Roof Framing 101
Pitch is the amount of angle or slope the roof has. This is denoted two ways, a 1/3 or 1/4 pitch in fractions, and a 6/12 pitch in inches, which means the roof rises 6 inches for each 12 inches of run. A framing square is traditionally used for laying out the roof and determining pitch.
This article will discuss a gable roof, without valleys or dormers, because it’s the most common and the easiest for a first-time builder to tackle. After you determine the rise, span, run, line length and pitch, the next step is to lay out the rafters, or mark the cuts on a pattern rafter to create the roof.
There are three basic cuts used in creating the rafter; the plumb cut at the top of the rafter where it fits against the ridge plate; the tail cut that creates the outside edge of the building eaves; and the bird’s mouth, which positions the rafter on the top of the wall plate. We will show two methods of laying out rafters; one using a traditional framing (2-foot) square, and the second using the new C.H. Hanson Pivot Square.
Hold the framing square with the manufacturer’s name up; this is called the “face” of the square, and the opposite side is the “back.” The long arm of the square is the “blade,” and the short arm is the “tongue.”
In the example we’ll use a 1/3 pitch as shown in the drawing below, this means a rise of 8 inches for each 12 inches of run (an 8/12 pitch roof). The first step is to lay the square on the end of the rafter board and locate 8 inches on the tongue (the rise), and 12 inches on the blade (the unit of run). Measure from the point on the blade to the point on the tongue—it should be 14-7/16 inches. Multiply this by the run of the building. We’re using 10 feet in this example, excluding the overhang. The resulting figure is 144-1/2 inches. We add 12 inches for the overhang to get a final figure of 156-1/2 inches.
A framing square is used to lay out the rafters.
Examine the rafter board to determine if there is any curve or “crown” in the board. You should make this first pattern rafter on the straightest board you can find. If there is any curve in the board, lay out the rafter so the crown is up or facing away from you. Experience has shown that the weight of the roof will gradually flatten this crown. (If the crown were to be positioned down, the roof could eventually sag.) Then lay out the rafter as shown on the next page.