Figure 2-56.-Lengths of dormer ridge.main-roof ridge. The shortening allowance amounts toone-half the thickness of the main-roof ridge.View B shows that the length of the ridge for anunequal-span addition varies with the method offraming the ridge. If the addition ridge is suspendedfrom the main-roof ridge, the length is equal to thelength of the addition top plate, plus one-half the spanof the building. If the addition ridge is framed by thelong and short valley rafter method, the length is equalto the length of the addition top plate, plus one-half thespan of the addition, minus a shortening allowanceone-half the 45° thickness of the long valley rafter. If theaddition ridge is framed to a double header set betweena couple of double main-roof common rafters, the lengthof the ridge is equal to the length of the addition sidewallrafter plate, plus one-half the span of the addition, minusa shortening allowance one-half theinside member of the double header.thickness of thethe2-34Figure 2-57.-Shed roof framing.Figure 2-56, view A, shows that the length of theridge on a dormer without sidewalls is equal to one-halfthe span of the dormer, less a shortening allowanceone-half the thickness of the inside member of the upperdouble header. View B shows that the length of the ridgeon a dormer with sidewalls is the length of the dormerrafter plate, plus one-half the span of the dormer, minusa shortening allowance one-half the thickness of theinside member of the upper double header.SHEDA shed roof is essentially one-half of a gable roof.Like the full-length rafters in a gable roof, the full-lengthrafters in a shed roof are common rafters. However, thetotal run of a shed roof common rafter is equal to thespan of the building minus the width of the top plate onthe higher rafter-end wall (fig. 2-57). Also, the run ofthe overhang on the higher wall is measured from theinner edge of the top plate. With these exceptions, shedroof common rafters are laid out like gable roof commonrafters. A shed roof common rafter has two bird’s-mouths, but they are laid out just like the bird’s-mouthon a gable roof common rafter.For a shed roof, the height of the higher rafter-endwall must exceed the height of the lower by an amountequal to the total rise of a common rafter.Figure 2-58 shows a method of framing a sheddormer. This type of dormer can be installed on almostany type of roof. There are three layout problems to besolved here: determining the total run of a dormer rafter;determining the angle of cut on the inboard ends of thedormer rafters; and determining the lengths of thedormer sidewall studs.To determine the total run of a dormer rafter, divideheight of the dormer end wall, in inches, by the