Rafter Length Calculator

The horizontal distance from the outside of the exterior wall to a point directly under the peak. On a standard gable roof that is half the building width, plus whatever overhang you are carrying past the wall. Run is measured flat, not along the slope, which is the distinction that trips people up.

Because the overhang adds to the run, and the rafter is the diagonal across that run. A bigger overhang does not just extend the eave, it lengthens the whole rafter, and on a steep roof it lengthens it by more than the overhang itself.

No. What you get is the theoretical length along the top edge, straight out of the Pythagorean theorem. Subtract half the ridge thickness if you are running a structural ridge beam. Birdsmouths and tail cuts come later, as layout off this base length, and they are worth marking on one rafter and test-fitting before you cut the rest.

Roof Pitch answers questions about the roof as a whole: the angle, the percentage grade, and the slope factor you need for ordering material. This one takes the same rise-and-run relationship and solves for a single rafter in feet and inches, which is the number you want standing at the saw.