Rafter Length Calculator
Rafter length, rise, angle, and hip rafter — with roof diagram
Common Roof Pitches:
Roof Dimensions
Full width of building (wall to wall)
e.g. 6 for 6/12 pitch
Typically 1.5" for 2x lumber
Roof Framing Diagram
Cross-section showing rafter, rise, run, overhang, and ridge
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Frequently Asked Questions
How do I calculate rafter length?
Use the Pythagorean theorem: Rafter = √(Run² + Rise²). Run = half the building span minus half the ridge board thickness. Rise = Run × (pitch rise / 12). Add overhang to the run for total rafter length.
What is a 6/12 roof pitch?
A 6/12 pitch rises 6 inches for every 12 inches of horizontal run. The angle is arctan(6/12) = 26.57°. This is one of the most common residential pitches.
What roof pitch presets are standard?
3/12 (14.0°) is low/flat. 4/12 (18.4°) is low slope. 6/12 (26.6°) is medium — most common. 8/12 (33.7°) is steep. 12/12 (45°) is very steep. Pitches below 4/12 need special waterproofing.
What is a birdsmouth cut?
A birdsmouth is a triangular notch cut into the rafter bottom where it rests on the wall plate. It consists of a plumb cut (vertical) and a seat cut (horizontal). The seat cut depth should not exceed 1/3 of the rafter depth.
How do I calculate hip rafter length?
A hip rafter runs diagonally from the ridge to the corner of the building. Its run = √(common_run² + common_run²) = common_run × √2 ≈ 1.414 × common_run. Then apply the Pythagorean theorem with the same rise.
What is the rafter length multiplier?
Each pitch has a multiplier: multiply the run by this factor to get rafter length. 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414. These are derived from √(1 + (pitch/12)²).