Simple Roof Trusses

SIMPLE ROOF TRUSSES. A simple structure which is sometimes misunderstood is a common king-post roof truss, such as is shown at A in Fig. 171.

If two rafters are placed as at B, they will, Fig. 171. Simple Roof Trusses.

of course, tend to spread at the bottom and push the walls apart. This is prevented by a wooden tie-beam, as at C, or by an iron tie-rod, as at D. But after a certain span has been exceeded, the tie-beam commences to sag and has to be supported either by a king-post or by a bolt from the apex of the rafters, as at A and E.

The writer has found many carpenters with the impression that the king-post rests on the tie-beam to support the ridge; but a little reflec tion and a study of these diagrams should con vince even the beginner in roof framing of the fallacy of that view. As a matter of fact, both the tie-beam and king-post are in tension; that is, they are being pulled, and their work can be, and often is, done equally well with a wrought-iron rod.

But, as the span becomes so great as to neces sitate the support of the tie-beam from above, the rafters will also become so long as to tend to sag in the center, and they iu turn must be supported. This is done by struts from the foot of the king-post or rod, as at F. It is clear that these struts are in compression, and that they must be of wood or some other stiff material, such as angle or T-iron, in the case of an iron roof or other roof of heavy construction.

There are thus two kinds of stresses in such a truss, the tie-beam and king-post being pulled, or in tension; and the struts and rafters being pressed, or in compression. For the former, a flexible material, such as a wrought-iron rod, is suitable; but for the latter, a stiff and unyield ing material must be employed, so as to resist the tendency to buckle or bend under the weight of the roof.