About this calculator
Common rafter length from building span and pitch, with ridge height, plumb and seat cut angles, and a rafter count for the whole roof.
How it’s calculated
Run = half the span − half the ridge thickness. Rafter length = run ÷ cos(pitch). The overhang slope length = horizontal overhang ÷ cos(pitch). Plumb cut = pitch angle; seat cut = 90° − pitch.
Worked example
With these inputs:
- Building span (wall to wall)
- 24 ft
- Pitch entered as
- Rise per 12 (e.g. 6/12)
- Rise per 12
- 6
- Overhang (horizontal)
- 12 in
- Ridge board thickness
- 1.5 in
- Building length (for count)
- 40 ft
- Spacing (on centre)
- 16" OC
the result is:
- Rafter length (with overhang)
- 14' 5-9/16"
- Pitch
- 26.6° = 6/12
- Main rafter (ridge to wall)
- 13' 4-3/16"
- Overhang (slope length)
- 1' 1-7/16"
- Ridge height above wall plate
- 6'
Tips from the trade
- Measure the span outside to outside of the wall plates.
- Cut one pattern rafter, test it at both ends of the building, then gang-cut the rest.
- Rafter sizing (depth and grade) comes from span tables, not from this length.
Common questions
How do I convert 6/12 pitch to degrees?
6/12 = atan(6 ÷ 12) = 26.57°.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.