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Roof Framing 101

Construction How-To, Roofing March 27, 2008 Sonia


1a101RoofTT03 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.

Rafter Layout

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.

 TGtools Roof Framing 101

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.

1a101RoofTT01A Roof Framing 101

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.

 Rafter Layout with a Framing Square

This example is for a roof with an 8/12 pitch

  • Position the square at the end of the rafter board, with the tongue on your left and facing away from you. Position the square with both the 8 inch on the outside edge of the tongue and the 12 inch on the blade on the upper edge of the board. Mark along the backside of the tongue. This is the plumb cut for the roof ridge.
  • Measure form the top of this line down the board to determine the line length, or length of the rafter, less ½ the ridge board. This commonly is a 2-by or 1-1/2-inch board, so the measurement is less ¾ inches. Also exclude the overhang at this point. Holding the square in the same position as before, mark down to the side of the tongue. This marks the plumb cut at the inside of the house wall for the notch (called a bird’s mouth) to seat the rafter one the wall plate.
  • Add the length of the overhang beyond this mark and mark it. Note the square is turned over. In the example shown this is 12 inches. Cut the rafter at the ridge line and at the overhang line.
  • Then hold the square on the plumb line that marks the bird’s mouth. Determine the wall thickness or depth of the bird’s mouth cut and make a mark. Cut the notch, first with a handsaw or portable circular saw, and then finish the cut with a handsaw.

Another method of laying out the rafter with the square is called “stepping off.” Once the plumb cut is marked, simply move the square 12 inches and mark another plumb cut. Continue moving down the rafter and marking plumb cuts, including any odd figures.