Birdsmouth Cut Calculator

e.g., 6 for 6/12 pitch
in
Construction

Calculate the dimensions of a birdsmouth cut (seat cut and plumb cut) for a roof rafter based on roof pitch.

Formula

Angle = atan(rise/run); Seat = rafter width × cos(angle); Plumb = rafter width × sin(angle)
  • Seat cut is horizontal; plumb cut is vertical.
  • Birdsmouth should not exceed 1/3 of rafter depth.

6/12 pitch, 3.5 in rafter

Inputs
  • Roof Pitch (Rise): 6
  • Roof Pitch (Run): 12
  • Rafter Width: 3.5 in

Angle = 26.57°. Seat = 3.5×cos(26.57) = 3.13 in. Plumb = 3.5×sin(26.57) = 1.56 in.

Frequently asked questions

What is a birdsmouth cut?
A notch cut into a rafter that allows it to sit flat on the wall plate.