Calculator
Rafter Calculator
Estimate common rafter length, pitch, rise, run, roof angle, and overhang geometry. Use it for early roof layout planning, lumber/timber takeoffs, and checking approximate material lengths before structural verification.
Rafter calculator
Common roof geometry planning only. This does not calculate loads, spans, bracing, tie-downs, snow loads, wind loads, or code compliance.
Result
Enter roof geometry, then calculate the rafter estimate.
These results are estimates only. Verify measurements, material specifications, structural requirements, safety requirements, and local building rules before buying materials or building.
How this calculator works
This calculator estimates common rafter geometry from span, pitch, optional overhang, rafter count, and optional length pricing. It supports metric and imperial inputs and displays lengths in both metres and feet.
It is for roof geometry planning only. It does not calculate loads, spans, bracing, tie-downs, snow loads, wind loads, structural sizing, or code compliance.
Formula
- For pitch rise per 12: run = span / 2; rise = run x pitch_rise / 12; rafter length = square root of (run squared + rise squared).
- For angle mode: rise = run x tan(angle); rafter length = run / cos(angle).
- With overhang: overhang rafter extension = overhang / cos(angle).
- Total rafter length = rafter length without overhang + overhang extension.
- Roof angle = arctangent(rise / run), converted to degrees.
Worked example
For a 6 m span, run is 3 m. With a 6-in-12 pitch, the angle is about 26.57 degrees and rise is 1.5 m.
The rafter length before overhang is about 3.35 m. A 300 mm horizontal overhang adds about 0.34 m along the rafter.
Common mistakes
- Using total span as the rafter run instead of half the span for a simple common rafter.
- Forgetting overhang, ridge details, birdsmouth cuts, and trimming.
- Treating geometry estimates as structural design.
- Ignoring roof loads, connections, bracing, and local rules.
FAQs
Does this calculate structural rafter size?
No. It estimates geometry only and does not calculate loads, spans, bracing, tie-downs, snow loads, wind loads, or compliance.
Why is run half the span?
For a simple common rafter on a symmetrical roof, the rafter runs from wall line to ridge, which is half the building width.
Can I include an overhang?
Yes. The calculator converts a horizontal overhang into an extension along the rafter using the roof angle.
Can I estimate total rafter material cost?
Yes. Enter the number of rafters and an optional price per metre or foot.
Calculator disclaimer
These results are estimates only. Verify measurements, material specifications, structural requirements, safety requirements, and local building rules before buying materials or building.