Calculate the precise dimensions for a birdsmouth notch in roof rafters.
Measurements
in
in
Cut Dimensions
Enter measurements to see cut dimensions
About this calculator
Overview
A birdsmouth cut is a notch made in a rafter so it can sit flat on the top plate of a wall. It consists of two cuts: the seat cut (horizontal) and the heel cut (vertical). This joint ensures stability and proper load distribution in roof construction.
💡
Pro Tips
Use a speed square to quickly mark the angles on your rafter.
Always cut on the waste side of your line for the most accurate fit.
If your roof pitch is steep, you'll need a deeper seat cut to maintain the same heel depth.
!
Fun Facts
"The name comes from the notch's resemblance to the open beak of a bird."
"A common rule of thumb is that the heel cut should not exceed 1/3 of the rafter's width to maintain structural integrity."
"The seat cut length usually matches the width of the wall's top plate (e.g., 3.5 inches for a 2x4 wall)."
Formula: H = S × tan(θ)
S = H / tan(θ)
θ = atan(Rise / 12)
