This is not a simple tasks. It requires careful calculation and precise execution. Here’s how you can do it and avoid mistakes.
You will need a framing square. It is used bu carpenters to mark the desired dimensions. Place the rafters on a sawhorse. Be very careful with the measurements.
Go to the top side of the rafter and then place the square at the angle between the rafter length and width. Use the pencil to draw a line. This is the plumb but, or the area that will be in contact with the board of the roof.
Place a meter tape at the plumb cut and then mark the length of the rafter. Use the square tongue to draw the line. This is where the birdsmouth will lie.
Measure the width of the wall place and then mark it on the framing square. Slide the square down the line that you have marked previously until the distance between the line and the side of the rafter is equal to the width of the wall place. Now draw a second line to enclose the triangle.
The overhang cut is the cut at the bottom of the rafter that will hang over the exterior wall of the house. Mark the overhang desired length. Then, draw a line parallel to the first. This will be the overhang line.
Cutting the rafters
For cutting the rafters, a worm drive circular saw would be the most appropriate cutter for this operation. You can use any saw and cutter, but with great care.