Rafter Length Calculator
Estimate a straight common rafter from horizontal run, roof rise and horizontal overhang.
Formula
Length = (run + overhang) × √(1 + (rise/run)²).
Horizontal run to ridge: 12 ft · Rise over that run: 6 ft · Horizontal overhang: 1 ft · Common rafter length ≈ ((12 ft × 0.3048 m/ft)+(1 ft × 0.3048 m/ft))×sqrt(1+((6 ft × 0.3048 m/ft)÷(12 ft × 0.3048 m/ft))^2) · Length in feet ≈ (4.430097877)÷0.3048 · Roof angle ≈ atan((6 ft × 0.3048 m/ft)÷(12 ft × 0.3048 m/ft))×180÷pi
How to use
- Enter wall-to-ridge horizontal run and rise.
- Add horizontal eave overhang.
- Read theoretical rafter length before cut allowances.
Examples
- Run 4 m, rise 3 m and overhang 0.8 m give 6 m.
Frequently asked questions
Is run the full roof span?
For a symmetric gable roof, run is half the span.
Are ridge and seat cuts included?
No. Ridge thickness, birdsmouth, cuts and stock allowance are excluded.
Does this size the timber?
No. It gives geometry only, not structural capacity.
Sources
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