_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 15_s._, or about threepence per foot.”