Wood & Trim
Rafter Length Calculator
Estimate common-rafter slope length from span, ridge, overhang and pitch.
Enter your measurements
Results update as you type.
About this calculator
Enter the building span, ridge thickness, overhang and roof pitch (X:12). The tool computes the horizontal run and the sloped rafter length, with optional overhang. It is a geometric estimate, not a structural design.
How to use this calculator
- Measure the wall-to-wall span and the ridge board thickness.
- Enter the pitch as X:12.
- Add the horizontal overhang if the rafter tails extend past the wall.
- Calculate for run, rise and rafter length.
Formula
Adjusted run = (span − ridge) ÷ 2
Slope factor = √(1 + (pitch ÷ 12)²)
Rafter length = (adjusted run + overhang) × slope factor
Rise = adjusted run × pitch ÷ 12
Assumptions
- A symmetric gable roof; the ridge is centered.
- Ridge thickness must be less than the span.
- Overhang is measured horizontally, then extended along the same slope.
Common mistakes
- Confusing full building span with the half-span run.
- Entering sloped overhang where horizontal overhang is requested.
- Treating geometric length as a final structural cutting schedule.
Worked example
Try it: press “Load example” above the form.
A 24 ft span with a 1.5 in ridge and 6:12 pitch gives an 11.9375 ft run; with a 1 ft horizontal overhang, the rafter is about 14.46 ft.
Limitations
No lumber sizing, structural design or code compliance. Geometry only — consult a professional for framing.
Frequently asked questions
Where is the horizontal run measured?
Measure the horizontal distance from the relevant support line to the ridge reference in the stated geometry. Follow the diagram and keep any horizontal overhang separate.
Does rafter length include cuts and structural allowances?
The result includes only the ridge-board thickness adjustment and horizontal overhang you enter in the stated symmetric-roof geometry. It does not design birdsmouth or end cuts, bearing, fasteners, timber section or structural span; it is not a fabrication cut length.
What does the worked example show?
A 24 ft span with a 1.5 in ridge and 6:12 pitch gives an 11.9375 ft run; with a 1 ft horizontal overhang, the rafter is about 14.46 ft.
Which assumptions should I check before using the result?
No lumber sizing, structural design or code compliance. Geometry only — consult a professional for framing.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
Calculation documentation · Updated 2026-11-11 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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