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Roofing Calculators

Rafter Length Calculator

Common rafter length from span and pitch, including overhang, plus the hip and jack rafters.

Enter values

Results update as you type.

Result

Common rafter length

5.0807 m

Ridge height above the plate
2.309 m
Run (half span)
4 m
Number of rafters
42
Total rafter timber
213.39 m
Hip rafter length
6.4663 m
Plumb cut angle
30 °
Seat cut angle
60 °

About the Rafter Length Calculator

Common rafter length from span and pitch, including overhang, plus the hip and jack rafters.

Also known as: rafter size calculator, common rafter length, roof timber length, hip rafter length.

How to use this calculator

  1. Enter building span, roof pitch, eaves overhang, rafter spacing and the other fields.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

rafter = (run + overhang) ÷ cos(pitch)

Worked example

Using the values pre-filled in the calculator above:

  • Building span = 8 m
  • Roof pitch = 30 °
  • Eaves overhang = 0.4 m
  • Rafter spacing = 600 mm
  • Roof length = 12 m

the calculator returns:

  • Common rafter length: 5.0807 m
  • Ridge height above the plate: 2.309 m
  • Run (half span): 4 m
  • Number of rafters: 42