Rafter Length Calculator — Common Rafter Size

How the rafter length calculator works

Rafter length = run × pitch factor, where the pitch factor is √(1 + (rise/12)²). A 6:12 roof has a factor of 1.118, a 12:12 roof is 1.414.

Run = 12 ft. Rafter = 12 × 1.118 = 13.4 ft, plus overhang.

angle = arctan(rise / 12). 4:12 = 18.4°, 6:12 = 26.6°, 8:12 = 33.7°, 12:12 = 45°.

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