In the discussion of roofs and roof framing which has already been given here, only those roofs have been considered which were of so short a span that they could easily be covered with a framework of ordinary rafters, spaced from 1 to 2 feet apart, between centers, but it is very often necessary to build roofs of larger span, for which ordinary rafters, even if supported by dwarf walls and collar beams, are not sufficiently strong. In this case a different method of framing must be employed.

Instead of a number of rafters spaced fairly close together, and all of equal strength, we will have a few heavy "trusses," placed at intervals of 10 or more feet, and spanning the entire distance between the two side walls. On top of the trusses are laid "purlins," running parallel to the walls, which in their turn support the common rafters, running perpendicular to the side walls, as in the case of simple rafters in an ordinary roof. There may be one or more purlins in each slope of the roof, depending upon the size of the span, since the purlins must be spaced near enough together so that a small rafter can span the distance between them. Usually there will be a purlin at each joint of the truss and the joints will be determined by the safe span for the rafters.

Fig. 236. Roof Plan Showing Use of Heavy Trussed Purlins and Common Rafters

Fig. 236. Roof Plan Showing Use of Heavy Trussed Purlins and Common Rafters.

This arrangement is shown in plan in Fig. 236, in which A are trusses, B are the purlins, C are the common rafters, and D is the ridge.

There are many different kinds of trusses in common use for various kinds of buildings, which differ from each other chiefly in the arrangement of the tension and compression pieces of which every truss is built up. Some trusses are built entirely of timber, while in others timber is employed only for the compression pieces, and wrought iron and steel for the tension pieces.