Terms of the Trade: What is a Birdsmouth?
Also known as a bird's beak cut, a birdsmouth is a wooden joint that forms a triangular shape, often to connect the rafter of a roof to the upper plate of the wall beneath.
It consists of an indentation cut into the top beam, allowing it to rest flatly against the support below for a firm attachment (usually with toenails or rafter tie down plates). The cut is usually no more than a third of the depth of the rafter for structural safety, though building codes vary from place to place.
The part of the cut that rests on the wall is referred to as the seat and runs horizontally, while the heel (or plumb) cut is vertical, running parallel to the wall.
Manufactured metal rafters usually don't have birdsmouths, nor should they. Attempting to add one to pre-engineered roofing structures can be dangerous.