The run of valley jack Nos. 9 and 10 is equal tothe spacing of jacks OC. Therefore, the length ofone of these jacks is equal to the commondifference of jacks.The run of valley jacks Nos. 11 and 12 is twicethe run of valley jacks Nos. 9 and 10, and thelength of one of these jacks is therefore twice thecommon difference of jacks.The run of valley cripple No. 13 is twice thespacing of jacks OC, and the length is thereforetwice the common difference of jacks.The run of valley cripple No. 14 is twice the runof valley cripple No. 13, and the length is there-fore four times the common difference of jacks.SHORTENING ALLOWANCES.— A hip jackhas a shortening allowance at the upper end, consistingof one-half the 45° thickness of the hip rafter. A valleyjack rafter has a shortening allowance at the upper end,consisting of one-half the 45° thickness of the ridge, andanother at the lower end, consisting of one-half the 45°thickness of the valley rafter. A hip-valley cripple has ashortening allowance at the upper end, consisting ofone-half the 45° thickness of the hip rafter, and anotherat the lower end, consisting of one-half the 45° thicknessof the valley rafter. A valley cripple has a shorteningallowance at the upper end, consisting of one-half the45° thickness of the long valley rafter, and another at thelower end, consisting of one-half the 45° thickness ofthe short valley rafter.SIDE CUTS.— The side cut on a jack rafter can belaid out using the same method as for laying out the sidecut on a hip rafter. Another method is to use the fifth lineof the unit length rafter table, which is headed SIDECUT OF JACKS USE (fig. 2-41). If you follow that lineover to the figure under 8 (for a unit of rise of 8), youwill see that the figure given is 10. To lay out the sidecut on a jack set the square faceup on the edge of therafter to 12 inches on the tongue and 10 inches on theblade, and draw the side-cut line along the tongue.BIRD’S-MOUTH AND PROJECTION.— A jackrafter is a shortened common rafter; consequently, thebird’s-mouth and projection on a jack rafter are laid outjust as they are on a common rafter.Ridge LayoutLaying out the ridge for a gable roof presents noparticular problem since the line length of the ridge isequal to the length of the building. The actual lengthincludes any overhang. For a hip main roof, however,the ridge layout requires a certain amount of calculation.As previously mentioned, in an equal-pitch hip roof,the line length of the ridge amounts to the length of thebuilding minus the span. The actual length dependsupon the way the hip rafters are framed to the ridge.As indicated in figure 2-54, the line length ends ofthe ridge are at the points where the ridge centerline andthe hip rafter center line cross. In the figure, the hip rafteris framed against the ridge. In this method of framing,the actual length of the ridge exceeds the line length, ateach end, by one-half the thickness of the ridge, plusone-half the 45° thickness of the hip rafter. In the figure,the hip rafter is also framed between the commonrafters. In this method of framing, the actual length ofthe ridge exceeds the line length at each end by one-halfthe thickness of a common rafter.Figure 2-55, view A, shows that the length of theridge for an equal-span addition is equal to the length ofthe addition top plate, plus one-half the span of thebuilding, minus the shortening allowance at theFigure 2-55.—Lengths of addition ridge.2-33

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