Birdsmouth cuts provide better seating for rafters on the top plates of the walls. Ceiling joists are usually nailed to the rafters to provide the necessary structural support for any loads on the roof, and together with the help the birdsmouths, they maintain the walls in position, resisting any outward forces. In addition to any structural advantages, birdsmouths also help with the installation of rafters by making it easier to position the walls in relation to them.
Lay the rafter on a pair of sawhorses with the upper edge away from you and the ridge board's plumb cut to your right. Measure from the long point of the plumb cut to mark the location of the birdsmouth's plumb cut.
Place a framing square on top of the rafter with the heel away from you and the square's blade to your right. If you're laying out a common rafter, line the 30 cm (12 inch)mark on the outer edge of the square's blade with the rafter edge nearest you. If it's a hip rafter, line up the 42.5 cm (17 inch) mark instead. Find the mark on the square's tongue that corresponds to the pitch of the roof and line it up with the same edge. With these two points lined up with the edge, slide the square along the rafter until the outer edge of the tongue lines up with your plumb-cut mark. Run a pencil along the tongue to mark the plumb cut.
Mark the depth of the seat. If you're laying out a common rafter, measure along the plumb-cut line 3.7 cm (1 1/2 inches) from the edge nearest you. Since hip rafters are usually made with wider lumber than common rafters, the seat mark will be greater than 3.7 cm (1 1/2 inches), but the distance from the seat to the top will be the same. Therefore, measure from the far edge of a common rafter to the seat mark and use that same measurement to lay out the seat mark on hip rafters.
Position the square as you did in Step 2 and slide the square until the blade lines up with the seat mark. Run your pencil along the blade from the plumb-cut line to the edge of rafter nearest you. This line should be at a right angle to the plumb-cut line. These two lines together with the edge of the rafter should form a small right triangle that needs to be removed to make the birdsmouth.
Use a circular saw to cut along the seat mark from the edge of the rafter to the plumb-cut line. Don't cut past the plumb-cut line, but rather finish the cut with a handsaw.
Move to the other side of the sawhorses and set the angle of a circular saw to match the plumb-cut line. Set the depth of the saw so that it doesn't cut past the seat mark. Turn the rafter on edge and cut along the plumb-cut line to remove the triangular piece of wood.
Wear eye protection and take all necessary precautions when using a circular saw.