a. Basic Triangle. The basic triangle is the most elementary tool used in roof framing (see Figure 3-25 ). When framing a roof, the basic right triangle is formed by the horizontal lines (or run), the rise (or altitude), and the length of the 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

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 ).

c. Cut of a Roof. The cut of a roof is the rise over the run (such as 4/12 roof) or the pitch of the roof (see Figure 3-27 ).

d. Span of a Roof. The span of any roof is the shortest distance between the two opposite rafters' seats (see Figure 3-27 ).

e. Line Length. In roof framing, line length is the hypotenuse of a triangle whose base is the run and whose altitude is the total rise (see Figure 3-27 ).

f. Horizontal Line. A horizontal line is one level with the building foundation.

g. Overhang. The overhang is that part of a rafter that extends past the outside edge of the walls of a building. When laying out a rafter, this portion is in addition to the length of a rafter and is figured separately. The overhang is often referred to as the tailpiece.

h. Total Rise. The total rise is the vertical distance from the wall plate to the top of the ridge.

i. Run. Run always refers to the level distance any rafter covers--normally, one-half the span.

j. Unit of Run (or unit of measurement). The unit of measurement, 1 foot (or 12 inches), is the same for the roof as for any other part of the building. Using this common unit of measurement, the framing square is used in laying out large roofs.

k. Pitch. Pitch signifies the amount that a roof slants and the ratio of rise (in inches) to run (in inches). Using this method, 4, 6, or 8 inches of rise per foot of run would give a pitch of 4:12, 6:12, or 8:12. There are two methods of indicating pitch.

l. Rise. The rise of a rafter is the vertical (or plumb) distance that a rafter extends upward from the plate.

m. Plumb Line. The line is the line formed by the cord on which the plumb bob is hung (see Figure 3-28 ).

n. Plate. The plate is the wall-framing member that rests on the top of the wall studs (see Figure 3-8 ).

o. Ridge. The ridge is the highest horizontal roof member. It ties the rafters together at the upper end (see Figure 3-8 ).