When the rafter is cut properly and set in place it will be found that the plane of its top surface does not lie in either of the two roof surfaces. The surface of the top lies at an equal angle to each roof surface and one edge extends up above the other rafter in both roofs.
Fig. 202A shows how the edges of a hip rafter extend over the plate at the bottom. To overcome this the rafter can be backed or cut shorter as shown in Figs. 202B and 202C. To back the rafter, lay the square on the bevel at the bottom end in the position the plate will occupy. Now mark the points C D and draw the lines D F and CG from these points parallel to the edge of the rafter and cut away the triangular part A /3 D FIT and A EC G H. This is an expensive means of making the rafter conform to the roof surface and most carpenters merely shorten the rafter until the outside corners conform to the surfaces, as shown in Fig. 203.
If the rafter is not to be backed, the effect of backing can be easily obtained by nailing a thin board on the top of the rafter and giving this board the proper bevel. The method is illustrated in Fig. 204.
The clapboard A is nailed at the edges and the one side wedged up to the angle the backing would take, care being taken to allow the square to touch the edge B B of the rafter at C C. The square used on the side of the rafter gives the plumb cut C D, and C M over the clapboard gives the side bevel. In cutting, the saw is held at an angle to coincide with both CD and C M.
In the rafter B E, Fig. 201, the horizontal cut at E is obtained by using 23 feet 11 inches and 13 feet 4 inches 'and is the same as the cut at C. The length of the rafter can be found by using proportion or by finding the length of B D.
In using proportion, it is evident that B E, the run of the short valley, is to CF as 7 feet 6 inches, or 90 inches, is to 13 feet 4 inches, or 160 inches. • The rise is 7 feet 6 inches and the run is 13 feet 51 inches.
Another way in which the problem may be solved, is to find where the ridge of the ell intersects the main roof surface. The inter section is at a height of 7 feet 6 inches which is 2,4,- of the run of A C, or 9 of 16 feet and the distance B D is just 9 feet. Hence, the run of B E is 13.45=13 feet 51 inches, and this is of the run of the rafter C F. Hence, the run of C F is 13.45X 23 feet 11 inches.
The end cut at B on B E, that is, the bevel that fits against CF, to be cut accurately, must be handled like the side bevel at F. First cut the bevel at the plate and get the backing line that makes B E lie in the main roof surface. Now, at B, either back the rafter a
short distance, or use a clapboard as in Fig. 20-4.
Jack Rafters. Fig. 205 shows the plan of the roof in which there are, in addition to hip and valley rafters, sets of jack rafters. A B and B D are hip rafters, C E is a valley rafter, and the other rafters are common and jack rafters. At B E and E II are shown the ridge boards. Of the jacl rafters there are three different kinds: those like IJ which run from the valley rafter to the ridge board; those like KL which run from hip rafter to plate; and those like N T, which run between the hip and valley rafters. These jack rafters differ only in respect to the bevels which have to be cut on them. The rafter IJ is a simple plumb cut at the top, similar to the cuts at the top of the common rafters, and at the bottom where the rafter meets the valley there are two cuts—a plumb cut and the side cheek cut—which are similar to the cuts in a valley rafter where it comes against a ridge board. This cut has been previously explained.
The rafter KL has a simple horizontal cut at the bottom like that used on the common rafter, but at the top there are two cuts similar to those at the foot of rafter IJ. The rafter N T has two cuts at both top and bottom. All these bevels are obtained just as the bevels for the hip and valley rafters.
The length of a jack rafter is proportional to its distance from the ridge or plate to which it is parallel. The longest jack rafter is equal in length to a common rafter, and the length steadily decreases as the distance of the rafter from its first full length rafter. The exact difference in length between the first jack rafter and the next, is determined by finding how far apart the jack rafters are to be placed, and comparing this distance with the distance from the top of the first full length jack rafter to the point where the hip or valley rafter rests on the ridge board or plate. Suppose, for instance, that the rafters are to be spaced 2 feet apart, and the length of the com mon rafter is 10 feet. If the distance from the top of this rafter to the point where the valley rafter is fitted against the ridge is 12 feet, it is evident that each rafter will be 2 feet shorter. That is, the second rafter will be S feet and the next 6 feet and so on. We use six spaces, although there are only five rafters, there being no rafter used where the valley and ridge join.