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 small roofs which have to cover only narrow buildings and in which the length of the rafters is short, there is no necessity for any interior support, and when the rafters have been cut to the correct length, set up against the ridge, and secured in place, the roof framing is complete. In long spans, however, the roof would sag in the middle if it were not strengthened in some way, so it is customary to support long rafters as near the center as possible. This support may be formed by placing a piece of studding under each rafter, somewhere between the plate and the ridge, and if this is done very much lighter rafters can be used than would otherwise be considered safe. It is claimed by some that it is cheaper to do this than to use the heavy rafters. A more common method is to use fewer upright pieces and to place a horizontal piece A on the top of them, running the whole length of the building and supporting each rafter. This is shown in Fig. 181. An upright piece B should be placed under every sixth or seventh rafter in order to give the necessary stiffness to the whole construction. For the uprights, pieces of ordinary studding 2X4 inches or 2X3 inches in size may be used. When there is to be a finished attic in the building, these upright studs may be made to form the side walls of, the attic rooms, and are then spaced about 1G inches on centers to receive the laths. Such walls are called dwarf walls.
Fig. 180. Ridge Pole Extended above Roof.
Fig. 181. System of Interior Supports for Rafters.
Fig. 182. Collar or Tie Beams as Interior Support for Rafters.
Another form of interior support is the collar beam or tie beam. This is a piece of timber which extends between the rafters on opposite sides of the roof and ties them together, as shown at A in Fig. 182. It may be a piece of board about 1 inch thick and 8 or 10 inches wide, which is nailed onto the side of the rafter at each end. It is placed as near the center of the rafter as may be practicable, and in the case where a finished attic is required it forms the support for the ceiling. For this reason it must be at a considerable height from the attic floor, and can not always be placed very near the center of the rafter. The important point is to see that it is well nailed at each end.
DINING ROOM IN HOUSE FOR MR. C. M. THOMPSON, CAMBRIDGE, MASS.
FIREPLACE IN DINING ROOM OF HOUSE FOR MR. C. M. THOMPSON, CAMBRIDGE, MASS. Cram, Goodhue & Ferguson, Architects, Boston and New York.
Fig. 184. Rafter and Wall Framing for Double Gable Roof.