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
