What is a roof truss?

What it is

A truss does with geometry what a rafter does with depth. Its members are small — often two-by-fours — but arranged as triangles, which cannot change shape without a member changing length, so the frame is rigid and light. The top chords carry the roof deck, the bottom chord carries the ceiling and ties the walls together, and the webs zigzag between them; galvanised plates with punched teeth are pressed into every joint in the factory. Trusses are designed by the manufacturer's engineer for the specific house from the plan, span and local loads, and arrive as a numbered set with a layout drawing.

The IRC, Section R802.10 requires wood trusses to be designed and manufactured to the national truss standard (ANSI/TPI 1), supplied with truss design drawings that show the loads, spans, bearing points and required bracing, braced as those drawings and the manufacturer's instructions require, and — the clause every remodeller learns — not cut, notched, drilled, spliced or otherwise altered without the approval of a registered design professional. The SBCA publishes the handling and bracing guidance the drawings refer to.

What it costs and who does it

Trusses are priced per set by the manufacturer from the plan, installed by the framing contractor; repairs to a damaged or altered truss are priced as an engineer's fee plus the carpentry to the engineer's drawing, and are a framer's job, not a roofer's, though a roofing crew's carpenter often does small ones while the deck is open. Truss uplift — the seasonal lifting of the bottom chord that cracks the drywall at interior walls — is a finish repair, not a structural one, and is dealt with by floating the ceiling corners.