How do you cut a birds mouths in a rafter?

First is the angle of the roof rafter. This MUSt be established BEFORE any cutting takes place.

Identify the wall plate (normally a 4 x 2 (2 x 4 if in the US)timber across the top of the wall of the building. The plate sits with the 4 inch face flat on the wall with the 2 in high side vertical to the wall.

Measure the horizontal cut, called the seat. Trace a horizontal line straight across the surface of the rafter or stable brick. This line marks where the top of the joist (or other brick) must meet flush with the rafter (or stable brick).Subtract the thickness of the wall. Measure the wall -- six inches, for example -- and subtract it from the length of the seat line. Dot this mark on the seat line.Then mark the plumb, or vertical, line. Align the measuring tape vertically with the dot on the seat line. Trace the vertical line, straight down to determine where the side of the joist (or other brick) meets flush in the rafter (or stable brick). Cut out the "bird's mouth" angle to fit in the frame.