Roof Truss Span Calculator

Take half the span as the run, then multiply by the slope factor √(1 + (pitch ÷ 12)²). For a 24 ft building at 6/12: run = 12 ft, factor = √(1 + 0.5²) = 1.1180, so each top chord is 12 × 1.1180 = 13.42 ft. Double it for the total truss length from heel to heel across the ridge — 26.83 ft. This is the same arithmetic the solver at the top of the page runs, and the same number the Simple Geometric tab reports as rake length when the overhang is set to zero.

Divide the measured rise by half the span, then multiply by 12. A 24 ft building with a ridge 5 ft above the wall plates gives 5 ÷ 12 × 12 = 5 → a 5/12 pitch. Measuring rise is the honest field method when plans are missing: stretch a level from the wall plate toward the ridge, measure the vertical gap, and you have the one number the pitch arithmetic actually needs. The solver accepts the resulting x/12 value directly.

degrees = arctan(pitch ÷ 12), and pitch = 12 × tan(degrees). The common anchors: 4/12 = 18.43°, 6/12 = 26.57°, 8/12 = 33.69°, 12/12 = 45.00°. Truss drawings often print degrees while the job site speaks X/12, and this one formula is the whole translation layer between them — a phone calculator and ten seconds catch any transcription error between plan and order sheet.

Span is the full width of the building, wall to wall. Run is half of it — the horizontal distance from wall plate to ridge. Every truss geometry formula uses the run, and every input field on this site asks for the span, which is exactly why the error is so common: a builder who enters half the building width as 'span' gets a truss for a building half as wide, with every length halved. Rule of thumb: if the number you entered is the distance to the middle of the building, you have typed the run.

The plumb cut is the vertical cut at the ridge end of the top chord — plumb means truly vertical in the finished roof. Its angle equals the pitch angle measured from horizontal: 26.57° on a 6/12. On a framing square, set the square so the blade reads 12 on the run leg and the pitch value on the rise leg; the tongue side then lies along the plumb line. The mating cut at the heel is the seat cut (the level notch that bears on the wall plate), cut at the same pitch angle rotated 90° — together the two cuts are the birdsmouth that grips a rafter. On trusses the heel is usually a butt joint against the plate, so the seat cut belongs to stick framing, not truss work.

The top-chord length from wall plate to ridge does not; the rafter length to the fascia does. The two differ by the overhang measured along the slope: a 12 in. horizontal overhang on a 6/12 roof is 12 × 1.1180 = 13.4 in. of extra member. Truss manufacturers quote a 'truss span' plus an 'overhang' figure separately for exactly this reason — order by both, and keep the solver's two outputs distinct when comparing quotes.

Ridge height is (span ÷ 2) × pitch ÷ 12, and it drives three real constraints: zoning height limits (measured to the midpoint or average of the roof plane), interior volume (a 6/12 on 24 ft leaves a 6 ft ridge versus 4 ft at 4/12), and shipping (a 26.8 ft truss standing 6 ft tall ships on edge; the same truss at 12/12 stands 12 ft and may travel on a taller trailer with permits). Steepening the pitch multiplies height linearly — every 2/12 added on a 24 ft span buys exactly 2 ft of ridge.