Estimate common rafter length from horizontal run, rise and overhang.
Calculator
Rafter Geometry
Review the main result, supporting measurements, and calculation details.
Length Breakdown
Calculation Details
| Metric | Value | Note |
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How to calculate common rafter length
Enter the roof pitch as rise per 12 inches, the horizontal run from wall support to ridge, and the horizontal eave overhang. The calculator applies the same slope factor to the run and overhang.
slope factor = √(pitch² + 12²) / 12; rafter length = (run + horizontal overhang) x slope factor
Rafter-length example
For a 6:12 roof with a 12 ft horizontal run and 12 in horizontal overhang, the base rafter length is about 13.42 ft and the total sloped length is about 14.53 ft. The roof rise over the 12 ft run is 6 ft.
Horizontal versus sloped overhang
This calculator expects the overhang's horizontal projection. A 12 in horizontal overhang is longer than 12 in when measured along a sloped rafter. If your drawing already gives the tail length along the slope, do not enter that value as horizontal overhang.
Geometry is not structural sizing
The result is theoretical line length before ridge, birdsmouth, plumb-cut, seat-cut, fascia, and field-fitting adjustments. It does not select lumber size or verify loads. The American Wood Council span-calculator guidance explains that allowable spans depend on strength, stiffness, shear, bearing, species, grade, spacing, and loads.
Frequently asked questions
Do I subtract half the ridge-board thickness?
Often the theoretical run is adjusted for the ridge detail, but the exact layout depends on the plans and framing method. This calculator leaves that job-specific adjustment to the user.
Does the total linear footage include waste?
No. It multiplies the calculated length by the entered rafter count. Add an appropriate cutting and procurement allowance separately.