Allusion has been made in 3 previous column, to the various materials used for the covering of roofs, with reference to the different degrees of inclination suitable for them. Thatched roofs have been considered by some to maintain the most equable temperature in the buildings covered by . keeping out alike the extreme heat of summer, and cold of winter. They are objectionable on account of their harbour ing vermin, being easily damaged by wind, and dangerously combustible. The frequent repairs required, make thatch also an expensive material. Besides straw, reeds and heath are sometimes used for thatching, and possess the adavntage of greater durability. Tiles admit heat and moisture more than good slates. Pantiles having no holes for nailing through, are simply hung by ledges, upon laths nailed to the rafters. Plain tiles, laid in mortar, and overlapping so as to be double thickness everywhere, make a very good, though heavy covering. Tiles of a peculiar form, called hip-tiles, are used for covering salient angles; and gutter-tiles, which are similar to them, but placed with the concave side upwards, in the valleys or receding angles. Slates are laid in various ways. They arc sometimes nailed down on across boarding : or if large, on battens, or pieces of wood from two and a half to three inches wide, and three-quarters of an inch to an inch thick, which are nailed to the rafters at intervals regulated by the length of the slates. Lozenge-shaped slating is occa sionally used, and has an ornamental appearance, but is easily injured, as there is but one nail through each slate. It is always laid on boarding. For what is called patent slating, the best large slates are selected, and fixed without either boarding or battening, the common rafters being placed at such a width as to come under the joints. The slates are screwed down, the courses over-lapping about two inches. The meeting-joints are covered by fillets of slate, about three inches wide, set in putty, and screwed down ; and the hips and ridges are sometimes covered in the same manner, though it is best, in all such cases, to use lead. Patent slating, when well executed, is water-tight, with as low a slope as one in six. In some districts, of stone are used in lieu of slates or tiles. Shingles, which arc like slates, but made of wood, were formerly much used in covering pyra midal steeples, and in roofs of steep pitch. They are still used in the United States, and arc usually laid on boarding, in a similar manner to common slates.
Sheets of metal are very convenient for covering domes, and curved angular surthces generally, and also for flat roofs, or such as have too little slope for slating. Lead is the most common material for such purposes, though copper, iron, tinned-iron, and recently zinc, are also used. Lead terraces, or flats, are commonly laid on boarding or plaster. The joints are sometimes soldered, but the most approved method is to roll or wrap the edges into each other, making allowance for expansion and contractiom A fall of a quarter of au inch in a foot, is sufficient for surfaces covered with sheet metal.
The weight of lead is somewhat against its use for roof. covering ; it is used in sheets weighing from 4 lbs. to S lbs. per foot super. Copper is much lighter, and is used in sheets weighing about 1 lb. per foot super, but its expensive ness precludes its adoption for general purposes. Zinc is much lighter than lead, weighing from 12 to 20 ounces per foot super ; and of late has been very widely employed on this account, as well as because of its cheapness ; it forms a very useful covering. Iron galvanized, or coated with zinc, has also been very extensively used for this purpose, for which it is also well adapted. Sometimes it is corrugated, by which it is considerably strengthened, and thus some of the strength of the roof itself may be dispensed with, espe cially in small spans, where curved roofs of corrugated iron may be safely employed. Tiles of this material also, are
sometimes used ; they are treated much in the same way as common tiles, being simply nailed to battens or boarding. The following table, giving the weight per square of 100 feet for different kinds of roof-covering, may prove serviceable :— Having now given a description of the several varieties of roofs covering a square or rectangular plan, it may not be improper to give a few examples of domes, and show how they may be constructed under various circumstances. If the dome to be constructed be on a circular plan, with no lantern above, the ribs may be built in the following manner, with planks of convenient lengths, in three or more thick nesses. Having ascertained the length of the ribs. and the number of pieces in that length, and having properly shaped all the pieces to the curve, the middle piece at tne bottom may be one of these lengths ; to each side may be joined two other pieces, one reaching to a third of the middle piece, and the other to two thirds from the bottom, so that by continuing with planks of the whole length to the other extremity of the rib, the middle thickness will always be covered two•thirds from the bottom on one side, and one-third on the other ; the deficiency at the top must be filled up with pieces, one of a third, and the other of two-thirds, as at the bottom ; the whole being well bolted together, and strapped across the joints, will be nearly as strong as a solid rib.
Plate VIII. Figure 1, shows the manner of constructing this kind of dome; No. 1 being the semi-plan ; No. 2, the eleva tion ; No. 3, the manner of building the rib. In domes of this kind, it may sometimes be necessary to discontinue the ribs, that the spaces may be more equally divided for the horizontal ribs. It is evident that a dome built in this man ner, may be carried to almost any extent, provided it have a sufficient number of horizontal ribs. Of this construction is the Halle du Ble, at Paris, of 200 feet in diameter, the invention of a judicious carpenter, the Sieur Molineau, a man of little scientific education, but of considerable mechanical experieneo. Being convinced that a very thin shell of tim ber might not only be so shaped as to he nearly in equilibria but that, if well connected with horizontal ribs, it would have all the requisite stiffness, he presented his scheme to the magistracy of Paris. The grandeur of the idea pleased them but they referred it to the Academy of Sciences. The members, who were competent judges, were struck with the justness of M. Molineau's principles, and astonished that a thing so plain, had not become familiar to every house carpenter. It quickly became a universal topic of conver sation, dispute and cabal, in the polite circles of Paris. But the Aeadamy haying given a favourable report of their opinion, the project was Immediately carried into execution, and soon completed, and now stands as one of the greatest exhibitions in Paris. The circular ribs, which compose this dome, con sist of planks 9 feet long, 13 inches broad, and 3 inches thick, made in three thicknesses, as in that already described. At various distances, these ribs are connected horizontally by purlins and iron straps, which make so many hoops to tile. whole. When the work had reached such a height, that the distance of the ribs was two-thirds of the original distance, every second (now consisting of two ribs, very near each other,) was discontinued, and the void glazed. A little above this, the heads of the ribs arc framed into a cir cular ring of timber, which forms a wide opening in the middle, over which is a glazed canopy, or umbrella, with an opening between it and the dome, for allowing the heated air to get out. All who have seen this dome agree in describing it as the most beautiful and magnificent object they ever beheld.