Fig. 364 gives an example adapted for spans of from 30 to 40 feet, in which the rafter is supported at two intermediate points. An example of a roof of this form with details is given in Plate VII. An example of a truss with two struts at right angles to each principal rafter is given in Plates VIII. and IX.
1 Unwin's Wrought-iron Bridges and Roofs. 2 Sometimes called English roofs.
Fig- 365 shows an iron roof arranged in the form of the ordinary wooden queen-post1 roof. The principal rafters and straining piece are of T iron, the struts also of T iron, the queen rods and tie rod of circular rod iron.
The head of the rafters is secured to the straining piece by plates of iron covering the joint and riveted to both. The roof boarding is carried by horizontal common rafters, or, as they are usually called, "purlins," of angle iron filled in with wood. On the boarding may be laid slates, corrugated iron, sheet iron, or zinc.
The end of the tie rod is fastened to the rafter by a bolt, which, passing through both, secures them to a cast-iron shoe fixed upon the wall.
The tie rod may be slightly altered in length by the action of a cottered joint, which is described at p. 191.
The dotted lines show cross braces, which are often added in roofs of more than 30 feet span. In some cases the straining piece is supported in the centre by a curved T iron springing from the feet of the vertical bolts.
This roof is surmounted by a ventilator, the construction of which is obvious from the figure.
1 In iron roofs the term "Queen-rod" roof is generally applied to those having several suspending rods similar to the Princesses in wooden roofs.
The feet of the struts are secured to the tie rod by bolts with double nuts, figured and described at p. 190.
This roof is surmounted by a ventilator, consisting' of cast-iron louvred standards supporting T-iron rafters, which carry angle-iron purlins similar to those of the main part of the roof.
The ventilator is strengthened by a tension rod passing across it and secured to the sides and centre standard by cottered joints.
An example of a saw-tooth roof trussed in the queen-rod form is given in Plate X.