mass timber exterior walls in the 2021 IBC and summarizes the results of the two NFPA 285 fire tests. In so doing, it provides a solution that any design team can replicate, both to remove a barrier to the use of mass timber for exterior walls and help others design safe and efficient mass timber buildings.

Download the test summary

Accompanying Documents

Construction documents for the tested assemblies and test reports developed by the Southwest Research Institute can be downloaded here: