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Figure 2-15.—A “bird’s-mouth” is formed by the heel plumb line and seat line. various  calculations,  layouts,  cutting  procedures,  and assembly  requirements  required  for  efficient  con- struction. GABLE Next to the shed roof, which has only one slope, the gable roof is the simplest type of sloping roof to build because  it  slopes  in  only  two  directions.  The  basic structural members of the gable roof are the ridgeboard, the  common  rafters,  and  the  gable-end  studs.  The framework is shown in figure 2-13. The ridgeboard is placed at the peak of the roof. It provides  a  nailing  surface  for  the  top  ends  of  the common  rafters.  The  common  rafters  extend  from  the top wall plates to the ridge. The gable-end studs are upright framing members that provide a nailing surface for siding and sheathing at the gable ends of the roof. Common Rafters All common rafters for a gable roof are the same length. They can be precut before the roof is assembled. Today, most common rafters include an overhang. The overhang (an example is shown in fig. 2-14) is the part of the rafter that extends past the building line. The run of the overhang, called the projection, is the horizontal distance from the building line to the tail cut on the rafter. In figure 2-14, note the plumb cuts at the ridge, heel, and tail of the rafter. A level seat cut is placed where the rafter rests on the top plate. The notch formed by the seat and heel cut line (fig. 2-15) is often called the bird’s-mouth. The width of the seat cut is determined by the slope of the roof: the lower the slope, the wider the cut. At least 2 inches of stock should remain above the seat cut. The procedure for marking these cuts is explained later in this chapter. Layout is usually done after the length of the rafter is calculated. CALCULATING   LENGTHS   OF   COMMON RAFTERS.— The length of a common rafter is based on the unit of rise and total run of the roof. The unit of rise and total run are obtained from the blueprints. Three different  procedures  can  be  used  to  calculate  common rafter length: use a framing square printed with a rafter table; use a book of rafter tables; or, use the step-off method where rafter layout is combined with calculating length. Framing squares are available with a rafter table printed on the face side (fig. 2-16). The rafter table makes it possible to find the lengths of all types of rafters for pitched roofs, with unit of rises ranging from 2 inches to 18 inches. Let’s look at two examples: Example 1. The roof has a 7-inch unit of rise and a 16-foot span. Figure 2-16.—Rafter table on face of a steel square. 2-11

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