Which roof framing configuration is particularly vulnerable to collapse in a fire due to lightweight components?

• A. Lightweight truss roof framing
• B. Conventional rafters
• C. Steel joists
• D. Heavy timber roof framing

Answer: (A)

The thing being tested is how fast certain roof systems can fail in a fire. Lightweight roof framing uses slender members and small gusset plate connections designed to save weight, not to resist lengthy heat exposure. In a blaze, heat quickly weakens both the wood members and these joints, and the gusset plates can lose shear and the chords and webs can fail. Because the system relies on a lightweight, triangulated network, the loss of a few key connections can cause a rapid, progressive collapse with little warning as loads no longer have a stable path to transfer through the roof structure. This makes it particularly vulnerable compared to heavier systems.

Heavy timber, by contrast, chars and forms an insulating layer that helps preserve strength longer, and conventional rafters are bulkier and better at maintaining a load path under fire conditions. Steel joists can fail if not protected, but the characteristic immediate, catastrophic failure in a fire is most closely associated with the lightweight truss configuration due to its minimal mass and connection design.