There are as many base feet in a Jack rafter as there are number of feet on the plate from the corner to the center of the rafter. (See fig. 21.) The base 12" and the same rise used in the common rafter laid on the rafter as many successive times as there are base feet to meet the hip rafter, gives the length of the Jack rafter.
(See fig. 30.) Mark on the base for the plate cut, and on the rise for the perpendicular cut to meet the hip rafter. (See fig. 35.) Where a heel or support is let on see fig. 31.
The face or angular cut to meet the hip varies with the pitch and every 1" rise. To obtain the angular cut to meet the hip, take the number of inches in the hypotenuse or the length of rafter in one base foot of the common rafter. (See fig. 33.)
Taking the length of the hypotenuse (14 1/2") with the base (12") on the square instead of the rise (see fig. 34), and mark back on the length of the hypotenuse 14 1/2" for the angular cut. (See fig. 35.) This rule is reliable in any pitch of roof.
Fig. 33. 12 And 14 1/2 Angular Cut