How it Works: Roof Trusses
The roofs on most new houses in the United States are built with trusses.
Synopsis: The roofs on most new houses in the United States are built with trusses. In this “How It Works” column, structural engineer Rob Munach explains how trusses carry their loads and how T-bracing and roof sheathing add strength. He identifies the different parts of a truss–top chord, bottom chord, web, and nail plates–and how each functions. An illustration shows the compression and tension ratings of a truss that has been modeled with a combined dead, live, and snow load of 40 lb. per sq. ft. Munach stresses that because trusses are built with no redundancy, cutting them can have consequences ranging from damage to interior finishes and materials to catastrophic roof failure. Another illustration shows how cutting the web of a truss increases the deflection potential of the bottom chord.
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.
Truss performance by the numbers
In a truss, the bottom chord and the top chord are under tension and compression forces as a result of the loads on the roof. Which chord carries tension and which carries compression depends on the direction of the overall loading on the truss. The loads on the truss are generally in a downward direction, but may turn upward during extreme wind. The numbers below are the actual compression and tension ratings of a truss that has been modeled with a combined dead, live, and snow load of 40 lb. per sq. ft. They illustrate the push-pull relationships of all the truss members.
For more photos and details on how roof trusses work, click the View PDF button below.