Figure 2-51.Valley cripple Jack and hip-valley cripple jack.

Jack Rafters

Figure 2-53.Jack rafter framing diagram.

Builder 3&2 Volume 02 - Construction manual for building structures
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Figure 2-51.—Valley cripple Jack and hip-valley cripple jack. valley cripple jack extends between two valley rafters in the long and short valley rafter method of framing. A hip-valley cripple jack extends from a hip rafter to a valley rafter. LENGTHS.— Figure 2-52 shows a roof framing diagram for a series of hip jack rafters. The jacks are always on the same OC spacing as the common rafters. Now, suppose the spacing, in this instance, is 16 inches OC. You can see that the total run of the shortest jack is the hypotenuse of a right triangle with the shorter sides each 16 inches long. The total run of the shortest jack is therefore: Suppose that a common rafter in this roof has a unit of rise of 8. The jacks have the same unit of rise as a common rafter. The unit length of a jack in this roof is: This means that a jack is 14.42 units long for every 12 units of run. The length of the shortest hip jack in this roof is therefore the value of x in the proportional equation 12:14.42::16:x, or 19.23 inches. This is always the length of the shortest hip jack when the jacks are spaced 16 inches OC and the common rafter in the roof has a unit of rise of 8. It is also the common difference of jacks, meaning that the next hip jack will be 2 times 19.23 inches. The common difference for hip jacks spaced 16 inches OC, or 24 inches OC, is given in the unit length Figure 2-52.—Hip jack framing diagram. rafter table on the framing square for unit of rise ranging from 2 to 18, inclusive. Turn back to figure 2-41, which shows a segment of the unit length rafter table. Note the third line in the table, which reads DIFF IN LENGTH OF JACKS 16 INCHES CENTERS. If you follow this line over to the figure under 8 (for a unit of rise of 8), you’ll find the same unit length (19.23) that you worked out above. The best way to determine the length of a valley jack or a cripple jack is to apply the bridge measure to the total run. The bridge measure of any jack is the same as the bridge measure of a common rafter having the same unit of rise as the jack. Suppose the jack has a unit of rise of 8. In figure 2-41, look along the line on the unit length rafter tables headed LENGTH COMMON RAFTER PER FOOT RUN for the figure in the column under 8; you’ll find a unit length of 14.42. You should know by this time how to apply this to the total run of a jack to get the line length. 2-31

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