Of late years, many roofs have been constructed of iron, a material which at first began to be introduced for particular members, such as tie and suspension.rods, but afterwards became employed for the entire truss, and sometimes for the covering likewise. Iron roofs are for die most part of similar construction to those already described of timber, those members which are subjected to tension, such as ties and suspending rods, being of wrought-iron rods, and those which suffer compression, such as principals amid struts, of cast-iron. Such roofs have been very extensively employed in railway works, for the covering of passenger and engine sheds, and such like.
The roof of the passengers' arrival and departure shed at Euston Grove has a very light and elegant appearance ; it is constructed principally of wrought-iron, the bressumers, columns, and gutters, only being of cast-iron. The entire width is SO feet, formed in two spans of -10 feet each, a row of iron columns and bressumers supporting the rafters in the centre and outside, and on the opposite side by iron corbels built in the wall, and further secured thereto by strong bolts and nuts; the rafters are six feet eight inches apart, and are of wrought iron, in form of the letter T; the slate battons, as they may be called, are of angle-iron, firmly riveted to the back of the rafters, at such a distance as the slates require, and to which they are secured by strips of copper. The roof is firmly tied from side to side by a tension-red of one inch and a quarter diameter, to each pair of rafters, and is further supported and braced by struts of T iron and suspension-rods, with nuts and screws to adjust their length. The entire length of the roof is 200 feet ; the gutters are cast in lengths of 10 feet each, joined together by flanges and bolts, and so fixed as to form an incline towards each column, which, being cast hollow, and having a pipe connected with a drain, they form a convenient and easy conveyance for rain from the roof The truss consists of two principals—a tie-beam, which is sustained by three suspending-rods and four struts, two on either side • one from the head of the central suspending rod to the foot of the outer one, and another thence to the principal.
The roof of the locomotive engine-house at Camden Town is of very similar construction to the one above described : the rafters are of T iron, and the slates are supported by angle-iron riveted to the rafters. There are cast-iron chairs
secured down to a stone coping on the walls, and from which the rafters spring; each pair of rafters is tied by means of a tension-rod, and otherwise supported and braced by struts of T iron, and suspension-rods of round iron, which make the whole very firm, and gives it a light and pleasing appearance.
Both these roofs were manufactured and erected by Messrs. Cubit, of Gray's Inn Road, under the direction of C. Fox, Esq., now resident engineer to the Company.
Of the numerous roofs of this material which have been erected over railway works, the following examples will give some of the principal varieties. The first, for a roof 30 feet span, consists of two principals, with tie-rod, which is sus pended at two points, by two oblique rods meeting at the apex, by which means the king-rod is dispensed with. At the meeting of the suspension-rods with the tie-rod, a strut branches out at right angles to the principal, which it meets at half way. The rafters and rods are connected at the ridge in a separate casting, or the two ends of the rafters may be cast to half lap over each other, and the rods secured by wrought pins and nuts.
The rafters are of cast-iron of the section, the dimen sions of the lower table or flange being, in the centre of the length, 31 inches wide, and at the two extremities, la inch, with a thickness of z inch ; and those of the upper table 21 inches throughout the entire length, and inch thick. The depth is the same throughout, being -1/ inches.
The tension-rods are 1 inch round at the outer ends, and inch midway ; and the oblique suspension-rods 4- inch round throughout.
The second example for the same span consists of two principals and tie-rod, which is suspended at three points by a king-rod and two queen-rods, the bearings into three equal parts : struts rise from the foot of the king-rod to the heads of the queen-rods.
The rafters are formed of two parallel bars, 3 inches by inch, having a wooden rib 11 inch thick between them. The struts are of T iron, 21 inches wide over the top table, and g- inch thick ; rib 2i inches deep, and inch in thickness.