Asphalte or bitumen has come into use as a material for roofs. It had been employed for various purposes in France for many years, but did not attract much attention till within the last eight or ten years. It is now in very general use in that country for foot pavements, flat roofs, and water-cistern linings; and in England it has also been a good deal used for the same purposes, and for barn-flooring. The particular modes in which it is employed for floors and pavements we need not here consider, but it has been used for roofs in the following manner. Mr. Pocock has patented a “flexible Asphaltic roofing,” intended to supersede the use of slates, tiles, zinc, thatch, &c., in the covering and lining of farm-buildings, sheds, cottages, and other erections; and it is approved for its durability, lightness, and economy. The weight of this material being only sixty pounds to the square of one hundred feet, the walls and timbers to support it need to be but half the usual substance; it is also a non-conductor of heat, impervious to damp, and will bear a heat of two hundred and twenty degrees without injury. This peculiar material is said to be formed of asphalte mixed with the refuse felt of hat manufactories, compressed into thin plates.
Scotch Fir Roofs.
Scotch fir roofs are occasionally made. The method of giving durability to the timber for this purpose consists in first cutting the wood to the required size, and then steeping it for a fortnight in a pond of lime-water; it is found that the acid contained in the wood becomes crystallized by combining with the alkali of the lime. Sir Charles Menteath is said to have some farm buildings which, although roofed with Scotch fir forty years ago, are as well protected now as when the roofs were first laid on; the wood having been previously steeped in lime-water. The sulphate of copper, the chloride of zinc, the corrosive sublimate, and the various other chemical substances which have been recommended of late years as means for preventing the decay of timber, will possibly render the use of timber roofs more practicable than it has been hitherto considered.
Iron Roofs.
Roofs of iron are in great request at the present time. One of these sorts of roofs may be formed of three kinds of cast-iron plates. The first, called the “roof-plate,” is shaped with three of its sides turned up and one turned down, and is made tapering narrower towards one end; the second, called the “low-ridge plate,” has two of its sides turned up and the other two turned down; the third, called the “high-ridge plate,” has all its sides turned down, and is formed with an angle in the middle, so as to slope each way of the roof. Such a roof may be made very flat, inasmuch, that for a house twenty feet wide, the height of the roof in the middle need not exceed two feet; no boarding is required, the plates resting without either cement or nails on the rafters. From the manner in which the edges of the plates overlap, there is no risk of contraction or expansion.
Some of the iron roofs recently made are on the principle of those used in Russia, of which the following description has been given in the Repertory of Patent Inventions:—“Sheet-iron coverings are now universally made use of in all new buildings at Petersburgh, Moscow, &c. In the case of a fire, no harm can come to a house from sparks falling on a roof of this description. The sheets of this iron covering measure two feet four inches by four feet eight inches, and weigh twelve pounds and a half avoirdupois per sheet, or one pound five ounces each superficial square foot. When the sheets are on the roof, they measure only two feet wide by four feet in length: this is owing to the overlapping. They are first painted on both sides once, and, when fixed on the roof, a second coat is given. The common colour is red, but green paint, it is said, will stand twice the time. Small bits or ears are introduced into the laps, for nailing the plates to the two-inch square laths on which they are secured. It takes twelve sheets and a half to cover one hundred feet, the weight of which is one hundred and fifty pounds—the cost only £1 15s., or about threepence per foot.”
Iron roofs are now often made of corrugated or furrowed sheet-iron. In this form the iron is impressed so as to present a surface of semi-circular ridges with intervening furrows lengthwise of the sheets. By this means, a piece of sheet-iron, which, as a plain flat surface, has no strength but in its tenacity, becomes a series of continued arches abutting against each other; and the metal, by this new position, acquires increased strength. Iron so furrowed is deemed preferable to common sheet-iron for covering a flat-roof, because the furrows will collect the water and carry it more rapidly to the eaves. But there are greater advantages than this. If the furrowed sheets be bent into a curved surface, convex above and concave below, they will form an arch of great strength, capable of serving as a roof without rafters or any other support, except at the eaves or abutments. Iron roofs measuring two hundred and twenty-five feet by forty have been constructed in this manner. To increase their durability the iron sheets are coated with paint or tar.
Zinc and other Metallic Roofs.
Additions are made every year to the number of contrivances for forming metallic roofs, among which is one now the subject of a patent, for the use of galvanized iron. In this case the aid of the electric agent is employed to give iron sheets an amount of durability which they do not possess in their natural state.
Zinc has been much employed within the last few years as a material for roofs. Its availability for this purpose rests partly on its superior lightness as compared with lead, and its superior condition under the action of the atmosphere as compared with iron. The latter quality arises thus; after the zinc has been covered with a thin film of oxide by the action of the atmosphere, it suffers no further change from long exposure; so that the evil of rust checks itself. At the temperature of boiling water, zinc sheets, which are brittle when cold, become malleable, and their availability for roofs is thereby increased. The property which zinc has, however, of taking fire at a temperature of about 700° Fahr., rather detracts from its value as a material for roofs.