In designing a roof for a building, the first point requiring attention is the location of the trusses. These should be so placed as to secure solid bearings upon the walls; care being taken not to place either of the trusses over an opening, such as those for windows or doors, in the wall below. Ordinarily, trusses are placed so as to be centrally over the piers between the windows; the number of windows consequently ruling in determining the number of trusses and their distances from centres. This distance should be from ten to twenty feet; fifteen feet apart being a suitable medium distance. The farther apart the trusses are placed, the more they will have to carry; not only in having a larger surface to support, but also in that the roof-timbers will be heavier; for the size and weight, of the roof-beams will increase with the span over which they have to reach.
In the roof-covering itself, the roof-planking may be laid upon jack-rafters, carried by purlins supported by the trusses; or upon roof-beams laid directly upon the back of the principal rafters in the trusses. In either case, proper struts should be provided, and set at proper intervals to resist the bending of the rafter. In case purlins are used, one of these struts should be placed at the location of each purlin.
The number of these points of support rules largely in determining the design for the truss, thus:
For a short span, where the rafter will not require support at an intermediate point, Fig. 59 or 64 will be proper.
For a span in which the rafter requires supporting at one intermediate point, take Fig. 60, 65, or 66.
For a span with two intermediate points of support for the rafter, take Fig. 61 or 67.
For a span with three intermediate points, take Fig. 63.
Generally, it is found convenient to locate these points of support at nine to twelve feet apart. They should be sufficiently close to make it certain that the rafter will not be subject to the possibility of bending.