Local building codes will govern the spacing of roof trusses, which can vary with the width of the building, weather conditions, type of roofing used and other factors. Three load factors affect spacing: dead load or the weight of the roof itself; live load, snow and ice and other accumulations; and wind load, the force of wind against the roof. The type of lumber in the truss also can affect spacing.

Space Variations

Trusses are used in about 80 percent of new houses and virtually all are made of pine or fir lumber. Most trusses are made with 2-by-4-inch boards. Some roofs with heavier load factors or special conditions may use 2-by-6-inch or even 2-by-8-inch lumber. Heavier lumber can affect spacing. A 2-by-4-inch truss may require closer spacing than a 2-by-6 in the same weather areas.

Standard Spacing

Most trusses are spaced 24 inches on center, which means the centers of the truss members are 2 feet apart laterally. Building codes in some severe weather areas or governing roofs in special situations may require 16- or even 12-inch spacing, but 24 inches is the universal basic dimension.

How to Space

Spacing trusses correctly requires marking the cap plates on the outside walls. Two techniques are employed: using either a framing square or a tape measure. A framing square has two arms, a tongue 1 1/2 inches wide -- which is the width of a standard truss board -- and a blade 24 inches long, which is the standard spacing. Mark the truss width with the tongue, and use the blade to mark the spacing. Using a tape measure requires stretching the tape across the top of the wall cap.

Marking Spaces

Mark the first truss from the back end of the roof, then measure 23 1/4 inches to mark the outside line of the second truss. Mark the inside of that truss at 1 1/2 inches, then measure from that line 24 inches down the roof, marking the 1 1/2-inch width at every location. The last truss dimension on most roofs will be slightly under 24 inches.

Other Spacing

Use the same technique if 16-inch spacing is required. Mark the first truss, measure 15 1/4 inches in from the end wall to mark the outside line of the second truss, then proceed down the roof. The same procedure would apply to 12-inch or any other spacing.