Rafter length calculator
This is geometry — the length to cut a rafter over the run and pitch you enter, plus the overhang along the slope. It is not a span rating and does not say what size the rafter must be or what load it will carry; that belongs to your plans, a span book, or an engineer. Mark the ridge and seat cuts on the stock before you saw.
Enter the horizontal run and the roof pitch and the tool returns the sloping rafter length over that run, then extends it by your overhang along the same slope. Pitch is the rise per 12 of run, so the rise is simply the run scaled by pitch over twelve.
Worked example
A 12-foot run at a 6/12 pitch rises 6 feet, so the rafter body is √(12² + 6²) ≈ 13.42 ft at about 26.6° from horizontal. A 1-foot horizontal overhang stretches along the slope to 1 × (13.42 ÷ 12) ≈ 1.12 ft, so the whole rafter is roughly 14.54 ft before the cuts.
The formula
rise = run × (pitch ÷ 12) · rafter = √(run² + rise²) × (1 + overhang ÷ run)
The body of the rafter is Pythagoras over the run and rise. Because the overhang sits on the same slope, it grows by the same run-to-rafter ratio, so multiplying the run length by that ratio and adding it gives the full cut length.
Questions
- How do I work out a common rafter length?
- The rafter is the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise. Its length over the run is the square root of run squared plus rise squared. For a 12-foot run at a 6/12 pitch the rise is 6 feet, so the rafter is √(144 + 36) ≈ 13.4 feet before the overhang.
- Is the run the same as the span?
- No. The span is the full distance the roof crosses; the run of a common rafter is the horizontal distance from the wall to the ridge, which for a symmetrical gable is half the span less half the ridge thickness. Enter the run, not the span, and this tool gives the sloping length over that run.
- Does this size the rafter or tell me the maximum span?
- No — it gives a cut length only, the same way a stair tool gives a stringer length. Whether a rafter of a given size can span a distance under snow, wind and its own weight is a structural question answered by span tables and an engineer, and it is deliberately not something this tool outputs.
- How do I add the overhang and the ridge cut?
- Enter the horizontal overhang and the tool extends the rafter along the slope by the same multiplier. It does not deduct for half the ridge thickness or add for the plumb and seat cuts — those are small adjustments you mark out on the stock once the overall length is known.
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