Rafter Calculator
Plan common-rafter reference geometry from a symmetric gable span or a single-slope run.
Geometry Tool 1 of 6 · Output: Combined reference line
2Calculate & check
Enter measurements to see the result and its calculation schedule.
3Limits
These are geometric reference lengths, not finished cut lengths or lumber purchase specifications. Birdsmouth, member depth, bearing, fascia, ridge construction, connections and fabrication allowances are excluded. This tool does not size rafters, calculate loads or allowable spans, or solve hip and valley rafters.
How to use this calculator
- Choose a symmetric gable or a single-slope roof. Select your measurement units.
- For a gable, enter the outside-wall span and actual ridge thickness. For a single slope, enter the horizontal run directly.
- Enter the pitch and horizontal eave overhang. Enter zero explicitly if no ridge thickness or overhang is being allowed for.
- Calculate and read the wall-to-ridge length separately from the eave extension.
Formula & method
S is the outside-wall span; t is actual ridge thickness; O is horizontal overhang; p is rise per 12 horizontal units. Single-slope mode takes R directly. The gable calculation ends at the ridge face, not its centreline.
A worked example
Example inputs, not recommendations
For a 20 ft span, a 1.5 in ridge, a 6:12 pitch and a 1 ft horizontal overhang, effective run is 9.9375 ft. Rise is 4.96875 ft and the combined reference line is approximately 12.2285 ft.
Assumptions & limitations
Assumes
- The gable is symmetric with a centred ridge, or the entered single-slope run describes one straight segment.
- The eave extension continues at the same slope. Dimensions refer to the labelled reference line.
Does not cover
These are geometric reference lengths, not finished cut lengths or lumber purchase specifications. Birdsmouth, member depth, bearing, fascia, ridge construction, connections and fabrication allowances are excluded. This tool does not size rafters, calculate loads or allowable spans, or solve hip and valley rafters.
Geometric planning estimate. This does not size structural members or verify loads, connections, or code compliance. Verify structural and code requirements against local code or with a qualified professional.
Displayed values are rounded; subsequent calculations retain full precision. Numerical limits and accepted formats are documented in Calculation methods.
Common questions
Is span the same as run?
No. Span is the full outside-wall width of the symmetric gable. Effective run here is half the span after subtracting actual ridge thickness.
Why subtract ridge thickness?
A centreline-to-wall triangle would use half the span. This worksheet instead places the upper endpoint at the ridge face, reducing the horizontal run by half the ridge thickness.
Can I cut a rafter to the combined length?
Do not treat this reference line as a cut instruction. Cut geometry depends on member depth, bearing details and other construction decisions excluded from this calculation.