After the lines of the roof have been decided upon a plan should be drawn of the framing timbers. In drawing this the first step should be to draw all the hips and valleys and then the common and jack rafters, leaving proper opening for chimneys, dormers, etc. Unless there is some special reason for doing otherwise the common rafters should be drawn perpendicular to the walls or ridge, as this brings a large part of the weight of the roof directly on to the walls, and the boards, being put on horizontally, offer a support or stage for the workmen.
Fig. 77 shows the framing plan of an ordinary house roof, the common and jack rafters being indicated by single lines, spaced 16 inches apart.
The spacing of the common rafters varies much with the custom of the locality; in some places 16 inches is the usual spacing, and in others a spacing of from 20 inches to 2 and even 3 feet is adopted. When the spacing is only 16 inches the rafters are often lathed on their under side, but when over that distance apart they must be furred for lathing. It will require less lumber to obtain the same strength and stiffness in the roof when the rafters are spaced 2 feet apart than when they are only 16 inches apart, even including the furring strips. In Boston the usual spacing for dwellings is 20 inches, and in Denver, Colo., it is 16 inches, but in the latter place the attic lathing is applied directly to the rafters, while in Boston the laths are nailed to furring strips. The Boston method gives the straightest work and the most economical construction as far as material is concerned, but it involves a little more labor than the Denver method.