Rafter Calculator
Plumb-to-plumb common rafter plus overhang — not a cutting list. Click any i for detail.
Roof geometry
Span (ft)
i
Span
Building width outside-to-outside. Run is span ÷ 2. |
|
|---|---|
Pitch (in 12)
i
Pitch
Inches of rise per 12 in of run (6 = 6/12). |
|
Overhang (in)
i
Overhang
Horizontal tail beyond the wall, in inches. |
How to use this calculator
- Enter the building span.
- Enter pitch as X-in-12.
- Add overhang in inches.
- Compare with the roof pitch calculator’s factor.
Results explained
Run = span/2. Rise = run × pitch/12. Common = √(run² + rise²). Total = common + overhang (in feet).
Quick reference: pitch
A 6/12 roof is about 26.6°.
| Item | Detail |
|---|---|
| 4/12 | ≈ 18.4° |
| 6/12 | ≈ 26.6° |
| 8/12 | ≈ 33.7° |
| Rafter factor 6/12 | 1.118 × run |
Hip rafters are longer; they are not this page.
How the estimate is built
run = span/2; rise = run×(pitch/12); length = √(run²+rise²) + overhang_ft.
Example scenario
24-ft span, 6/12, 12 in overhang → run 12, rise 6, common 13.42, total 14.42 ft.
FAQ
Ridge board?
Subtract half the ridge thickness from run if you want plumb-cut to the ridge face.
Birdsmouth?
Does not change the theoretical hypotenuse; it changes the seat cut.
Roof pitch only?
See the roof pitch calculator.