Length is along-the-slope from ridge to overhang tip. Subtract half ridge-board thickness for actual cut. Verify rafter SIZE with engineered plans or AWC tables.
About this calculator
This rafter length calculator gives you the cut length for a common rafter on a gable roof based on building width, roof pitch, and eave overhang. Run is half the building width (rafter spans from outside the wall plate to the ridge). Rise is run × pitch/12. The diagonal length (ridge length minus 1/2 ridge board thickness, but uncorrected here) is the Pythagorean √(run² + rise²) — this calculator returns the total along-the-slope length including the eave overhang, which is the length to mark on the rafter stock. You still need to subtract half the ridge board thickness from the upper end for a precise plumb cut, and lay out the bird's-mouth seat cut at the wall. ESTIMATE ONLY — verify with a stamped roof plan; structural rafter sizing depends on snow load and ceiling joist tie capacity.
Multiply the run (half the building width) by the pitch factor √(pitch² + 144) ÷ 12. For a 24-ft-wide building with 6/12 pitch, run is 12 ft and the pitch factor is √(36+144)/12 ≈ 1.118, so the slope length from ridge to wall is 12 × 1.118 ≈ 13.4 ft. Add the eave overhang along the slope to get the total rafter length to cut. Always verify against the architect or engineer's roof plan.
Do I subtract the ridge board thickness?
Yes — at the upper end of the rafter, you cut a plumb cut that lands flush against the ridge board. The actual horizontal run of the rafter is half the building width minus half the ridge board thickness (typically 3/4" for a 1-1/2" ridge). For a 6/12 pitch that shaves about 0.84" off the slope length per rafter — small, but real. This calculator returns the un-corrected length; subtract the ridge offset before final cut.
What size rafter do I need for a 24-foot-wide house?
This calculator gives length only, not size. Rafter size depends on snow load, lumber species/grade, spacing, and whether the ceiling joists tie the rafters together. AWC and IRC R802.5 span tables are the right reference. For 24-ft buildings under 30 psf ground snow with 16" o.c. spacing, 2x8 #2 SPF rafters are typical, but 2x10 or 2x12 are needed in heavy-snow regions or with cathedral ceilings. Always verify with engineered plans.