Rafter Length Calculator

Get precise rafter measurements for your roofing project. Enter building width, roof pitch, and overhang to calculate rafter length, pitch angle, rise, run, and hip rafter dimensions. Works with any common roof pitch from 2/12 to 24/12.

Total width of the building from outer wall to outer wall

Rise in inches per 12 inches of run. Common: 4/12 (low slope) to 12/12 (steep).

How far the rafter extends past the wall

Thickness of the ridge board (typically 1.5 in)

Rafter Length = √(Run² + Rise²)

Run = (Building Width × 12 − Ridge Thickness) ÷ 2
Rise = (Pitch ÷ 12) × Run

Pitch Angle = arctan(Rise ÷ Run)
Hip Rafter Length = Common Rafter × 1.4142
24-ft wide building, 6/12 pitch, 1-ft overhang, 1.5-in ridge: Run = (288-1.5)/2 = 143.25 in. Rise = (6/12)×143.25 = 71.6 in. Rafter = √(143.25²+71.6²) = 160.2 in = 13.35 ft. With overhang: total = 14.39 ft. Pitch angle = 26.6°.

How do I calculate rafter length?

Use the Pythagorean theorem: rafter length = √(run² + rise²). The run is half the building width minus half the ridge board thickness. The rise is (pitch / 12) × run. For example, a 24-ft wide building with 6/12 pitch has a run of 11.94 ft and rise of 5.97 ft, giving a rafter length of 13.35 ft before overhang.

What is the standard roof pitch for residential homes?

Most residential roofs use a pitch between 4/12 and 9/12. A 4/12 pitch (18.4°) is common for asphalt shingles and is considered the minimum for proper water runoff. A 6/12 pitch (26.6°) is the most common standard pitch. Pitches above 9/12 (36.9°) are considered steep and may require special safety equipment.

How does ridge board thickness affect rafter length?

The ridge board thickness reduces the run by half its thickness. For a 1.5-inch ridge board on a 24-ft wide building, the run is (288 - 1.5) / 2 = 143.25 inches instead of 144. This small reduction saves about 0.1 ft per rafter but matters for precise fit. Always account for the ridge board in your measurements.

What is the birdsmouth cut and why is it important?

A birdsmouth is a notch cut into the rafter where it sits on the top plate of the wall. It provides a stable bearing surface and transfers the roof load to the wall. The seat cut should not exceed 1/3 of the rafter depth. A 2×6 rafter (5.5 in deep) should have a seat cut of about 1.5 inches max for structural integrity.

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