Fig. 363 is an example of one of the earliest, and still one of the best and simplest forms of iron roof for small spans. In it each rafter is trussed by means of a strut supporting it in the centre, the stress on the strut being taken up by tension rods which connect the head of the strut with the extremities of the rafter. The thrust upon the walls is counteracted by a horizontal tie rod joining the feet of the two struts, and which holds the trussed rafters together.
In this example the rafters are of T iron, the struts each of two T irons riveted together (see Fig. 384), with feet formed to receive the ends of the tension rods, as shown, and also those of the tie rod, which unites the two sides of the roof. The higher end of the upper tension rod (Y) is secured to the cast-iron head H (see p. 188), and the foot of the lower tension rod (Z) passes through an iron shoe secured to the wall. Both tension rods can be slightly altered in length by means of cottered joints, and the tie rod by means of the union joint (X), so that they may be brought into a proper state of tension when the roof is fixed.
1 Sometimes called Framed Roofs.
In the example given the upper purlin is arranged so as to support the lower side of the skylight, otherwise it would be better placed immediately over the head of the strut, so as to cause no cross strain upon the rafter.
In some varieties of this roof the intermediate tie is kept too high, which leaves a strain on the rafter similar to that experienced in a collar-beam roof (see p. 156).
"The merit of this truss is that the bracing is nearly all in tension. "Mr. Bow has shown that, if the members are proportioned to the stress, it is more economical of material than any other form." 1
This form of truss is adapted for spans of from 20 to 30 feet; it is, however, frequently used for much larger spans. Professor Unwin gives an instance in which it has been adopted for a span of 87 feet, but recommends at the same time that it should be restricted to spans of 60 feet.
As the span of the roof increases, the length of the rafters becomes such that they require support at more points than one.
The roof, Fig. 363, is old-fashioned in detail; a better example of a roof of this form is given in Plate IV.