Figure 2-15.—A “bird’s-mouth” is formed by the heel plumbline and seat line.various calculations, layouts, cutting procedures, andassembly requirements required for efficient con-struction.GABLENext to the shed roof, which has only one slope, thegable roof is the simplest type of sloping roof to buildbecause it slopes in only two directions. The basicstructural members of the gable roof are the ridgeboard,the common rafters, and the gable-end studs. Theframework is shown in figure 2-13.The ridgeboard is placed at the peak of the roof. Itprovides a nailing surface for the top ends of thecommon rafters. The common rafters extend from thetop wall plates to the ridge. The gable-end studs areupright framing members that provide a nailing surfacefor siding and sheathing at the gable ends of the roof.Common RaftersAll common rafters for a gable roof are the samelength. They can be precut before the roof is assembled.Today, most common rafters include an overhang. Theoverhang (an example is shown in fig. 2-14) is the partof the rafter that extends past the building line. The runof the overhang, called the projection, is the horizontaldistance from the building line to the tail cut on therafter. In figure 2-14, note the plumb cuts at the ridge,heel, and tail of the rafter. A level seat cut is placed wherethe rafter rests on the top plate. The notch formed by theseat and heel cut line (fig. 2-15) is often called thebird’s-mouth.The width of the seat cut is determined by the slopeof the roof: the lower the slope, the wider the cut. Atleast 2 inches of stock should remain above the seat cut.The procedure for marking these cuts is explained laterin this chapter. Layout is usually done after the lengthof the rafter is calculated.CALCULATING LENGTHS OF COMMONRAFTERS.— The length of a common rafter is basedon the unit of rise and total run of the roof. The unit ofrise and total run are obtained from the blueprints. Threedifferent procedures can be used to calculate commonrafter length: use a framing square printed with a raftertable; use a book of rafter tables; or, use the step-offmethod where rafter layout is combined with calculatinglength.Framing squares are available with a rafter tableprinted on the face side (fig. 2-16). The rafter tablemakes it possible to find the lengths of all types ofrafters for pitched roofs, with unit of rises ranging from2 inches to 18 inches. Let’s look at two examples:Example 1. The roof has a 7-inch unit of rise anda 16-foot span.Figure 2-16.—Rafter table on face of a steel square.2-11
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business