Woodworking & Construction Calculators

Carpentry & framing

Roof Pitch & Rafter Length Calculator (Common Rafter)

Enter the building span, pitch as inches of rise in 12 in of run, ridge thickness and overhang. The tool returns the common-rafter triangle and the sloped overhang.

Inputs

Units
ft

Outside-to-outside width the pair of rafters covers.

Inches of rise per 12 in of run.

in
in

Results

Run
143.25in
Rise
71.63in
Pitch angle
26.6°
Common rafter
160.16in
Rafter with overhang
173.57in

Recommended gear

Calculation method

Each common rafter is a right triangle. Run is half the span minus half the ridge. Rise follows the pitch ratio. The hypotenuse is the birdsmouth-to-ridge length; the overhang is stretched by the same slope.

Exact formula

run = (span − ridge) / 2. rise = run × (pitch / 12). common rafter = √(rise² + run²). overhang along the slope = overhang × √(pitch² + 12²) / 12.

Pitch vs millimetres

Pitch stays rise-per-12 even in metric mode. Span, overhang and ridge convert; the 6/12 triangle does not become 6/300.

Frequently asked questions

Is 6/12 a 26.6° roof?

Yes. Angle = arctan(6/12). The toggle does not change pitch ratio — only the displayed lengths.

Does this size hip or valley rafters?

No. Those are longer and sit on a different plan angle. This tool is common rafters only.

Should I add a ridge allowance twice?

No. Ridge thickness is subtracted once from the span, then split between the two rafters.

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