Calculate rafter length from rise, run and building span — includes overhang and birdsmouth options.
Calculate rafter length from rise, run and building span — includes overhang and birdsmouth options.
Enter values above and click Calculate — results will appear here with the formula explained.
Rafter length is the hypotenuse scaled to your building's half-span, plus overhang. For a 24-foot span, half is 12 feet (144 inches) of run.
Add overhang after scaling — it extends beyond the wall, not part of the triangle.
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Rafter Length Calculator: Calculate rafter length from rise, run and building span — includes overhang and birdsmouth options. Formula: Rafter = sqrt(rise²+run²) × (span/2 / run) + overhang. Example: 6:12 pitch, 24-foot span, 12-inch overhang: rafter ≈ 13.
Rafter length is the hypotenuse scaled to your building's half-span, plus overhang. For a 24-foot span, half is 12 feet (144 inches) of run.
Add overhang after scaling — it extends beyond the wall, not part of the triangle.
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6:12 pitch, 24-foot span, 12-inch overhang: rafter ≈ 13.42 feet + 1 foot = 14.42 feet.
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