Prefabricated trusses have improved roof framing. They are made in factories where angles can be precisely cut and set, and joints are fastened with steel gussets that are stronger than traditional framing nails. Trusses also are easier to install, because they go up as a unit, rather than individual boards. One element has not changed. Trusses must be laid out precisely to accommodate roof decking and shingling.
Standard truss boards are spaced 24 inches apart, from one end of the roof to the other. This conforms to the 4-by-8-foot dimensions of plywood or oriented strand board (OSB) sheets used to deck or cover the roof in preparation for shingles. Improperly spaced trusses will not provide correct nailing points to secure decking.
Use a ladder, tape measure, speed square and pencil or marker to lay out the truss spacing on the wall caps, the top boards on the two outside walls. Start at the back on one side. Measure 1 ½ inches in from the wall end and draw a line across the cap using the speed square. That marks the inside edge of the first truss. Truss members are 1 ½ inches thick.
Measure 23 ¼ inches in and draw another line to mark the outside line of the second truss. Make another line 1 ½ inches in from that. Measure 24 inches from the inside line of the second truss point and mark another pair of parallel lines 1 ½ inches apart. Continue down the roof, measuring 24 inches between inside and outside truss lines and 1 ½-inch spacing. The last space will be slightly under 24 inches.
Move to the other wall, start at the back and repeat the process. Once both wall caps are marked, double check the spacing by stretching builder's twine across a pair of marks and check the angle with the speed square. If the string line is square on both wall caps, the truss layout will be good. Be careful when measuring the 24-inch spacing, which must be exact.