Rafter Length Calculator
Calculate common rafter length from run and rise, with overhang adjustment.
Formula & Explanation
Rafter length is calculated using the Pythagorean theorem: Rafter Length = √(Run² + Rise²) For overhangs, the same proportions apply: Overhang Rise = Overhang Run × (Rise ÷ Run) Overhang Length = √(Overhang Run² + Overhang Rise²) Total Length = Rafter Length + Overhang Length Example: 20-ft run, 10-ft rise, 2-ft overhang - Rafter = √(400 + 100) = 22.36 ft - Overhang rise = 2 × (10÷20) = 1 ft - Overhang length = √(4 + 1) = 2.24 ft - Total = 22.36 + 2.24 = 24.6 ft For cutting, measure the actual rafter during installation using a framing square or angle gauge.
Assumptions
- •Uses Pythagorean theorem for straight-line distance (plumb cuts not included)
- •Overhang is estimated at standard 2 feet; adjust for custom overhangs
- •Does not account for bird's mouth cuts, tail cuts, ridge angle adjustments, or other framing details
- •Actual rafter length varies slightly due to cut angles and connection details
- •This is a preliminary estimate; measure on-site during framing
Example Calculation
A rafter with 20-ft run and 10-ft rise is 22.36 feet long; 24.60 feet with a 2-foot overhang.
Inputs
Results
Rafter Lengths by Pitch
| run | rise | rafterLength |
|---|---|---|
| 20 ft | 0 ft | 20.00 ft |
| 20 ft | 5 ft | 20.62 ft |
| 20 ft | 10 ft | 22.36 ft |
| 20 ft | 15 ft | 25.00 ft |
| 20 ft | 20 ft | 28.28 ft |
Frequently Asked Questions
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