Rafter estimating uses a right-triangle relationship between run, rise, and rafter length. The details change with roof type, overhangs, ridge thickness, cuts, and layout.
Use Rise and Run
For a simple common rafter estimate, square the rise and run, add them together, then take the square root.
This gives the diagonal length before adjustments such as overhangs, ridge details, birdsmouth cuts, and trimming.
Keep It an Estimate
Roof framing is sensitive to pitch, spans, loads, material, connection details, and local requirements.
Use early calculations for planning, then verify the actual design before buying or building.
Rafter Geometry Formula
For a simple common rafter, run is usually half the span. With pitch as rise per 12, rise = run x pitch_rise / 12.
Rafter length = square root of (run squared + rise squared). If using roof angle, rise = run x tan(angle), and rafter length = run / cos(angle).
Worked Example
For a 6 m span, the run is 3 m. With a 6-in-12 pitch, the roof angle is about 26.57 degrees and the rise is about 1.5 m.
The simple rafter length before overhang is about 3.35 m. Overhangs, ridge details, birdsmouth cuts, and trimming need separate checks.
Common Mistakes
Do not use the full building width as the run for a simple symmetrical common rafter.
Do not treat a geometry estimate as structural design. Loads, spans, bracing, tie-downs, wind, snow, and local rules need separate verification.
FAQs
Does roof pitch affect rafter length?
Yes. A steeper pitch increases rise over the same run, which increases rafter length.
Does a rafter calculator replace structural design?
No. It provides estimates for planning and measurement checks only.
Estimates only
These results are estimates only. Verify measurements, material specifications, structural requirements, safety requirements, and local building rules before buying materials or building.