Calculator · Framing, fixing & movement
Rafter Calculator
Estimate common rafter length, pitch, rise, run, roof angle, and horizontal-overhang geometry. Use it for early roof layout planning, lumber/timber takeoffs, and checking approximate material lengths before structural verification.
Rafter calculator
Result
Enter roof geometry, then calculate the rafter estimate.
How it works
This calculator estimates common rafter geometry from span, pitch, an optional horizontal overhang projection, rafter count, and optional length pricing. It supports metric and imperial inputs and displays lengths in both metres and feet.
The entered overhang is the horizontal projection from the outside wall line to the end of the overhang, not a distance measured along the sloping rafter. The calculator converts that horizontal input to a sloped rafter extension using the selected roof pitch.
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.
- 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).
- Sloped overhang extension = horizontal overhang projection / cos(roof angle).
- Total rafter length = rafter length without overhang + sloped 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 projection converts to about 0.335 m of sloped overhang added along the rafter, giving a total near 3.69 m.
Common mistakes
- Using total span as the rafter run instead of half the span for a simple common rafter.
- Entering a distance measured along the sloping rafter where the calculator expects the horizontal overhang projection.
- Forgetting overhang, ridge details, birdsmouth cuts, and trimming.
- Treating geometry estimates as structural design.
- Ignoring roof loads, connections, bracing, and local rules.
Common questions
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. Measure the horizontal projection from the outside wall line to the end of the overhang. Do not enter an along-rafter measurement; the calculator converts the horizontal input into the sloped extension 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.