Like tapering, cutting a birdsmouth into a rafter reduces the load-carrying capacity of the member. A common error with low-slope rafters is excessive cutting of the rafter seat.
This leaves the rafter bearing not on the heel of the seat, as it should, but on the toe. This reduces the effective size of the rafter, producing stresses that can create splits at the bearing point, and eventually, a sagging rafter.
Figure 1: Avoid sagging rafters by properly cutting birdsmouths
Cut the rafters so the heel of the joist becomes the bearing point on the plate. Not only will this maintain the integrity of the joist, it will provide extra inches between the top of the exterior wall and the roof sheathing. This translates into more room for attic insulation to extend over the outside wall, reducing cold spots that can cause condensation at the eaves.