Adding a bird's mouth to a roof rafter should be the last step in the completion of your rafter but is the most difficult to get right.
This cut allows the rafter to sit squarely on the top plate and when cut right gives a solid foundation for attaching the rafter to the wall.
Step 1 - Layout of the Rafter
At this phase of construction, a few things should already be determined.
The rafter angle or pitch.
The length of the overhang.
Depth of bird mouth. That is usually 1 1/2 inches so the rafter can rest at exactly the depth of the top plate.
With this information determined and the rafter ends cut to the proper pitch, layout of the bird's mouth is fairly easy.
Start by measuring perpendicularly from the tail end of the rafter to the point the overhang length meets the edge of the inside part of the rafter, forming a triangle.
This point is the start of the bird's mouth.
Step 2 - Layout of the Bird's Mouth
From the marked point draw a line at the same angle as the pitch of the rafter up and make a mark at 1 1/2 inches.
From that line across the rafter draw a perpendicular line at the 1 1/2 inch mark out towards the inside part of the rafter. This 90-degree angle forms the bird's mouth.
Step 3 - Cut out the Bird's Mouth
With the bird's mouth marked out, use a circular saw to cut the marked lines. These will come close to, but will not cut all the way through, the bottom of the bird's mouth.
Avoid over cutting through the lines to cut out the bird's mouth; this overcut can take away from the strength of the rafter and its seat on the top plate.
After cutting to the lines with the circular saw, use a hand saw to finish the cut through the bottom and clean up any lips that may be left to ensure a clean seat.
Step 4 - Templates
To make cutting a bird's mouth easier, use the first finished rafter as a template for the remaining ones. This ensures all the rafters will be the same.
Before using it as a template though, make sure it sits properly and fits snugly. Then take two blocks of wood and nail them to the top edge of the rafter.
These blocks will act as slides to allow the rafter to be placed on the lumber and adjusted so cutting around knots can be avoided.