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.
As soon as the attic floor is on, the roofs will be raised. In ordinary country houses the roof should be supported where possible by the interior partitions where they extend down to first floor girders over basement piers, in which case no complicated framing or truss work will be required. The ordinary form of roof consists of a series of rafters supported at the bottom by the plate of the house and at the top by the ridge pole. Intersecting roofs are supported by larger timbers called valley rafters and these should always continue up to the ridge. If the rafters are over eighteen feet long it will be necessary to support them near their center, this is done by partitions or by collar beams spiked across from rafter to rafter. (Fig. 27.) In large buildings they are supported by purlins resting on trusses or on posts. The spacing of the rafters varies from sixteen inches to two or three feet, twenty inches being the most usual.
For any roof of less than 30-foot span with the plate securely tied, no interior supports will be needed, and above this span, purlins should be used. The size of rafters not over 12 feet long should be 2 X 6 inches, from 12 feet to 18 feet 2 X 7 inches and 2 X 8 inches, and over that length 2 X 10 inches. On the whole it will be cheaper to reduce the length to 10 or 12 feet by means of purlins. An examination of the framing plans shows that our rafters are about fifteen feet long and two by seven inches in size, set twenty inches on centers. The roof is a hip roof, that is, a roof which draws in from all sides, which is the strongest kind of roof, so that we shall not have to provide any special supports, and shall only have to see that the proper pitch is given according to plans, that the valleys are properly put in and are extended to the ridge or to the hips, and that the openings are of the right size and in the right positions. All portions of the roof must be well spiked together, the ridges perfectly straight and level and in the center, and the rafters all set exactly to a line.