Enter your building’s span, roof pitch, overhang, and rafter spacing to get each common rafter’s cut length and how many rafters the roof needs. Works for sheds, garages, and houses alike.
How to use this calculator
- Enter the building width (span) in feet and the roof pitch as rise per 12 (e.g. 6 for 6/12).
- Add the eave overhang in inches and pick your rafter spacing (16 in on center is typical).
- You get the rafter length in feet and inches plus the total rafter count.
The math behind it
Half-span run = span ÷ 2. Rise = run × pitch ÷ 12. Rafter length = √(run² + rise²), extended by the overhang along the same slope. Count = ceiling(span in inches ÷ spacing) + 1. The plumb-cut angle equals the roof angle — find yours with our roof pitch calculator.
FAQs
How long should a rafter be for a 24 ft span at 6/12?
About 14 ft 4 in per rafter with a 12-inch overhang — try the calculator above with your exact numbers.
Do I subtract the ridge board?
Yes — subtract half the ridge board’s thickness from the calculated length to get the true cut length.
Is this a structural approval?
No. This estimates lengths and counts; always verify spans against local code span tables and an engineer for structural work.
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