Rafter Length Calculator

The span is the full width the roof crosses — for a simple gable, the outside-to-outside width of the building. The run is the horizontal distance one rafter travels, which on a symmetrical gable is exactly half the span. A 24 ft wide house has a 24 ft span and a 12 ft run per rafter. Pitch is always expressed against the run, never the span, so a 6/12 roof rises 6 inches for every 12 inches of run. If your ridge sits off-center, measure each side separately and calculate the two rafters independently.

The theoretical rafter runs from the outside of the wall plate to the centerline of the ridge. A real rafter stops at the face of the ridge board, so half its thickness has to come off — and it comes off measured along the slope, not horizontally. For a 2x ridge (1½ inches actual) that is ¾ inch horizontally, which on a 6/12 pitch becomes about 0.84 inch along the rafter. Skip the deduction and every pair of rafters pushes the ridge up slightly and spreads the wall plates. For a 1¾-inch LVL ridge, enter 1.75.

A birdsmouth is the notch cut near the lower end of the rafter so it seats flat on the top plate — a horizontal "seat" cut and a vertical "heel" cut. Building code limits how deep it can go: at least two-thirds of the rafter depth must remain, so on a 2x8 (7¼ inches actual) the seat cut should not exceed roughly 2⅜ inches. This calculator gives the length along the slope from the plate line up to the ridge, plus the overhang. You lay the birdsmouth out on that line — it does not change the number.

Most American houses fall between 4/12 and 9/12. The 4/12 to 6/12 band is the sweet spot for asphalt shingles: it sheds water well and is still walkable. From 7/12 (30.3°) up, crews normally need roof jacks or staging to work safely. 2/12 is the absolute minimum for asphalt shingles and only with doubled underlayment; below that you need a membrane roof. Steeper costs more both ways — a 12/12 roof has 41% more surface than its footprint, versus about 12% for a 6/12 — and steep-slope labor typically adds 25–50%.

No. This gives you a cut length, not a structural size. Whether a 2x8 or a 2x10 carries your span depends on lumber species and grade, rafter spacing, snow and wind loads, and whether the ceiling below is finished — all set out in the IRC rafter span tables and your local amendments. As a rough feel, No. 2 southern pine 2x8s at 16 inches on center often reach the mid-teens in feet under a 30 psf snow load. Treat everything here as a planning estimate and have a licensed builder or engineer confirm the member size before you cut.