Rafter Length Calculator

Calculate rafter length, birdsmouth cut, and tail length from roof span, pitch, and overhang.

📐 Rafter Length Calculator

Results

Run11.94 ft
Rise5.97 ft
Rafter Length13.35 ft
Tail Length1.12 ft
Total Length14.46 ft
Birdsmouth Seat3.5 in

💡 How to Calculate Rafter Length

Rafter length is calculated from the building span, roof pitch, and overhang. The run = (span ÷ 2) minus half the ridge board thickness. The rise = run × (pitch ÷ 12). Rafter length = √(run² + rise²). Add the tail length for the overhang.

The birdsmouth cut notches the rafter to sit flat on the wall plate. The seat cut (horizontal) should be at least 1.5 inches for a 2×4 wall. The HAP (Height Above Plate) should be at least 3.5 inches to maintain structural strength. Use 2×8 or larger rafters.

24 ft span at 6/12 pitch with 12-inch overhang: run = 11.94 ft, rise = 5.97 ft. Rafter = 13.35 ft. Tail = 1.12 ft. Total = 14.47 ft — order 16 ft lumber.

Rafter Length Calculator FAQ

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