ft
Total width — run = span ÷ 2Enter span (4–120 ft)Rise inches per 12 in of run
in
Typical: 12–24 inchesCommon Rafter Length
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How to Calculate Rafter Length
The rafter is the hypotenuse of a right triangle where the legs are the run (horizontal, = half the span minus half the ridge) and the rise (vertical, = run × pitch/12).
Run = (Span ÷ 2) − (Ridge ÷ 2)
Rise = Run × (Pitch ÷ 12)
Rafter Length = √(Run² + Rise²) + Overhang along slope
Overhang along slope = (overhang_in / 12) × rafter_factor, where rafter_factor = √(pitch² + 144) / 12.
💡 Bird's mouth depth: The seat cut (notch on top plate) should not exceed ⅓ of rafter depth. For 2×8 (7.25" actual) max seat cut ≈ 2.4".
FAQ
Place a level horizontally on the roof, mark 12 inches along it, then measure straight down to the roof surface. That vertical distance is your pitch number. A 6/12 pitch rises 6 inches for every 12 inches of horizontal run.
The plumb cut angle = arctan(pitch/12). For 8/12: arctan(8/12) ≈ 33.7°. Set your circular saw bevel to this angle. The seat cut (bird's mouth) is perpendicular to the plumb cut.
Use IRC span tables based on span, spacing (12/16/24" OC), species, grade, and snow load. Rough guide at 16" OC: 2×6 up to ~12 ft run, 2×8 to ~14 ft, 2×10 to ~18 ft, 2×12 to ~22 ft. Always verify with your local building department.
A common rafter runs perpendicular from the wall plate to the ridge board — it's the standard rafter this calculator computes. A hip rafter runs at a 45° angle from a corner of the building to the ridge, forming the diagonal edge of a hip roof. Hip rafters are longer and cut at compound angles. Valley rafters are similar but form the inside corner where two roof planes meet.
The bird's mouth is a notch cut into the rafter where it sits on the top plate of the wall. It consists of a plumb cut (vertical, parallel to the rise) and a seat cut (horizontal, parallel to the run). The seat cut depth should not exceed ⅓ of the rafter depth. Use a framing square to lay out: set the pitch number on the tongue and 12 on the blade, mark along the tongue for the plumb cut and along the blade for the seat cut.
For standard residential roofs using prescriptive IRC span tables, a structural engineer is not typically required. However, an engineer is recommended (and sometimes required by local code) for: spans exceeding IRC table limits, unusual loads (heavy tile roofing, large snow loads), complex roof shapes, or modified/non-standard framing. Always verify with your local building department before starting.
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