Fire performance in mass timber isn't a gap to explain away. For architects and engineers who understand how the material actually behaves, it's one of the more compelling reasons to specify it.
When design professionals first work with mass timber, the learning curve around fire is less about physics and more about perception. The material looks like it should burn easily. The reality — documented through decades of research and a growing body of full-scale test data — is more nuanced, and more useful, than that assumption suggests.
Understanding how mass timber performs in fire doesn't just help clear a code hurdle. It changes how teams approach exposed timber as a design element, how they write specifications, and how they make the case to clients and building officials who are still catching up to what the research shows.
Fire Station 76 | Photo credit: Hennebery Eddy Architects
How the Material Behaves
Mass timber chars when exposed to fire. That char layer — dense, carbonized, and slow to conduct heat — forms on the surface and protects the structural core beneath it. The inner wood remains solid and continues to support loads while the outer layer chars — a performance that differs from light-frame construction, where smaller members can lose strength more quickly due to their size.
What makes this practically useful is that the char rate is predictable. Engineers can calculate how much of a beam or column remains structurally effective after a defined fire exposure period. That calculation is not theoretical — it is the basis for structural fire design in heavy and mass timber and has been recognized in U.S. building codes for decades.
What the Testing Showed
As part of a broader body of research, five full-scale compartment fire tests conducted by the American Wood Council and the U.S. Forest Service put glulam and cross-laminated timber (CLT) systems through conditions designed to reflect real residential occupancy. The findings consistently suggested:
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Encapsulated, or mass timber fully protected by sufficient gypsum, showed no significant charring after three hours of sustained fire exposure.
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In rooms with partially or fully exposed mass timber surfaces, the timber self-extinguished once furnishings and contents had burned — the char layer formed and protected the wood beneath it.
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With a properly designed sprinkler system operating as intended, a single sprinkler head controlled the fire in a room with all mass timber surfaces exposed.
For design teams looking to go deeper on how these findings translate into real-world detailing and code pathways, courses like Design and Construction of Taller Wood Buildings walk through the same test data and its application at the building scale.
Parallel research in Canada, led by FP Innovations, tested glulam and CLT against the benchmark of two-hour-rated noncombustible construction. A CLT stair and elevator shaft — exposed on its interior face — showed no smoke or heat penetration after two full hours of fire exposure. It also confirmed that standard fire stops and sealing products already in use for light-frame and concrete construction work equally well in mass timber assemblies.
Photo credit: Cutler Development
What the Code Now Reflects
The 2021 International Building Code introduced three new construction types for mass timber buildings — IV-A, IV-B, and IV-C — permitting structures up to 18 stories depending on the degree of encapsulation and the fire-resistance ratings achieved. The tiered structure gives design teams real flexibility: how much timber is exposed, and where, becomes a design decision informed by performance data rather than a blanket restriction.
Several jurisdictions, including Oregon, Washington, Colorado, California, and Virginia, were already approving mass timber projects ahead of the 2021 IBC publication. The technical record supported it.
The takeaway for design teams is that mass timber fire performance has moved from an open question to a documented, testable system — but "documented" doesn't mean uniformly accepted. Jurisdictions differ in how much encapsulation or additional justification they require, and some remain cautious given the relative newness of tall mass timber construction in practice. Design teams should treat exposure and encapsulation as decisions to be worked out with local code officials early, using the technical record as leverage rather than assuming it closes the conversation.