This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
In all roofs the pieces which make up the main body of the framework are called the rafters. They are for the roof what the joists are for the floor, and what the studs are for the wall. The rafters are inclined members, spaced from 16 to 24 inches apart on centers, which rest at the bottom on the plate, and are fastened at the top in various ways, according to the form of the roof. The plate, therefore, forms the connecting link between the wall and the roof and is really a part of both of them. The size of the rafters varies, depending upon their length and the distances at which they are spaced. It is sometimes allowable to use them as small as 2X4 inches, but this should be done only for the lightest work. The size of the rafters for an ordinary dwelling house is usually 2X8 inches. In larger buildings, such as school houses, it may be found necessary to use rafters as large as 2X10 inches, when they are of a considerable length.
The material usually employed for rafters is the same as that used for the joists and for the studding. This is generally spruce in the eastern states and yellow pine in the Mississippi Basin, but may be hemlock in very cheap work. The size and spacing of the rafters vary to some extent with the location of the building, as in the northern part of the country the roof is called upon to carry a considerable weight of snow and ice, while in the south there is little or no snow and the roof is not called upon to carry so much weight. If snow freezes and packs on the roof it may weigh as much as twenty-five or thirty pounds per square foot of roof surface. The wind pressure must also be considered, as well as the weight of the material composing the roof itself.
The connection of the rafters to the wall is the same in all the types of roofs described above. They are not framed into the plate but are simply spiked to it, being cut at the bottom so as to rest on top of it. Usually they extend out for a considerable distance beyond the wall to form the eaves, as shown in Fig. 170, and they are then cut over the plate and allowed to continue beyond it. The usual length for the eaves is about 1 foot, but it may vary to suit the taste of the designer of the building. Sometimes the rafter itself is not extended beyond the plate, but is cut off just as though it was not intended that it should continue beyond the wall line, and a separate piece called a "false rafter" is nailed against it alongside to form the projection for the eaves, as shown in Fig. 171. This piece does not always continue in the same line with the real rafter, but may, and usually does, make an angle with it, as shown in the figure, so as to give a break in the roof line near the line of the eaves.
Fig. 170. Rafter Extended to Form the Eaves.
Fig. 171. False Rafter.
It is sometimes desired to form a concealed gutter around the eaves, and in order to do this the joists are allowed to extend 10 or 12 inches and on the ends of these a 2X4 is laid flat and nailed, and the rafters rest on this piece. The scantling is nailed directly above the plate and the gutter is run in notches cut in the overhanging joists which also support the cornice. The general appearance is shown in Fig. 172 and the details of the construction in Fig. 173.
Fig. 172. Roof Showing Concealed Gutter.
There are four different kinds of rafters used in framing roofs, all of which may sometimes be found in a single roof frame, if the roof is of complicated design, while ordinary roofs may be framed with only the more simple forms of rafters. In Fig. 174 is shown in plan the framing for a roof in which all four kinds of rafters are to be found. AAA are the common rafters, which extend from the plate to the ridge and which are not connected with or crossed by any of the other rafters. B B B are jack rafters, which are shorter than the common rafters and which do not extend from the plate to the ridge, but are connected at one end to a hip or valley rafter.
C C are the valley rafters, which are needed at every corner between the main building and an ell or other projection, while the hip rafters D D are found at the outside corners. At the points where the valley rafters are situated there are troughs or valleys formed by the roof surfaces - as these pitch downward on both sides toward the valley rafter - while at the outside corners, where the hip rafters are found, the roof surfaces pitch upward on each side to the hip rafter. This may be seen and perhaps better understood by looking at any hip and valley roof as actually constructed, and as this type of roof is very common there will be no lack of examples.