(Continued from Part I.)

THE king-post roof and simpler forms described in Part I., are adapted for spans up to 30 feet. This Advanced Course includes the trusses ordinarily used for spans of from 40 to 60 feet.

Gothic and other roofs adapted for special styles of architecture, or for particular situations, will not be referred to.

Trusses involving the use of curved or built-up beams are also excluded.

N.B. - In all the figures illustrating timber roofs, the distinctive letters for different parts are as follows : -

King And Queen Post Roofs

King-post trusses will do very well for roofs up to about 30 feet span, but for wider roofs it is found that the tie beam requires support at more than the one central point; additional vertical ties, called queen posts, have therefore to be introduced, as at QP, QP, Fig. 76.

The common rafters being longer, require support at more than one point, two purlins are therefore introduced on each side of the roof.

Queen-Post Roof With King Bolts

This excellent construction is shown in Fig. 96, p. 49.

When a flat top is not required, purlins with common rafters running down the slope of the roof are adopted, as in Fig. 76, and the apex of the roof finished as there shown.

Queen-Post Roofs

When rooms have to be formed in the roof, and frequently besides, the king post is omitted, in which case, to prevent the heads of the queen posts from being forced inwards, a straining beam is placed between them, as shown at SB in Fig. 77, and their feet are kept apart by a straining sill, SS.

This form of roof is well adapted for spans of from 30 to 45 feet.

The ends of the straining beam sometimes receive additional support from cleats, as at C, secured to the queen posts. The strap above C is omitted in the figure in order to show the joint.

Roof With Queen Posts And Princesses

In roofs of a greater span than 45 feet, the tie beam requires to be upheld at more than two intermediate points.

The extra support necessary is furnished by the introduction of additional suspending posts, PP, known as Princesses.

Such a construction as that shown in Pig. 80 may be used for spans between 45 and 60 feet.

In roofs of above 50 feet span the straining beam between the heads of the queen posts is so long that it would sag without support, and this may be afforded by means of a small king tie, suspended from the junction of the principal rafters, which are prolonged above the straining beam, as dotted in the figure.

In a roof of this kind the space between the queen posts affords convenient accommodation.

Roofs Of Spans Greater Than 60 Feet

The consideration of such roofs does not fall strictly within the limits of this course; and in these days they would generally be constructed of iron; it will be sufficient, therefore, to give one or two skeleton examples of old timber roofs of large span, before dismissing the subject.

In these figures the lines all represent timber in scantling, framed and put together in a similar way to the members of the trusses depicted in Figs. 76-80.

Fig. 81 nearly resembles the roof of the old Birmingham Theatre, and is recommended by Tredgold as a good truss for roofs of from 75 to 90 feet span.

Fig. 81.

In this case the triangular portion above the straining beam, being of considerable dimensions, is formed into a regular kingpost truss.

The length of rafter between the queen post and princess being so great as to require support, this is afforded by means of a second strut, S2.

In this roof the straining sill, SS, was keyed and bolted to the tie beam, and the tie beam was scarfed between A and B.

The scantlings adapted for the trusses shown in Figs. 77-81 are given at pages 51 and 5 2.

Fig. 8 2 shows one of the principal trusses of the old roof of Exeter Hall.

This truss is of 76 feet span, and includes a second set of princesses.

In other respects it is similar to the roof last mentioned, except that the straining sill is trussed as described at page 42.

The scantlings of this roof were as follows : -

Fig. 83 shows a similar form of roof, but with the apex removed, and a lead flat substituted, the central portion, which carries the flat, being strengthened by the introduction of a king post and cross bracing.

Roofs Composed Of Wood And Iron For Spans Of More Than 40 Feet

As already stated in Part 1., the chief use of wrought iron in composite roofs is as a substitute for the wooden posts or suspending pieces which uphold the tie beam.

Cast iron is also used, in the form of shoes, heads, etc., for receiving and connecting the members of the truss.

It is not considered worth while to give any illustrations of composite roofs of wide span, as they are similar in principle to those illustrated in Part I., and have not been much used since the introduction of iron roofs.