【New】How it Works: Roof Trusses

A roof truss is an engineered building component designed to span longer distances than dimensional lumber without relying on interior partition walls for support. The most common truss, a 2×4 Fink truss, is designed to support several different loads. This issue, Paul Johnson and Nathan D. Young show how to build complex roofs with trusses. Whether you’re framing a new roof or remodeling an existing truss roof, it’s important to know what components make up a truss and how it works.

Dead loads: The weight of the roof, the truss itself, attic insulation, floor sheathing, ductwork, and drywall used for the ceiling.

Top chord: Supports the dead load of the materials used to construct the roof, the live load of workers building or maintaining the roof, and wind and snow loads.

Bottom chord: Supports the dead load of ceiling materials and the live loads of homeowners accessing the attic as well as the weight of the items stored in the attic. The bottom chord also serves as a rafter tie and keeps the top of the exterior walls from thrusting outward.

Web: Helps to support and to transfer the compression and tension forces from one chord to the other. Individually, the web members also are in tension and compression, depending on their location in the truss and the direction of the loads on the truss.

Nail plates: Components are fastened together with toothed metal plates that are machine-pressed to the members.

Better bracing: Nailing 2×4 T-bracing to the top edge of long web members prevents them from deflecting sideways. T-bracing takes more time to install than continuous lateral bracing and is more expensive, but it’s more effective and easier to install correctly.