Rafter Length Calculator

What this rafter length calculator does

This calculator estimates common rafter length for a simple gable roof using practical framing inputs such as roof pitch, full span, rise and run, or roof angle. It also adds the rafter tail from horizontal overhang and can apply a ridge deduction so the cut length better matches how many site-built rafters are actually measured.

How rafter length is calculated

Common rafter length comes from the right triangle formed by the rise and run of one roof side. Once the rise and common run are known, the main rafter length is the hypotenuse of that triangle. The overhang is then added along the same roof slope, so the tail is longer than the horizontal overhang distance by itself. If a ridge board is used, one side of the roof usually deducts half the ridge thickness from the run.

Why ridge deduction matters

Without ridge deduction, the calculated length reaches the centerline of the roof. In real framing, each common rafter typically lands against a ridge board or ridge beam, so the working run for one side is shortened by half the ridge thickness. That small adjustment matters when you want a cleaner cut list and a closer field estimate.

Planning note

Use the results here as a framing estimate for common rafters. Final birdsmouth depth, heel height, ridge details, seat cut layout, and code-required bearing should still be checked against the plans, structural requirements, and local code before cutting lumber.