Rafter Cut
Free Rafter Cut calculator with clear step-by-step results.
About Rafter Cut
Sets out the two cuts every common rafter needs. The birdsmouth heel depth is the seat width multiplied by the tangent of the pitch, and the remaining depth above it must stay above two-thirds of the rafter for the section to retain its strength. The plumb cut at the ridge is the complement of the pitch, less half the ridge thickness measured along the rafter.
Formulas
Heel cut depth
Heel = seat width x tan(pitch)
Ridge shortening
Shortening = (ridge thickness / 2) / cos(pitch)
Worked example
With the default values below, Rafter Cut returns Birdsmouth heel cut depth of 57.7. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Roof pitchaccepts 1–70 degrees
- 30 degrees
- Rafter depthaccepts 50–500 mm
- 195 mm
- Birdsmouth seat widthaccepts 20–300 mm
- 100 mm
- Ridge board thicknessaccepts 0–100 mm
- 45 mm
Results
- Birdsmouth heel cut depth
- 57.7 mm
- Rafter depth left above the seat cut
- 137.3 mm
- Plumb cut angle from the rafter edge
- 60.00 degrees
- Shortening for half the ridge board
- 26.0 mm
Step by step
- 1Values used
- 2Heel cut depth
- 3Ridge shortening
- 4Birdsmouth heel cut depth= 57.7 mm
- 5Rafter depth left above the seat cut= 137.3 mm
- 6Plumb cut angle from the rafter edge= 60.00 degrees
- 7Shortening for half the ridge board= 26.0 mm
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- Roof framing is structural. Member sizes, notch limits, spans and fixings must comply with the applicable timber design code or be specified by a structural engineer.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Roof framing is structural. Member sizes, notch limits, spans and fixings must comply with the applicable timber design code or be specified by a structural engineer.
Frequently Asked Questions
How deep can a birdsmouth be?
A common rule is no more than one third of the rafter depth, leaving two thirds intact. Deeper notches need engineering justification because they remove material where shear is highest.
Why shorten for the ridge board?
The rafter length is calculated to the theoretical roof centreline. The ridge board occupies half its thickness on each side, so each rafter must be shortened by that amount measured along its slope.