Rafter Calculator – How Long Are My Rafters? | CalcLoft
Find out exactly how long your rafters are for any building width and pitch.
How to find rafter length
The rafter is the hypotenuse of a right triangle. For a 24 ft building, the run is 12 ft (half the span). At 4/12 pitch, the rise is 12 × 4/12 = 4 ft. Length = √(12² + 4²) = 12.65 ft, plus overhang.
Formula: rafter = √(run² + rise²) + overhang
Rafter length by span (4/12 pitch)
| Building width | Run | Rafter (no overhang) |
|---|---|---|
| 16 ft | 8 ft | 8.43 ft |
| 20 ft | 10 ft | 10.54 ft |
| 24 ft | 12 ft | 12.65 ft |
| 28 ft | 14 ft | 14.76 ft |
| 32 ft | 16 ft | 16.86 ft |
Number of rafters
Divide the roof length by the spacing and add one: a 30 ft roof at 24 in spacing needs about 16 rafters per side. Buy a few extra for cuts and mistakes.
Rafter calculator FAQ
How long is a rafter for a 24 ft wide building?
At 4/12 pitch: about 12.65 ft plus your overhang.
What is roof run?
Run is half the building width — from the outside wall to the ridge.
Do I need a ridge beam?
Rafters either bear on a ridge board (stick framing) or are pre-engineered trusses. Follow your local code and plans.
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