Birdsmouth Cut
A notch cut into a rafter where it sits on the top plate of a wall, consisting of a horizontal seat cut and a vertical plumb cut. Provides a stable bearing surface for the rafter.
In Detail
The seat cut should be no more than 1/3 of the rafter depth to maintain structural integrity. IRC R802.6 limits the depth of the seat cut. For a 2×6 rafter (5.5" actual), the maximum seat cut is about 1.8". The rafter tail extends past the wall to form the eave overhang. Birdsmouth cuts are not used with raised heel trusses or when rafters are supported by hangers.