How this calculator works
The rafter segment to the ridge is the hypotenuse of rise and horizontal run. Pitch mode supplies the rise as run times the rise-per-12 ratio. Eaves overhang is a horizontal projection, so its sloped length uses the same multiplier as the main segment. Enter a run already adjusted for the ridge board if that deduction applies. This calculation gives layout length, not member capacity.
Formula
segment = sqrt(run² + rise²); rise = run x pitch/12; total rafter = segment x (1 + horizontal overhang/run); pitch = 12 x rise/run.
Worked example
A 3 m rise over 4 m run gives a 5 m rafter before overhang and a 9:12 pitch. A 6:12 roof over 4 m run rises 2 m and has a 4.472 m rafter segment.
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Frequently asked questions
Should I enter full building width?
No. Run is the horizontal distance for one roof side. On a symmetric gable roof that is about half the building width after ridge adjustment.
Does the result include birdsmouth cuts?
The length follows the slope. Seat, heel, ridge and fascia cuts need their own layout; use the birdsmouth calculator for that geometry.
How do I count rafters or trusses?
Use the roof truss calculator for maximum spacing and whole frame count along the roof length.