Rafter Length Calculator
Updated Formulas shown
A 24 ft span at 6/12 with a 1½ in ridge and 12 in overhang needs rafters about 14.5 ft long — buy 16 ft boards.
Rafter length
14.46 ft
Buy 16 ft boards
- Run (half span − half ridge)
- 11.938 ft
- Main rafter (to wall)
- 13.35 ft
- Tail (overhang)
- 1.12 ft
- Total rise
- 5.97 ft
- Plumb cut angle
- 26.6°
- Seat cut angle
- 63.4°
Rafter length = run × pitch factor. Measure along the top edge; cut lumber 1–2 ft longer than needed.
The formula
Run = span ÷ 2 − half the ridge thickness. Rafter length = run × √(rise² + 12²) ÷ 12, plus the overhang × the same factor.
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Frequently asked questions
What size lumber for rafters?
Depends on span, spacing, snow load and species — use your local code's span tables. 2×8 at 16 in on center is common for moderate spans.
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Estimates only. Check quantities with your supplier or contractor before ordering.