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Rafter Length Calculator

Common rafter length from building span, pitch and overhang.

How it's calculated

Run = half the span, less half the ridge thickness. Rise = run × pitch ÷ 12. Rafter length = run × √(1 + (pitch ÷ 12)²), and the overhang is measured the same way along the slope. Buy the next longer stock length to leave room for the plumb cut.

Example

A 24 ft span at 6/12 with a 1½-inch ridge: run 143.25 in, length 13.34 ft, plus a 1.12 ft overhang = 14.46 ft, so buy 16-footers.

Rafter length per foot of run

PitchLength per ft of run
4/1212.65 in
5/1213.00 in
6/1213.42 in
7/1213.89 in
8/1214.42 in
9/1215.00 in
10/1215.62 in
12/1216.97 in

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