Instructions on Ordering & Installing Roof Trusses
- 1). Measure the building to determine the span and run for the truss. Span is the distance between the side walls; run is half that, the space each truss half must cover. Decide a pitch, the angle of slope of the roof from side to peak. Most house roofs are 5/12 or 6/12, meaning they slope up 5 or 6 inches for every foot of run.
- 2). Choose a style, depending in part on how much space will be required under the bottom of the truss. The most common forms are fink, double W, fan and Howe. Those have sloped rafter sides with a horizontal bottom brace. The differences are in the internal bracing. Pick a different style for a special roof, with an arched ceiling, flat top or short pitch on one side, called bobtail or asymmetric. Use an open-style truss with a steeper pitch to provide living or storage space inside the truss area.
- 3). Order trusses from a roof or building supply store, using the span, run, pitch and style preferred. Divide the length of the structure by 24 to determine the number needed; trusses are spaced 24 inches apart. Specify two end trusses, which will have extra bracing. Specify a ridge board size, depending on the length of the roof and wind load. Most ridges will be either 2-by-4-inch or 2-by-6-inch boards. Trusses will be built with slots in the top to fit.
- 4). Install trusses one at a time. Raise the first truss and set it upright and plumb at the back of the building. Nail the ends to cap boards on the outside walls and brace it temporarily with 2-by-4s nailed to the truss and to stakes on the ground. Set other trusses at 24-inch intervals. Install a ridge board when all trusses are in place and nailed plumb to the walls. Add a center brace from the ridge board to the first roof joist for extra support.
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