In roofs of very large span it is often de sirable, in order to avoid running up to a great height, to form two or more ridges. Roofs without ties may be greatly strengthened by the use of parabolic curves of iron, notched into the rafters of each inclined face, and abutting on the wall-plates, which in such a case are firmly bolted together. Wrought iron straps of various forms are very useful, when judiciously applied, in strengthening the joints of a roof. The Norman roof is an ingenious contrivance for the construction of 'rode of large span with small pieces of wood ; in this arrangement all the rafters abut on joggled king-posts, of which there are several, their relative position being maintained by diagonal braces. Domes of wood, of great size, have been made without trussing, simply by forming the timbers into curved ribs abutting on the , wall-plates, which than form a circle, and are kept in their proper positions by horizontal circles framed with them at intervals, The wooden dome formerly existing at the Halle aux Bits, at Paris, was a remarkably bold example of this kind, being 200 feet in dia meter, and having a large opening in the centre.
Ail the different kinds of roof-coverings require certain fitting arrangements of the timber beneath, according to Weight, angle of , slope, &c. Cast iron, wrought iron, yellow I fir, thatch, tiles, pantiles, plain tiles, slates, ; shingles (thin boards shaped like slates), slabs of thin stone, plaster, asphalte and other cements, tarred paper, Sheets of lead; copper, • iron, tinned iron, and galvaniZed zinc iron • all are used for roof coverings, One of the largest roofs in existence is that over the Riding-school at MescoW; it is 233 feet span, with a slope Of and the external dimensions of the building are 1920 feet by 319.
the most remarkable roof, all things considered, is that which Messrs. Pox and Henderson have recently erected over the Exhibition Building in Hyde Park. The slender appearance of the many hundred columns which support it ; the elegant light ness of the girders which brace the columns together ; the ridge-and "furrow principle by which drainage is secured ; the matchless waggon•vaulted transept, upwards of 400 feet long by 72 in diameter ; and the acres of glass which form the roofing material—all serve to distinguish this roof from any before con-' structed.
The same engineers have recently con structed a magnificent iron roof at the Liver pool terminus of the Lancashire and Yorkshire Railway. It covers five lines of rails and three platforms, and a carriage road twelve yards wide, in one span, having no columns nor supports besides the outside walls ; the span varies from 136 feet to 128 feet, and the total length is 631 feet. The total area thus covered is 83,457 feet. The material used in the con struction of this roof is entirely iron. The framing consists of a series of trussed princi pals, placed at intervals of eleven feet from centre to centre ; and immediately over the principals are fixed wrought iron purlines; which support the covering ; this covering is of corrugated sheet iron, galvanized. The roof is both lighted and ventilated along the ridge by four continuous rows of large sky lights, and two rows of louvres; half the light is distributed along the ridge, and the remain ing half is equally distributed at the eaves ; the total area of light admitted being equal to one fourth of the entire area of the roof.