How this calc works

Rafter length is a right-triangle problem: the run is the horizontal distance from the wall to the ridge, the rise comes from the pitch, and the rafter is the hypotenuse. This calc takes your span and pitch and returns the line length of the common rafter — the cut length before you account for the ridge thickness and overhang.

Pitch is the rise over 12 inches of run — an 8/12 means 8 inches of rise per foot. The steeper the pitch, the longer the rafter for the same span, which is the same reason a steep roof has more surface area and more squares. Work in feet-inches-fractions and the answer comes back the way your tape reads it.

Common mistakes

The recurring error is cutting to the line length and forgetting the ridge adjustment — you subtract half the ridge thickness from each rafter, or the assembled ridge ends up wide by the full board.

The second is forgetting the overhang. The line length runs wall-to-ridge; if you want a tail past the wall, add the overhang length and remember the seat cut at the birdsmouth still lands on the wall plate.

RELATED FORMULAS