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Roof Pitch Calculator
Compute roof slope angle, pitch ratio and rafter length.
Formula
Pitch = rise / run ; angle = atan(rise/run)
How to use
- 1Measure the vertical rise from the top plate to the ridge.
- 2Measure the horizontal run (half the span for a symmetrical roof).
- 3Read the angle, ratio and rafter length.
About this calculator
The slope of a roof decides more than how it looks: it governs drainage, which materials you can use, loft headroom, and how long the rafters need to be. This calculator takes the roof’s rise and run and returns the pitch angle in degrees, the pitch ratio, and the exact rafter length.
Formula explained
Pitch angle = atan(rise ÷ run) in degrees. Pitch ratio = rise ÷ run, often quoted as “x in 12” (x units of rise per 12 of run). Rafter length = √(rise² + run²), since rise, run and rafter form a right triangle.
Worked example
atan(3 ÷ 6) = atan(0.5) ≈ 26.57°. Ratio 0.5 = 6 in 12. Rafter = √(3² + 6²) = √45 ≈ 6.71 m (before overhang).
Tips & notes
- Use half the span as the run for a standard gable roof.
- Add 600 to 900 mm of overhang to the rafter length before ordering timber.
- Confirm the minimum pitch for your roofing material; 22 to 30° suits most climates.