Rafter Length Calculator — Span, Pitch & Overhang
Who this is for: For framing crews and owner-builders laying out a gable roof — how long a 2×10 or 2×12 rafter stock you need before the lumber yard asks.
Enter building span, pitch, and overhang — get the pitched rafter length per side, the run it covers, and a cutting allowance note.
Quick answer: Rafter length = (half-span + overhang) × pitch multiplier. A 24-ft gable at 6/12 with 1-ft eaves → 14.5-ft rafters (order 16-ft stock). Ridge height = half-span × rise/12.
Wall to wall, outside faces
6 for a 6/12 roof
12-24 in typical eaves
| Formula | length = (span/2 + overhang) × √(1+(rise/12)²) |
|---|---|
| Example | 24' span, 6/12, 1' overhang → 14.53 ft |
| Stock | Order next standard length (14/16/18 ft) |
| Ridge height | (span ÷ 2) × rise ÷ 12 |
How is rafter length calculated?
A gable rafter runs from the ridge board down the slope to the wall plate, plus the overhang past the wall. The calculator takes the horizontal run (half the building span, plus the overhang projected on the slope) and multiplies by the pitch multiplier — the same √(1+(rise/12)²) factor as the pitch calculator — because the rafter is the hypotenuse of the run-rise triangle. Example: a 24-foot span at 6/12 with a 1-foot overhang gives a 13-foot run and a 14.53-foot rafter, which is why 16-foot stock covers most single-story gables. Add roughly a rafter-depth allowance for the plumb cut and birdsmouth, and lay out from the actual top plates — this number is for ordering and planning, not a cut list.
Common uses
- Ordering rafter stock length before framing a gable
- Checking whether shed or porch rafters fit standard lengths
- Estimating ridge height from span and pitch
- Sanity-checking a truss quote against stick framing