Some illu trations are appended of the most remarkable ix-might iron roofs °vomited of late years, principally to meet the requirements of the railway communications, They are, In their effect. light and elegant, even though costly and it is precisely on account of there artistic glialltles, and of the danger from the warping of complicated assemblages of timber, such AM must be used In roofs of large span, that the preference Is generally given to Iron over wood roofs for railway abode, wherein the principals are al*Ays exposed to view, and to the direct action of the atmosphere. The hest work upon the strength and the construction of this class of roofs 74 Morin's, ' Lecons scrP Ilicanigne Pratique; vol. iv. An Important remark to be maile with respect to these iron roofs is that the steam and smoke from the locomotives have a deleterious influence upon them; and for this reason they cannot be tired where the locomotives in steam are likely to remain long under them.
In the valuable practical works of Nicholson, Tredgold, &c., the methods of calculating the strength necessary in the various parts of a roof may be found and in the • Principles of Carpentry; by the latter author; t.ibles are given of the dimensions suitable for different spans. The table here quoted refers to a roof similar to .17.q. 7; the tresses being not more than ten feet apart, and the pitch at en angle of about 27' with the horizon, for a ewering of slates. The scantlings are suited for yellow fir, and must be somewhat increased for timber of inferior- quality.
For the strength of different materials, under various circumstances, the reader may consult MATI:RIALS. STUENGTII OF. As a general remark, it may be observed that oak, when exposed to tension, is weaker than fir, mid is therefore haw adapted for tics. Being, however, lees cotupreesible, it is usually preferred for rafters, straining pieces, and struts ; but Treagold observes that its greater tendency to warping in summer renders it less tit for rafters and puffins than foreign fir. Cast-iron is not much used in the framing of wooden roofs, excepting for shore, king-poet heads, bolt-heads, and collars at the feet of struts and straining pieces. Wrought-iron is very useful for straps and fastenings, and also for ties and trussing-poets ; but care is always necessary to guard against imperfections, which are more likely to pass unobserved than in wood. Wherever iron is applied, provision should be made for its expansion and contraction, and it is desirable to protect it from oxidation by painting. Though iron is far stronger for its sire than any kind of timber, it is neither so strong nor so cheap as yellow fir, aright for reight, provided the spans of the roofs are moderate.
The joints in the frame-work of a timber roof are of various kinds acconling to the nature of the strain they have to resist. They should be formed with great care, and with due regard to such probable changes of form as all constructions of timber aro liable to from shrink ing and warping. Cocking or cogg;ng is the name given to that kind of joining in which one piece of timber, in a state of tension, is eo attached to another that it cannot be drawn away without one piece breaking.
32 and 33 represent two methods of cocking the ends of tie bi.anis on the wall-plates, giving a plan and elevation of each. In both figures a represents the beam, and b the wall-plate. In the first plan, which was formerly much practised, the contraction of the dovetailed end of the beam would allow it to be drawn considerably out of its place, and would therefore permit the walls to spread: but in the second the amount of contraction is diminished, owing to the small width of the rectangular tongue that enters the tie-beam, while its position is such as to prevent the beam being drawn out of its place beyond the actual extent of the contraction of the tongue. The shrinking of the joggles of king-poets and queen-posts is often produc tive of serious derangement, a circumstance greatly in favour of the substitution of iron for wood for such parts, especially in large roofs. This inconvenience is sometimes avoided by making the upper ends of the principal rafters abut immediately upon each other, as repre sented in fe. 12. A similar arrangement is made, in some cases, where wooden king•posts are used, the king-poet and rafters being strapped together with iron. The sinking of a roof, particularly if it be of low pitch, is very injurious to the mortise.and-tenon joints of the struts and rafters, by throwing the strain on the shoulders of the tenons in such a way as to break off the tenous or splinter the wood. To guard against such injuries, it has been proposed by M. Perronct, a French engineer, instead of making the tenons and joggles square, to form them into circular arcs, the centres being at the opposite end of the strut or rafter. This plan appears worthy of general adoption, as it allows the joints to accommodate themselves to changes of form without injury. MI the timbers of a roof are usually fitted and framed together on the ground, and taken to pieces again before being elevated to the building.