How Rafter Length Is Calculated
Rafter length is found using the Pythagorean theorem, treating the run, rise, and rafter as a right triangle. The rise is determined by your roof pitch, expressed as inches of rise per 12 inches of run. Once rise and run are known, the rafter length is the hypotenuse of that triangle.
This calculator also adds your overhang to the run before calculating total rafter length, since the rafter needs to extend past the wall plate to form the eave.
What Is a Birdsmouth Cut?
A birdsmouth is the notch cut into the underside of a rafter where it meets the wall top plate. It lets the rafter sit flush against both the plate and the fascia. The seat cut angle shown above matches your roof's pitch angle, which is the correct angle for that notch.
Common Roof Pitches
- 3/12 to 4/12: Low slope, common on ranch style homes and additions
- 5/12 to 7/12: Standard slope for most residential construction
- 8/12 to 12/12: Steep slope, common in colder climates for snow shedding
- Above 12/12: Very steep, often seen on A-frame or chalet style structures
Why Rafter Length Matters
Getting rafter length wrong wastes lumber and throws off your ridge line. Cutting rafters even slightly short means the roof deck won't reach the ridge properly, and cutting them long means trimming on site, which slows down a framing job. This calculator gives you the exact theoretical length so you can mark and cut with confidence, then verify against your actual ridge board thickness before making final cuts.