Use the Rafter Calculator Canada | Roof Rafter Length
Enter your values below. The calculator runs in your browser and shows an estimate immediately.
How the estimate is calculated
Rafter length = √(run² + rise²), plus the slope length of the overhang
Example: A 12 ft run and 6 ft rise give a base rafter length of about 13.42 ft. A 12 in horizontal overhang adds about 1.12 ft along the slope.
When this calculator is useful
A geometric length estimate can help you prepare a cut list or check a roof sketch before discussing it with a builder. For a symmetrical gable, use half the full building span as the run. For other roof shapes, measure the actual horizontal projection for the rafter you are checking.
How to use this calculator
Measure run and rise in the same unit. For a standard symmetrical gable, the run is half the outside wall-to-wall span. The calculator adds the optional overhang along the roof slope and reports feet and inches.
What to keep in mind
This is a geometric estimate, not a framing plan. It does not account for ridge-board thickness, birdsmouth cuts, seat cuts, fascia details, lumber deflection, snow loads, wind loads or local building-code requirements. Have structural sizing checked by a qualified professional.
Frequently asked questions
What is the run for a gable roof?
It is the horizontal distance from the building’s outside wall line to the ridge line—usually half the building span for a symmetrical roof.
Does this include the overhang?
Yes. Enter the horizontal overhang in inches and it is added along the roof slope.
Does this size the lumber?
No. It estimates geometric length only. Rafter size and spacing depend on span, species, grade, snow load and code requirements.
Can I use inches instead of feet?
The run and rise fields are in feet. Convert inches to feet before entering them; the overhang field accepts inches.