Common rafters are fitted individually to a ridge beam and attached at the top of the wall. Rafters can be installed uncut, with the rafters tied to the top of the wall using a galvanised hurricane tie bracket. A better way is to cut a notch, known as a bird's mouth, where the rafter meets the wall cap so that the rafter nests more securely on top of the wall, making installation easier and creating a stronger, more precise joint. The best way to achieve accurate bird's mouth placement is to draw a full-size template of your roof line in chalk on your driveway or other concrete surface.
Use a chalk line to mark a horizontal line on your driveway. The line should be the width of the distance between the outside corners of the tops of your walls where they meet the rafters. Draw a perpendicular line from each corner of the line, using a framing square. This represents the outside line of your walls. Use a tape measure to locate the centre of the horizontal line and draw a perpendicular line up from this point the distance from the top of your walls to the top of the roof peak. Subtract the width of your rafters, which typically use 2-by-4 or 2-by-6 lumber, and make a mark at that point to represent the inside height of your roof peak.
Make diagonal lines from the peak point along the angle of your roof line the length of the top edge of the rafters, using the chalk line. Measure to the inside of this line the width of your rafters and make a parallel line under the first rafter line, representing the bottom edge of your rafter. This line will intersect the corners you drew with the framing square. The triangle created by this intersection represents the placement, angle and size of your bird's mouth.
Cut the angles at the top and bottom ends of a rafter to the correct length, using the chalk line drawing as a guide. Place the cut rafter into the parallel lines of the chalk template, with its end matched up to the peak of the drawing. Mark the rafter where it intersects the top corner of the wall in your template. Mark the top and sides of the wall clearly.
Align a framing square with these two marks so that the corner of the square forms a triangular notch with the edge of the board. Mark along this corner to outline the bird's mouth. Use a circular saw to cut this notch out. Repeat with each rafter.