Find a roof's pitch, angle, and rafter length from either the rise and run or a standard pitch ratio.
How Roof Pitch Works
Roof pitch is expressed as inches of rise for every 12 inches of run, like a "6/12" roof. That ratio converts directly to an angle using arctan(rise/run), and rafter length comes from the Pythagorean theorem once you know the rise and run. Hip and valley rafters run at a steeper angle since they span the diagonal between two roof planes, so their length uses the same rise but a run stretched by √2.
Worked Example
A roof with 6 ft of rise over 12 ft of run is a 6/12 pitch, about 26.57 degrees, with a common rafter length of 13.42 ft. The hip or valley rafter for that same roof comes out to exactly 18.00 ft, since it spans the diagonal between two roof planes.
Common Mistakes
- Confusing pitch (rise per 12" of run) with angle in degrees; they're related but not the same number.
- Forgetting hip and valley rafters use a longer run than common rafters, since they travel diagonally.
- Not accounting for overhang length separately from the rafter length calculated to the ridge.
Frequently Asked Questions
- What's the difference between roof pitch and roof angle?
- Pitch is expressed as rise per 12 inches of run, like 6/12. Angle is the same slope expressed in degrees. This calculator converts between them.
- Does this rafter length include the overhang?
- No, it calculates rafter length from the ridge to the exterior wall plate. Add your overhang length separately.
- Why is the hip rafter longer than the common rafter?
- A hip rafter runs along the diagonal intersection of two roof planes, so its horizontal run is about 1.414 times longer than a common rafter's run.