Principal Rafters

As these are in compression they were originally formed of cast iron, with the usual double-flanged section, frequently tapering in form, the lower flange being made wider in the centre of the length of the rafter than at the ends.

This material being, however, very heavy, and liable to snap suddenly, was soon generally abandoned.

Wrought-iron rafters for small roofs are most usually made of a T section, with the table 1 uppermost, so as to form a base on which to fix the purlins.

For verandahs and roofs of small span covered with glass, T-iron rafters may be used with the table downwards, thus forming sash bars, the glass being fitted into the angles and resting upon the flanges.

Rafters of I section though they are sometimes used are not convenient for connecting to struts, etc. Rafters of double angle iron are more adapted for roofs of larger span than 40 feet, and will be described in Part II.

Joints or Connections at Head of Principal Rafter. Heads or Crowns.

Heads

Wrouyht-iron Plate Joints. The simplest way of securing the upper ends of T-iron rafters is by riveting a flat plate on each side of them, as shown in Figs. 360, 366, and in detail in Figs. 367, 368. This plate also serves to carry the upper end of the king bolt, or of other tension rods. The ends either pass between the plates as in Plates IV., VII., IX., or are secured to them by being forked and passing on each side as in Fig. 360 and in Plate X.

Heads 100314

Fig. 367.

Heads 100315

Fig. 368.

Cast-iron heads were formerly often used to receive the rafters, but being clumsy and easily broken, have become almost obsolete.

A very simple form of cast-iron head is shown in Fig. 369; the ends of the rafters pass into slots in the side of the head and are there secured by bolts; the upper part of the head is formed to receive the ridge board, and the body receives the king rod, which is secured by a cottered joint arranged so that the rod may be slightly shortened in order to raise the tie rod and set up the truss when necessary.

1 In iron of T section the horizontal part of the T is called the table, and the vertical part the web.

Heads 100316

Fig. 369.

The head shown in Fig. 363 is more complicated and bad in form; a comparatively slight blow on the projecting joints would fracture them.

Fig. 371.

Heads 100317Heads 100318

Fig. 370.

Fig. 372. Joints or Connections at foot of Principal Rafter   Shoes.

Fig. 372. Joints or Connections at foot of Principal Rafter - Shoes.

Cast-Iron Shoes

The foot of the principal rafter is sometimes secured in a cast-iron shoe, in between the sides of which the vertical web of the rafter passes and is fastened by a bolt passing through it and the sides of the shoe.

Two or three examples of the commonest and simplest forms of these shoes are given in the figures 363, 365, and on a larger scale in Figs. 370, 371, 372, 379, 380; also in Plate VI., Figs. 405, 406, 414, Plate VIII., Figs. 431, 432, 433.