Rafter Length Calculator — Pitch, Overhang & Ridge
ROOF GEOMETRY
Common rafter measurements
Ridge face to eave
15 ft 0 5/16 in, rounded to the nearest 1/16 in
Geometric line length, not a finished cutting length. Birdsmouth, board depth, fascia details and cutting allowance are not included.
- Wall to ridge face
- 13.347 ft
- Sloping tail
- 1.677 ft
- Ridge deduction
- 0.070 ft
- Roof angle
- 26.565°
24 ft span · 6:12 pitch · 1.5 ft horizontal overhang · 0.125 ft ridge thickness
See how roof pitch, a horizontal overhang and the ridge board change your measurements.
The diagram is a geometric section: it does not choose timber sizes, connections, roof covering or safe loads. Centreline rise and ridge-face rise are measured from the same reference line at the wall, not from the floor.
Try a worked example
Try a worked example
What the measurements mean
What the measurements mean
For a symmetrical gable, the horizontal run to the ridge face is half the building span minus half the ridge thickness. Multiply each horizontal length by √(1 + (pitch ÷ 12)²) to find its sloping length.
Johnson CALC-1500 manual, page 16 provides the 27 ft / 7:12 worked example.
Estimate the roof covering
Estimate the roof covering
Once you have the roof dimensions, use the roof shingle calculator to combine rectangular, triangular and trapezoidal planes, apply product coverage and waste, and estimate bundles. Enter either measured sloping dimensions or horizontal projections with pitch; do not apply the slope twice. Starter strips and ridge caps are separate linear quantities.
For the seat, heel and height above the wall plate, use the birdsmouth cut layout calculator. It measures the notch separately from the rafter line length.
Calculate common rafter line length with roof pitch, horizontal overhang and ridge-board deduction. View the section and print your measurements.
