Rafter Length Calculator
Calculate common, hip, valley, and jack rafter lengths for any roof pitch and span.
Get the exact cut length including ridge and wall plate deductions.
What Is a Common Rafter? A common rafter runs from the ridge board at the roof’s peak down to the top plate (cap plate) of the exterior wall. It is the most frequently cut rafter in a gable roof. The rafter has two critical cuts: a plumb cut (vertical) at the ridge board end, and a bird’s-mouth cut (a notch) where it sits on the wall plate.
The Pythagorean Foundation Rafter length calculation is pure Pythagorean theorem. The run (half the building width) is one leg of the right triangle. The rise (run × pitch / 12) is the other leg. The rafter is the hypotenuse. For a 6:12 pitch on a 24-foot wide building: run = 12 ft, rise = 12 × 6/12 = 6 ft, rafter = √(12² + 6²) = √(180) = 13.42 ft.
The Rafter Multiplier Method Rather than recalculating the Pythagorean theorem every time, carpenters use a rafter multiplier (also called the rafter factor) — the hypotenuse length per foot of run. For 6:12 pitch, the multiplier is 1.118. Multiply your run (in feet) by 1.118 to get the theoretical rafter length. Speed squares have rafter tables printed on them listing these multipliers for every pitch from 1:12 to 24:12.
Ridge Deduction The rafter doesn’t reach all the way to the centerline of the ridge — it meets the side of the ridge board. The ridge deduction is: half the ridge board thickness divided by the run, multiplied by the rafter unit length. For a 1.5-inch ridge board (standard 2×8 or 2×10 lumber is actually 1.5 inches thick), the deduction is (0.75 / 12) × rafter_multiplier × 12 — a small but important measurement for perfect fit.
Overhang Addition The overhang (soffit) extends the rafter beyond the wall plate to create the eave. Add the overhang horizontal distance (in feet) multiplied by the rafter multiplier to get the additional rafter length needed beyond the wall.
Always Add Cutting Allowance After calculating the theoretical length, always add 2–3 inches to allow for the actual saw cuts (plumb cut at ridge, bird’s mouth at wall). Buy lumber in standard lengths — 10, 12, 14, 16, 18, 20 feet — and choose the next length up from your calculated measurement.