Roof Truss Calculator

How to calculate how many roof trusses you need

Trusses stand in a row along the length of the roof, one every so many inches. The number depends on the length of the roof and on the on-center spacing, which is the distance from the center of one truss to the center of the next. It does not depend on the span.

The plus one is the truss at the last end. If the roof is 40 ft long, that is 480 inches. At 24 inches on center that is 20 bays, so 21 trusses. Add one row for each separate roof length, like the main house and a garage wing, and the calculator adds them up. Measurements can be typed as 40'6" or 40 ft 6 in.

Example: a 25.5 ft roof at 24 inches on center

The roof is 25.5 ft long, which is 306 inches, and the trusses are on 24 inch centers.

Because 306 is not a multiple of 24, the last bay is short if you keep 24 inches between the first trusses. If you spread the 14 trusses equally, they sit 23.54 inches apart. Ask your supplier which layout they want.

Trusses by roof length and spacing

Trusses at the spacing in the column header, with no extra trusses added.

How far apart are roof trusses?

The spacing is decided in the design, not by this calculator. In a Journal of Light Construction answer, Frank Woeste, P.E., a professor emeritus at Virginia Tech, discusses spacings of 12, 16 and 24 inches. He says the roof snow load dictates the truss design, and that once the builder or architect specifies the loads, the spacing and the shape, truss engineering takes over. A 16 inch spacing means a lower lumber grade than 24 inches, and he says the wider 24 inch spacing is the most economical for residential construction. We read it in October 2026. The menu offers those three spacings and a custom one, so you can type the spacing from your own plans.

Peak height and top chord length

If you type the span and the pitch, the calculator also gives the geometry of a symmetric gable truss, with the span measured between the bearing points and no overhang. Half the span is the run of each side.

A 24 ft span at 6/12 has a half span of 12 ft. The peak is 12 × 6 ÷ 12 = 6 ft above the bearing line, and each top chord is 12 × 1.1180 = 13.42 ft long. These are theoretical line lengths. They do not include the heel height, the lumber depth or any overhang, and are not cutting lengths. To go from a pitch to degrees or the other way, use the roof pitch calculator. For the roof area and the roofing material, use the roofing calculator, the shingle calculator or the metal roofing calculator.

What this calculator does not do

It counts trusses from a length and a spacing, and it gives the plain geometry of one symmetric truss. It does not design trusses, check spans or loads, choose lumber, or count girders, bracing or hardware. It does not give prices. Trusses are engineered for each roof, so use your plans and your truss supplier for the final count.

Frequently asked questions