Rafter Length Calculator

Common rafter length = √(run² + rise²) where run = half the building width and rise = run × (pitch/12). For a 28 ft wide building with 6/12 pitch: run = 14 ft, rise = 14 × 0.5 = 7 ft, rafter = √(14² + 7²) = √(196 + 49) = √245 ≈ 15.65 ft. Add overhang length to get the full rafter from ridge to tail.

A bird's mouth is the notch cut into the underside of a rafter where it sits on the wall plate. It has two cuts: a plumb cut (vertical, parallel to the rafter's plumb line) and a seat cut (horizontal, sitting on the top plate). The seat cut should be no deeper than 1/3 the rafter depth for structural integrity. The plumb cut depth equals the wall plate width (typically 3.5 inches for 2×4 walls).

A common rafter runs perpendicular from the ridge to the wall plate. A hip rafter runs diagonally from the ridge corner to the building corner at a 45° plan angle. Because it runs diagonally, it travels a longer horizontal distance (the diagonal of the run) and uses a different unit length per foot of run — 13.42 inches per foot instead of 12 inches. Hip rafters require compound angle cuts at both ends.

Rafter size depends on span, spacing, and load. General guidelines: spans under 10 ft use 2×6, 10–16 ft use 2×8, 16–20 ft use 2×10, over 20 ft use 2×12 or engineered lumber. These are general rules — always verify with span tables in your local building code (IRC Table R802.4) or consult a structural engineer for loads greater than standard residential.