When the span gets beyond this, it is necessary to suspend the tic-beam at more points, and first of all, at two where the extreme bearings are not much more distant. In this case two suspenders, termed queen-posts, are employed, hold ing up the tie-beam at two points equidistant from each other and from the walls of the building ; the principals are not extended to meet each other at the apex, but terminate at the queen-posts against which they abut, and the queen-posts are retained at the required distance from each other by means of a straining beam, which also abuts against the head of the queen-posts, and by this means they are suspended. 'This description of truss is something like the first, if we suppose the king-post separated into two halves, with a horizontal strut placed between them, to prevent the principals from being pressed together ; struts are also placed at the foot of the queen-posts, to support the principals.
A truss for a still greater span is formed by suspending the tie-beam at three intermediate points, having a king-post between two queen-posts, without a straining-beam, the prin cipals being carried to meet each other at the apex, as in the first description of roof; struts are carried from the foot of the king-post to the head of the queen-posts, and from the foot of the queen-posts to the In many cases a roof with the same number of suspenders is somewhat differ ently constructed, and this consists, in the first place, of a roof precisely similar to the second above described, having, however, in addition, a king-post in the centre, carried up through and above the straining-beam, which is divided into two lengths, and abuts against the king-post on either side. Above and parallel to the principals is placed another pair of principals, which are continued to meet at the apex, where they abut against the king-post ; these alone are termed prin cipals, the other subordinate rafters which abut against the queen-posts, being termed straining braces, or auxiliary rafters. The principals are further supported at the upper part by struts, which spring from the king-post above the straining-beam, and below by a continuous bearing on the straining-braces, which, in their turn, are supported by the struts from the foot of the queen-posts. The last form of roof is calculated for a span of 60 to SO feet, and the pre vious one for a span of 50 or 60 feet. Sometimes, instead of a single king-post, this kind of truss will have the king-post, as it were, split in half and hung on either side of the truss, from the heads of the upper principals, extending down to the tie-beam ; in this case the straining or collar beam is in one piece, and passes between the halves of the king-post. Another variation consists in splitting the truss
into two half-trusses, and keeping the king-post between them both, so as to form one mass. A roof of this kind was constructed over the basilica of St. Paul at Rome more than four centuries ago.
This expedient may be entirely obviated by the use of iron king-posts, which, in all cases, would seem preferable to those of wood, both for king or queen.posts, or, indeed, for any mem ber of the truss which is subjected to tension, as for ties, where its employment-, to a very considerable extent, obviates the difficulty which is experienced by the sagging of tie-beams of timber. The number of suspenders, especially if composed of iron-rods, may be still further extended to seven or eight, or, indeed, any number, the only limit being found in the length of the principals.
For the strength of different materials the reader may con sult the article on that subject, but as a general remark it may be observed, that oak when exposed to tension is weaker than fir, and is therefore less adapted for tics. Being, how ever, less compressible, it is usually preferred for rafters, straining-pieces, and struts ; but Tredgold observes, that its greater tendency to warping in summer renders it less fit for rafters and purlins than foreign lir. Cast-iron is not much used, except in fire-proof roofs, and each piece requires to be well tested. Wrought-iron is very useful for straps and also for ties and trussing-posts; but care is always necessary to guard against imperfections, which are more likely to pass unobserved than in wood. Wherever iron is applied, pro vision should be made for its expansion and contraction, and it is desirable to protect it from oxidation by painting. Though iron is fir stronger for its size than any kind of timber, it is neither so strong nor su cheap as yellow-fir, weight or weight.
Plate VI. Figure 1, is the roof of the chapel of the royal hospital at Greenwich, constructed by Mr. S. Wyatt.
It is constructed with two queen-posts, and is similar to the second kind of roof above described, but it has in addi tion two struts from the foot of the queen-posts to the strain ing-beam, which abut against a second straining-piece under neath the first ; the tie•beam is also further suspended from the straining-beam by an irou-rod, which answers the pur pose of a queen-post.