Rafter Length Calculator to calculate common-rafter length from roof rise, horizontal run, and optional overhang, with roof pitch, angle, and slope multiplier for framing and roofing estimates.
Home & Construction Calculators
Calculate common-rafter length, roof pitch, angle, and overhang extension.
Use the same unit for rise, run, and overhang. The result is the theoretical sloped length before ridge, birdsmouth, and end-cut allowances.
About the Rafter Length Calculator
Use this rafter length calculator for a common rafter on a simple roof plane. Enter the vertical rise and matching horizontal run using the same unit. Add an optional horizontal overhang to estimate the sloped tail extension. The result includes rafter length to the wall line, total length including overhang, pitch per 12, roof angle, and slope multiplier.
Formula and assumptions
Rafter to wall line = √(rise² + run²). Slope ratio = rise ÷ run. Pitch per 12 = slope ratio × 12. Roof angle = arctan(slope ratio). Slope multiplier = √(1 + slope ratio²). Sloped overhang extension = horizontal overhang × slope multiplier. Total theoretical rafter length = rafter to wall line + sloped overhang extension.
Worked example
With a 108-unit rise, 120-unit horizontal run, and 12-unit horizontal overhang, the rafter length to the wall line is about 161.44 units. The 10.80:12 pitch has an angle of about 41.99° and a slope multiplier of 1.345. The sloped overhang adds about 16.14 units, giving a total theoretical length of approximately 177.59 units.
Measurement checklist
- Measure rise vertically and run horizontally.
- Use half the building span for one side of a symmetrical gable roof.
- Measure overhang horizontally, not along the roof slope.
- Add separate allowances for ridge cuts, birdsmouth seats, fascia cuts, and field trimming.
Manual calculation steps
- Square the total rise.
- Square the horizontal run.
- Add those values and take the square root.
- Calculate the slope multiplier from rise ÷ run.
- Multiply horizontal overhang by the slope multiplier.
- Add the sloped overhang to the wall-line rafter length.
Frequently asked questions
Does this include the ridge cut and birdsmouth?
No. The result is a theoretical centerline length. Actual layout requires allowances for ridge thickness, plumb cuts, birdsmouth geometry, fascia details, and local framing practice.
Should the run be the full building width?
For a centered symmetrical gable roof, use half the building span as the horizontal run for one common rafter.
How is an overhang added?
Enter its horizontal projection. The calculator multiplies that projection by the roof slope multiplier to estimate the sloped tail length.
Common mistakes
- Measuring the run along the slope rather than horizontally.
- Using the full span instead of half-span for one roof side.
- Mixing inches, feet, centimeters, or meters.
- Cutting lumber without accounting for ridge and birdsmouth details.