Calculators · Carpenter
Rafter Length and Roof Pitch Calculator
Short answer
A common rafter is the hypotenuse of a right triangle: its length is the run times the slope factor, where the slope factor is √(1 + (rise ÷ run)²). For a 4,000 mm run at 500 mm rise per 1,000 mm, the factor is 1.118 and the rafter is about 4,472 mm before overhang, measured along the line length.
- Slope factor
- 1.118
- Roof angle
- 26.6°
- Rafter line length
- 4,472 mm
- Including overhang
- 4,975 mm
Formula
Factor = √(1 + (rise ÷ 1000)²) · Rafter = Run × Factor
Worked example
Run 4,000 mm, 500/1000 slope: factor √(1 + 0.25) = 1.1180, rafter 4,472 mm; with a 450 mm overhang, 4,975 mm.
Before you rely on it
- Line length runs to the ridge centre line; deduct half the ridge thickness (measured along the slope) for the cut length.
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