How to Truss a Flat Roof

Order trusses for the width of the roof. Measure the length with a tape measure and divide by 24 to determine the number, with trusses spaced 24 inches apart. Base the style on the span or width of the roof, with the depth of a truss in inches approximately 7 percent of the span in inches; a truss for a 20-foot wide roof, for instance, will be about 17 inches deep.

Mark the wall sides for truss locations, using a tape measure, speed square and pencil. Measure 1 1/2-inches from the back wall end and draw a line across the wall cap for the inside of the first truss. Measure 23 1/4 inches in and draw another line, to mark the outside edge of the second truss. Mark 24-inch increments, based on that second line, the length of the roof. Mark walls on both sides.

Set the first truss in place and plumb it with a level. Put the bearing point on the end of the truss on top of the wall, with the end of the truss flush with the side on the wall on both sides of the roof. Secure the truss with 16d framing nails, driven diagonally through the truss bottom chord into the wall cap with a hammer. Put two nails on one side, one on the other.

Erect all trusses, maintaining spacing and plumb. Add "hurricane" clips on each end of every truss; these are metal brackets with one side nailed to the wall cap, the other to the truss bottom chord. Use 8d galvanized nails to fasten clips.

Add any required bracing, which will vary with the type of flat truss and roof size. Follow building codes or truss manufacturer's recommendations to install braces diagonally or laterally between truss chords. Use 1-by-4-inch or 2-by-4-inch boards for braces. Deck the flat roof trusses with plywood or oriented strand board.