Free Rafter Cut calculator with clear step-by-step results.
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.
Heel cut depth
Heel = seat width x tan(pitch)
Ridge shortening
Shortening = (ridge thickness / 2) / cos(pitch)
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.
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.
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.