12 feet. The roofs of dwellings can generally be sufficiently supported by carrying up partitions and by the use of collar beams. It often happens, however, that it is desired to use the attic space in a large house for a ball room or for some similar purpose in which posts or other supports for the roof would be very objectionable ; in such cases the roof can generally be supported in the following manner :
Hip-Roof Trussing. - Any room not exceeding 40x60 feet can be safely covered by a hip roof without trusses or interior supports of any kind, provided the plate can be made continuous around the four sides of the main roof. Fig. 88 represents the plate, hip rafters and deck beams of a hip roof, terminated by a deck. It can readily be seen that such a construction is practically a truss in itself, and if the several pieces are properly proportioned and connected it will carry an enormous weight and exert only a vertical pressure on the walls.
As ordinarily constructed the hips and plates of such a roof are made light and practically do not carry any load at all, but if all the common and jack rafters were spiked to the deck beams and hip rafters, so securely that there would be no tendency of the rafters to slide or push out at the bottom, the whole weight of the roof would then be thrown largely on the hips, and there would be no outward thrust on the plate. The thrust of the hips would be taken up by the plate and the walls under the plate would sustain the vertical load. The hips and deck beams must be of such size that they will not sag under their load, and the planks forming the plate must be fastened so securely, by tie irons at the angles, that the hip rafters cannot force them apart. No dependence should be placed upon the common rafters for supporting the hips and deck beams. If it is desired to carry the roof to a ridge it can be extended above the deck beams, the latter forming a new plate, as it were. In such a case, however, there should be collar beams between the deck beams to prevent their being pushed in.
Such a roof is entirely practicable, and the author nas often employed this principle in constructing hip and octagon roofs. If carefully built and proportioned this construction can be used where there is no ceiling at the plate level, but in such cases the plate should be made as wide as possible and not less than 12 inches when 30 feet long. It is obvious that if the angles of the plate should give way the whole roof would fall.
If there are projections and minor roofs they can be built on to the main roof and will strengthen it against spreading, but the main frame must be built as in Fig. 88, with the plate continuous around it. If the plate is interrupted by chimneys, it should be connected by iron ties, passing either through or around the chimney.