How Does Cross-Laminated Timber Achieve a Fire Rating?

I run the char calculation the way the wood design standards intend: start with the published one-dimensional char rate, multiply by the required fire duration, add the zero-strength layer behind the char front, and check the residual section under the fire-load combination. The fire-load combination is lighter than the design loads — the code recognizes that a building at full design occupancy during a fully developed fire is not the scenario being designed for — so members that look thin for everyday loads can still pass the fire check. I document the calculation member by member, because the plan checker will ask for it and the exercise catches the members where the architecture's exposed-timber ambitions exceed the rating the section can deliver. Panel layup affects the result. A five-ply panel of a given thickness outperforms a three-ply panel of the same thickness in a fire, because more of the section remains in the strong axis after charring. I coordinate the layup with the manufacturer early — changing ply count late in the project ripples through the gravity design, the connection design, and the cost. Edges and ends get the same treatment as faces: I detail edge protection where panels meet, because fire finds the joint the designer forgot.

Where the design calls for encapsulation, the gypsum layers are a fire-rated assembly and I treat them with the same rigor as a rated shaft wall. The layer count, fastener type and spacing, and joint staggering all follow a tested assembly — I do not invent encapsulation details. Fasteners must bite into the timber adequately without splitting it, and joints are staggered between layers so no continuous seam gives fire a path to the wood. Penetrations for MEP are the chronic failure point: every pipe, duct, and conduit through the encapsulation gets a listed fire-stop, and I coordinate the penetration layout with the MEP engineer so the field is not improvising holes in a rated membrane. I also think about what happens after the fire. Encapsulation that stays in place through the rating period has done its job even if it is destroyed in the process; the design question is whether the timber behind it is reusable. On projects where post-fire salvage matters to the owner, I document the expected char depths and protection performance so the post-fire assessment starts from engineering, not guesswork.

A CLT fire strategy is complete when the structure, the protection, and the concealed spaces are all addressed together. Timber burns predictably — the design has to be equally predictable. • Confirm the construction type and required ratings with the authority having jurisdiction before sizing a single member • Specify CLT with qualified heat-resistant adhesives and collect the manufacturer's certification at submittal • Run the char calculation member by member for exposed timber, including a zero-strength zone behind the char front • Detail encapsulation to a tested assembly — layer count, fasteners, and staggered joints exactly as tested • Protect every connection: conceal steel in timber, cover with encapsulation, or include the joint in the char calculation • Design concealed spaces against fire and smoke travel, and coordinate every MEP penetration with listed fire-stopping