Rafter Length Calculator for Common Rafters
Enter the span between the outside walls, the roof pitch and the horizontal overhang. The calculator gives the common rafter length along the slope, split into the part over the building and the overhang, plus the next standard board length. After switching units, re-enter your measurements.
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Formula
run = span / 2slope factor = sqrt(1 + (pitch / 12)^2)rafter over the building = run x slope factoroverhang length = horizontal overhang x slope factortotal = rafter over the building + overhang lengthstock length = total in feet, rounded up to the next even foot
Worked example
A 24 ft span, 6:12 pitch and 1 ft horizontal overhang:
- Run: 24 / 2 = 12 ft.
- Slope factor: sqrt(1 + 0.5^2) = 1.118034.
- Rafter over the building: 12 x 1.118034 = 13.42 ft.
- Overhang: 1 x 1.118034 = 1.12 ft.
- Total: 14.53 ft, so buy a 16 ft board. The roof angle is 26.57 degrees.
Assumptions
- A symmetric gable roof with common rafters and the span measured between outside wall faces.
- A ridge board thickness and the bird's-mouth cut are not deducted.
Frequently asked questions
How do you calculate common rafter length?
Take half the span as the run, then multiply by sqrt(1 + (pitch / 12)^2). For a 24 ft span at 6:12, the run is 12 ft and the rafter is 12 x 1.118 = 13.42 ft before any overhang.
How do I add the overhang to a rafter?
Measure the overhang horizontally and multiply by the same slope factor. A 1 ft overhang at 6:12 adds 1.118 ft, so the total is 13.42 + 1.12 = 14.53 ft.
What board length should I buy?
Lumber is sold in even-foot lengths, so the calculator rounds the total up to the next even foot. 14.53 ft means a 16 ft board. The plumb and bird's-mouth cuts and a ridge board can use some of the length, so check your framing plan.
Does this give the length for hip and valley rafters?
No. It is for common rafters that run square to the ridge. Hip and valley rafters are longer, because their run is the diagonal of the plan, and need a separate calculation. Check your plans and local rules before cutting.