To make a birdsmouth cut, first calculate the size and orientation of the cut based on the pitch of the roof. Then make the cut, using a chisel and a hand saw or power saw. The cut should measure no more than one-third of the width of the rafter or piece of wood, and it should be made using a sketch or template.
A birdsmouth cut in a rafter measures the same angle as the pitch of the roof. If the roof has a 30 degree pitch, the underside of the rafter and the vertical cut of the birdsmouth form a 120 degree angle. The cut itself measures 90 degrees, in order to rest snugly on the right angle of the wall plate.
Named for its beak-like triangle shape, builders use birdsmouth cuts to affix a rafter to the wall plate of a building or to give extra support to a fence. Hand-framed buildings commonly use this type of cut, while modern roof trusses come designed to sit flat on the wall plate, with no birdsmouth cut required.
The birdsmouth cut helps absorb stress from additional weight on a roof, such as snow, stopping the natural downhill motion of a roof by providing a flat surface where the truss rests.