Rafter Calculator

Size your rafters. A 24 ft wide building with a 6/12 pitch and 1.5 ft overhang needs rafters 15.09 ft long.

How rafter length is calculated

A common rafter is the hypotenuse of a right triangle. The run is half the span (plus overhang) and the rise is run x pitch / 12. Length = run x the pitch multiplier, which is the square root of 1 + (pitch / 12) squared.

For a 24 ft span at 6/12: run = 12 ft, the multiplier is 1.118 and the rafter with a 1.5 ft overhang is 15.09 ft. The ridge sits 6 ft above the wall plate. At 24 inch spacing along a 30 ft building you need 32 rafters. Buy 16 ft boards.

Notes

Subtract half the ridge board thickness from the length.
Rafter size (2×6, 2×8…) depends on span and snow load. Use span tables or an engineer.
Cut a bird's mouth where the rafter sits on the wall plate.

See also the roof pitch calculator and roofing calculator.