What Is a Birdsmouth Cut on a Rafter?
The notch that lets a rafter seat flush on the wall plate
What the cut does
A birdsmouth cut is a triangular notch cut into the underside of a rafter, near where it crosses the top of the wall plate. It lets the rafter sit flush and level on the plate, rather than balancing on its sharp lower corner at an angle — the notch has two faces: a 'seat cut' that rests horizontally on the plate, and a 'heel cut' (or plumb cut) that sits vertically against the outer edge of the plate.
Why it's necessary
Without a birdsmouth notch, a sloped rafter crossing a horizontal wall plate would only contact the plate at a single edge or corner, concentrating load poorly and making the rafter unstable and hard to fasten securely. The notch creates a flat, stable bearing surface matched to the roof's actual pitch.
Getting the layout right
The seat cut's depth and the heel cut's angle both depend on the roof's pitch and the rafter's dimensions — cutting the notch too deep weakens the rafter at exactly the point where it needs to carry load into the wall, so layout is normally done carefully using a rafter square or a calculated cut sheet rather than by eye.