Roof Truss Techniques

Preparation

• A major difference between using roof trusses and rafters is assembling the materials: buying one piece or individual lumber boards. With joists and rafters, a builder must order framing lumber, usually 2-by-4-inch, in a variety of lengths to fit the width of the roof to be framed. There inevitably is waste lumber. Using prefabricated trusses requires taking careful measurements for the width of the truss and the pitch, or slope, of the rafters and furnishing that information to a supplier.

Less Work

• Using joists and rafters requires measuring and cutting every board, then installing each with framing nails and a hammer. That requires skill in getting measurements right and cutting angles correctly with a circular saw. It also takes several workers to hold joists and rafters in place while they are secured. Prefabricated trusses are installed as a unit and eliminate all this individual work.

Less Time and Labor

• Joists and rafters are lifted into place on a roof one piece at a time and nailed in place. Trusses are lifted to the roof and secured in one step. The Wood Truss Council of America says framing a roof with trusses takes 40 percent less time and 16 percent less labor and material than framing a roof with individual elements.

Lifting Trusses

• Lifting and setting trusses requires special techniques. Some builders use machines -- cranes or forklifts -- to lift trusses to a roof to be secured by workers on the roof. Others use workers on ladders or scaffolding or on the ground with lifting poles to raise trusses for roof workers to erect and set. Trusses lifted with machines are lifted upright, with two lifting points. Trusses raised manually are lifted upside down for roof workers to erect and install. Either machine or manual installation requires careful marking of wall plates in advance so trusses can be set precisely, usually 24 inches apart.

Securing Trusses

• Trusses are secured to wall top plates with framing nails driven diagonally through the truss ends into the wall plates, two nails on one side of the truss and one on the other. Some builders use special metal brackets nailed to truss ends and plates, rather than basic nailing. Either method should be further secured with "hurricane" clips, metal brackets that overlap truss and plates joints to hold trusses solidly.