By Zencalcy Editorial TeamLast reviewed About us
Roof trusses are set at 24 inches on centre for most residential roofs and 16 for heavier loads or taller heels. The count comes from the building length, while the pitch sets the top chord length and the overall height the truss will be delivered at.
Truss count, rafter length and top-chord lumber.
Trusses = ⌈length × 12 ÷ spacing⌉ + 1 top chord = (width ÷ 2) ÷ cos(atan(pitch ÷ 12))
The +1 gives a truss at the far gable. Top chord length excludes any overhang built into the truss. A 30 ft span at 6/12 needs top chords near 16.8 ft, so delivery and crane access matter as much as the count.
| Symbol | Meaning |
|---|---|
| L | Building length. |
| s | Truss spacing. |
| N | Number of trusses. |
40 × 12 ÷ 24 = 20 spaces, so 21 trusses. At $180 each that is $3,780.
Continue with the Roof Pitch Calculator, Rafter Length Calculator, Roofing Calculator, or the Roof Shingle Calculator — or browse more roofing and all construction calculators.
24 inches is standard for asphalt shingles and light snow; 16 inches for heavy tile, tall heels or high snow loads.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026