The simplest form of iron roof with straight rafters is shown in Fig. 360.
The rafters are of T iron, united at the apex by a pair of overlapping platesl riveted to both, and from which is suspended the king bolt, the head of which is forked, so as to pass on each side of and embrace the plate.
The tie rod is bolted to the lower end of the rafter, and is supported in the centre by a double nut at the foot of the king bolt. The lower end of the rafter is itself secured to the head of the column supporting the roof.
As the rafter is entirely without support, except at the ends, this form of roof is not adapted for spans greater than from 15 to 20 feet.
The roof, when fixed, may be tightened up by screwing the nut at the foot of the king bolt, so as to shorten the latter and raise the tie rod.
In the king-rod roof, shown in Fig. 361, the principal rafters are of T iron, the struts of angle iron, the common rafters and purlins of wood. The inclination of the struts is bad, being too oblique to the principal rafter to enable them to take the thrust properly. The king rod is of circular rod iron, fixed at the top into a cast-iron head, its lower end being furnished with a screw, which, passing through holes in the feet of the struts and in the centre of the tie rod, is secured by a nut.
The upper end of each principal rafter enters the cast-iron head, and is secured to it, while the lower end is fastened by a bolt (which passes also through the forked end of the tie rod) to an iron chair which is secured to the wall.
1 Called Check Plates or Gusset Plates.
It will be seen that by screwing up the nut at the foot of the king bolt, the tie rod is raised and the roof tightened up.
Fig. 362 is a modification of the king-post roof constructed in iron. The dotted lines show additional suspending rods, which may be added in roofs of larger span, i.e. above 30 feet. An example of a roof of this form with details is given in Plates V. and VI.