53. The construction of a pitch roof over a large school building generally requires an arrangement of trusses and posts that is peculiar to this class of buildings, and while not difficult or complicated is yet worthy of description.
Such buildings are generally covered with a hip roof, terminating in a deck, and the size of the rooms and the length of the rafters usually necessitate the use of purlins to support the rafters and trusses to support the purlins.
Fig. 124 may be considered as representing the general arrangement of the supporting walls of an eight-room school building, and the method of construction applicable to this plan can generally be applied to any building with large rooms and brick partitions.
The roof of this building is supposed to be hipped, the pitch roof terminating in a deck 18 feet inside of the wall line. The pitch of the roof is 10 1/2 inches to the foot, and the wall plate is 3 feet 6 inches above the bottom of the ceiling joists.
An economical construction for such a roof requires the use of two lines of purlins, one under the edge of the deck and the other half way between it and the wall plate.
The most economical manner of supporting the purlins and the deck roof is shown in Fig. 125 the position of the purlins, trusses and posts being also shown on the plan Fig. 124.
To support the purlins over the school rooms two trusses are required over each room. For convenience of construction the top chord of the lower trusses may be made continuous or spliced over the post. Advantage should be taken of the partition walls to support the purlins at points directly above by means of posts, which should be braced as shown in the drawing. The top chords of the trusses serve as purlins to support the rafters, and the purlins parallel to the front and rear of the budding are placed at the same height and supported from the trusses by stirrup irons or patent joist hangers. The rafters of the deck roof generally have a rise of from 3/4 inch to 3 inches to the foot, and their inner ends should rest on a purlin, R, which can generally be supported by beams, B B, hung from the deck purlins. If the height of the purlin R does not give sufficient pitch to the roof it maybe blocked up from the beams B B. All parts of the roof should be well tied together, the posts tenoned into the purlins and the rafters well spiked to the purlins and wall plate. If the wall plate is raised above the ceiling joists it should be tied every few feet by tie-braces spiked to the rafters and to the ceiling joists, as shown at S.
The bottom chords of the trusses will naturally be used to support the ceiling joists over the school rooms, as shown in the figure. In some cases, it may be more economical to rest the tie beam on the top of the wall, and hang the ceiling from the trusses by means of rods and girders. If there are gables, the purlins can be extended to the gable walls, and the valley rafters will be supported by the purlins, as shown in Fig. 124.
54. Fig. 125A shows a method of supporting a school house roof, so as to utilize the attic space. This particular construction was used by the author to support the roof of a two-story school house 83 ft. 9 ins. wide, with school rooms on each side and a hallway about 25 ft. wide through the centre. As it was desired to obtain an assembly room in the attic, the author utilized the interior walls for supporting the roof, so that the only obstruction in the hall is the two rows of posts, 15'10" C to C longitudinally. In this way a large hall was obtained at very little additional expense over the ordinary roof construction. As may be seen, each pair of posts supports a cantilever truss on each side and a Howe truss in the centre. The top chord of the Howe truss extends over the posts to form the top of the cantilevers. The rafters of the side roof extend from the wall plate to the deck, and are supported by the purlins P, which rest on the bottom chords of the cantilevers. The sloping roof is covered with shingles and the deck roof with tin. The roof is braced longitudinally by trussing the rafters and ceiling joists. The truss timber is of Oregon pine.