Rafter Length Calculator
Calculate exact rafter length from run and rise measurements. Includes overhang, plumb cut angle, and seat cut angle for framing.
Dimensions
Half of total span
Ridge height above top plate
Eave extension beyond wall
Rafter Length
14.53 ft
hypotenuse + overhang
Roof Angle
26.6°
3.5:12 pitch approx
Overhang Length
1.12 ft
along rafter slope
Cutting Info
- • Order rafters at least 15' long (round up for safety)
- • Main rafter length (run to ridge): 13.42 ft
- • Plumb cut angle (top, where rafter meets ridge): 26.6°
- • Seat cut angle (bottom, where rafter meets wall): 63.4°
💡 Formula: Rafter length = √(rise² + run²) — the Pythagorean theorem applied to roof framing.
💡 Ridge board allowance: Subtract half the ridge board thickness from the rafter length for an accurate cut (typically 0.75-1.5 inches).
Understanding Rafter Calculations
A rafter is the structural beam that forms the slope of a pitched roof. Calculating rafter length accurately is critical for proper framing.
The calculation uses the Pythagorean theorem: rafter length = √(rise² + run²). The run is the horizontal distance from the wall to the center of the roof (half of total span). The rise is the vertical height from the top plate to the ridge board.
Plumb cut is the angled cut at the top where the rafter meets the ridge board.Seat cut (or birdsmouth) is the notch at the bottom where the rafter sits on the wall plate.