Figure 2-51.—Valley cripple Jack and hip-valley cripple jack.valley cripple jack extends between two valley raftersin the long and short valley rafter method of framing. Ahip-valley cripple jack extends from a hip rafter to avalley rafter.LENGTHS.— Figure 2-52 shows a roof framingdiagram for a series of hip jack rafters. The jacks arealways on the same OC spacing as the common rafters.Now, suppose the spacing, in this instance, is 16inches OC. You can see that the total run of the shortestjack is the hypotenuse of a right triangle with the shortersides each 16 inches long. The total run of the shortestjack is therefore:Suppose that a common rafter in this roof has a unitof rise of 8. The jacks have the same unit of rise as acommon rafter. The unit length of a jack in this roof is:This means that a jack is 14.42 units long for every12 units of run. The length of the shortest hip jack inthis roof is therefore the value of x in the proportionalequation 12:14.42::16:x, or 19.23 inches.This is always the length of the shortest hip jackwhen the jacks are spaced 16 inches OC and thecommon rafter in the roof has a unit of rise of 8. It is alsothe common difference of jacks, meaning that the nexthip jack will be 2 times 19.23 inches.The common difference for hip jacks spaced 16inches OC, or 24 inches OC, is given in the unit lengthFigure 2-52.—Hip jack framing diagram.rafter table on the framing square for unit of rise rangingfrom 2 to 18, inclusive. Turn back to figure 2-41, whichshows a segment of the unit length rafter table. Note thethird line in the table, which reads DIFF IN LENGTHOF JACKS 16 INCHES CENTERS. If you follow thisline over to the figure under 8 (for a unit of rise of 8),you’ll find the same unit length (19.23) that you workedout above.The best way to determine the length of a valley jackor a cripple jack is to apply the bridge measure to thetotal run. The bridge measure of any jack is the same asthe bridge measure of a common rafter having the sameunit of rise as the jack. Suppose the jack has a unit ofrise of 8. In figure 2-41, look along the line on the unitlength rafter tables headed LENGTH COMMONRAFTER PER FOOT RUN for the figure in the columnunder 8; you’ll find a unit length of 14.42. You shouldknow by this time how to apply this to the total run of ajack to get the line length.2-31

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