How to Locate a Roof Truss

Determine truss location based on the width or span of the truss, its pitch and its loads, dead load from the weight of the roof itself, live load from snow and other exterior weights, and wind load from force of wind. Type of lumber also can affect these calculations. Tables are available from construction sources and online references to guide this determination.

Locate trusses 24 inches apart in standard construction, but use 16-inch spacing for heavier loads. Use 36- or even 48-inch spacing with trusses of heavier lumber and such buildings as sheds and barns. Adapt the location to the size of truss lumber; most are built with 2-by-4-inch lumber but some use 2-by-6 or even 2-by-8; the thicker the truss lumber, the wider the locations can be.

Mark truss locations on the caps of the outside walls with a tape measure, pencil and speed square. Use a ladder to reach the wall cap. Start at the back end of one wall. Measure 1 1/2 inches from the end of the wall and use the square to draw a line across the cap board, to mark the inside edge of the first truss. Do this for all truss locations, regardless of spacing.

Measure 23 1/4 inches in from the wall end, for 24-inch spacing, and make another line for the outside edge of the second truss; make another line 1 1/2 inches farther. Measure 24 inches and mark another set of parallel lines 1 1/2 inches apart. Continue marking those locations to the end of the wall, then repeat the process on the other wall.

Use 15 1/4 inches to measure for the second truss mark on 16-inch spacing, 35 1/4 for 36-inch and 47 1/4 for 48-inch. Locate all other trusses in the same fashion as 24-inch spacing, allowing 1 1/2 inches for each truss. The last space in any truss locations will be slightly under the regular dimension.