Find how many trusses your roof needs, the top and bottom chord lengths, and a ballpark cost โ based on building span, length, pitch, and spacing.
Truss count is based on your building's length, not its width. Trusses run across the span (width) and are spaced down the length of the building at fixed on-center (OC) intervals โ typically 24 inches for residential roofs or 16 inches where snow load or spans are heavier.
Truss count formula: (Building Length ร 12 รท Spacing) + 1. The "+1" accounts for the fact that you need a truss at both the very start and end of the run, not just every interval in between. The two end trusses are usually built solid as "gable end trusses" to support siding.
Use the formula (building length in inches รท spacing) + 1. For a 40-foot building at 24" OC spacing: (480 รท 24) + 1 = 21 trusses.
24 inches on-center is standard for most residential roofs. 16 inches on-center is used for heavier snow loads, longer spans, or when supporting heavier roofing materials like tile or slate.
A single engineered truss typically costs $6โ$12 per linear foot of span, so a 30-foot span truss runs roughly $180โ$360 installed, before delivery and crane costs on larger projects.
For small structures like sheds or playhouses, simple king-post trusses can be field-built. For any habitable structure, trusses must be engineered and fabricated to meet local building code snow and wind load requirements.