Figure 2-35.-Backing or dropping a hip rafter.the top edge of the hip rafters. Dropping means todeepen the bird’s-mouth so as to bring the top edge ofthe hip rafter down to the upper ends of the jacks. Theamount of drop is taken on the heel plumb line (fig. 2-34,view D).The backing or drop required is calculated, asshown in figure 2-34, view B. Set the framing square tothe cut of the rafter (8 inches and 17 inches) on the upperedge, and measure off one-half the thickness of the rafterfrom the edge along the blade. A line drawn through thismark and parallel to the edge (view C) indicates thebevel angle if the rafter is to be backed. Theperpendicular distance between the line and the edge ofthe rafter is the amount of the drop. This represents theamount the depth of the hip rafter bird’s-mouth shouldexceed the depth of(view D).INTERSECTINGthe common rafter bird’s-mouthAn intersecting roof, also known as a combinationroof, consists of two or more sections sloping indifferent directions. A valley is formed where thedifferent sections come together.The two sections of an intersecting roof mayor maynot be the same width. If they are the same width, theroof is said to have equal spans. If they are not the samewidth, the roof is said to have unequal spans.SpansIn a roof with equal spans, the height (total rise) isthe same for both ridges (fig. 2-36). That is, bothsections are the same width, and the ridgeboards are thesame height. A pair of valley rafters is placed where theslopes of the roof meet to form a valley between the twosections. These rafters go from the inside cornersformed by the two sections of the building to the cornersFigure 2-36.-Intersecting roof with equal spans.2-25
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