Choose run or span.
Use a known horizontal run for direct calculation. Use span only when the ridge is centered and each side has half the entered horizontal span.
Estimate a common rafter’s geometric length from horizontal run, roof pitch and overhang. Use a known run or half the span of a symmetrical roof, with results in feet and meters.
This calculator processes inputs in your browser. It does not submit these measurements to a server.
Enter measurements to calculate.
These are geometric line lengths, not finished timber cutting dimensions. The overhang is assumed to continue at the same pitch as the main roof.
This tool does not calculate ridge-board deductions, birdsmouth cuts, fascia details, timber strength, allowable spans or structural compliance. Confirm framing and cutting dimensions from your project’s construction details.
Use a known horizontal run for direct calculation. Use span only when the ridge is centered and each side has half the entered horizontal span.
Enter the rise per 12 units of run. Add the horizontal overhang separately. Keep all length inputs in the selected unit.
Compare the main sloped length and the overhang extension. Use the total as a geometric starting point, then account for the actual framing details.
The horizontal run and vertical rise form two sides of a right triangle. The sloped rafter line is the hypotenuse. Pitch determines the ratio between rise and run.
In symmetrical-span mode, the calculator uses run = span ÷ 2. It assumes a centered ridge reference line and makes no ridge-thickness deduction.
For an offset ridge, unequal roof sides or known framing reference points, use the actual horizontal run instead.
Use a horizontal run of 12 ft, a pitch of 6:12, and a horizontal overhang of 1 ft.
The calculator uses unrounded values when adding lengths. The displayed total is not a finished cut length.
Understand run, span, overhang and the limits of a geometric length calculation.
Run is the horizontal distance associated with one sloped rafter line. Span can describe the full horizontal distance across a roof. For a symmetrical roof with a centered ridge, each side’s run is half the span.
No. The result is a geometric sloped line. Ridge details, bearing reference points, birdsmouth cuts, fascia cuts and other construction details can change the required cutting dimensions.
No. Symmetrical-span mode calculates to a centered reference line without subtracting ridge thickness. If you know the actual horizontal distance between your intended reference points, use horizontal-run mode.
Enter the horizontal overhang projection. The calculator applies the slope multiplier to find its sloped extension. Entering a sloped tail measurement would apply the adjustment twice.
Yes, for its straight common-rafter geometry. Choose horizontal-run mode and enter the horizontal distance between your reference points. Do not halve a shed roof’s run using symmetrical-span mode.
No. This calculator models a common rafter in one vertical plane. Hip and valley rafters have different plan geometry and require a calculation specific to their roof layout.
No. Length geometry does not determine timber size, spacing, load capacity or allowable span. Those decisions need the project’s structural requirements.
Existing valid length inputs are converted. Pitch remains unchanged because it is a ratio. Press Calculate to update the displayed results.