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 the lean-to roof the rafters rest at the top against the wall of the building of which the ell, or porch, is a part; and the work of framing the roof consists simply in setting them up and securing them in place with spikes or nails. The pitch roof, however, is formed on the principle that two pieces which are inclined against each other will hold each other up, and so the rafters must rest against each other at the top in pairs, as shown in Fig. 178. It is customary to insert between the rafters, at the top, a piece of board about 1 inch in thickness and deep enough to receive the whole depth of the rafter, as shown at A in Fig. 179. This piece of board is called the ridge or the ridge pole and extends the whole length of the roof. It serves to keep the rafters from falling sideways, and keeps the roof frame in place until the roof boarding is on. It is sometimes extended above the rafters, and forms a center for some form of metal finish for the ridge, as shown in Fig. 180.
Fig. 178. Construction of Ridge.
Fig. 179. Placing of Ridge Pole between Abutting Ends of Rafters.