Cast-iron shoes have to a great extent been superseded by simple joints constructed with flat plates to which the principal rafter is riveted, and the tie, if a rod, bolted, or if flat riveted to them. Examples of such joints are given in Figs. 394, 395, Plate IV., and Figs. 415, 416, 417, Plate VII, also Figs. 373, 374, 375, taken from an actual roof.
In large roofs, arrangements have to be made to allow free expansion and contraction of the iron, under changes of temperature, but these need not here be described; they will be referred to in Part II.
Tie Rods are generally of rod iron, circular in section. They may be flat bars on edge, which have an advantage, inasmuch as they are less liable to sag than a circular tie rod of the same strength. A flat bar, however, exposes a larger surface, and causes a heavy appearance in small roofs.
When flat bars are used, additional strength may easily be obtained by placing two or three bars side by side.
"Where bolts pass through a tie rod, the latter is widened out so as to leave sufficient substance round the hole, in order that its tensional strength may not be reduced (see Fig. 385).
The tie rod may be simply bolted or riveted to the rafter (see Fig. 360), or to a shoe of some description (Figs. 363, 366).
"When a king rod occurs, the centre of the tie rod is upheld by the king rod, which passes through it, and is secured by a nut on each side of it. The feet of the struts are attached to the tie rod in a similar way, as shown in Figs. 376, 383a.
In larger roofs the tie rod is generally severed in the centre (Fig. 377), and in circular rods the ends thus formed are shaped into eyes, through which pass the bolts securing the feet of the struts. The eyes are secured between flat plates, which may also take the end of any longitudinal tie rod, as in Figs. 377, 378, and Fig. 408, Plate VI. Flat tie rods are much more easily connected (see Plate IV., Fig. 396, Plate VII, Fig. 423, Plate IX., Figs. 444, 445).