rafter (the hypotenuse). Any part of the triangle can be computed if the other two parts are known.
Use the following equation:
The square of the hypotenuse of a right triangle is equal to the sum of the squares of the two
sides. In roofing terms--
Rafter length2 = run2 + rise2
Figure 3-25. Basic triangle
b. Bird's Mouth. A bird's mouth is a cutout near the bottom of a rafter, that fits over the top
plate. The cut that fits the top of the plate is called the seat; the cut for the side of the plate is called
the heel (see Figure 3-26).
Figure 3-26. Bird's mouth