What is a Birdsmouth?
A birdsmouth is the little triangular cutout in the bottom of a rafter that provides a flat area so the rafter can rest solidly on—and be solidly attached to—a wall top plate. The horizontal cut (where the rafter rests on the wall) is called the “seat cut”; the vertical cut (which snugs up to the exterior of the wall) is called the “heel cut.” The rule of thumb when cutting a birdsmouth is to never remove more than one-third of the depth of the rafter to maintain the rafter’s structural integrity.
The rafters are usually secured to the wall top plate with toenails driven through the birdsmouth or with metal “rafter tie down” plates. Manufactured trusses—because of the way they’re constructed—usually do not have birdsmouths. And, cutting a birdsmouth into a truss (because each is an engineered system) is an absolute no-no.