The methods of constructing the different parts for iron roofs of small span have already been described in Part I. This section will be confined to the consideration of the forms to be given to members of somewhat larger roof trusses.
It has already been mentioned in Part I. that the principal rafters of an iron roof of small span are generally of T shaped section.
Bars of similar sections, but of larger dimensions, are also used for larger roofs; but in these many other forms are also adopted, a few of which may now be mentioned.
Rafters of I section have been sometimes used for spans of over 6 0 feet, but are not convenient for the attachment of the struts.
When T iron is used for larger roofs the upper flange may be strengthened by adding plates to it, as in Fig. 285.
"Bulb iron with a thin web and a bulb somewhat larger than the top table, gives a greater resistance with the same weight of metal than T iron, but its cost is considerably greater, and it is not quite so easy to connect with the other part of the truss."1
The increased strength required in long rafters has sometimes been given without increasing the depth of the iron used by placing two bars side by side, parallel to one another, and kept an inch or two apart by means of cast-iron distance pieces.
Hip Rafters may be strengthened by the introduction of additional flat plates, thus without much increasing the bulk of the rafter.
Purlins for roofs of from 40 to 60 feet span may be of timber (Fig. 284); of angle iron (Fig. 282); T iron, or channel iron, as already described and illustrated in Part. I. The angle and channel iron may be filled in with wood (Fig. 290) in order that the roof covering may be more easily attached.
In larger roofs, where the principals are widely spaced, the purlins may be of I iron (Fig. 289), or trussed (see Plate IX. Fig. 345).