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. Thatch Roofs. The most common material employed as thatch is either the straw of wheat, rye, or other grain, or reed, stubble, or heather. The straw of wheat and rye, when well prepared and laid, forms the neatest and most secure thatching; the former being preferable to the latter in smoothness, suppleness, and durability. Barley-straw is placed next to rye in fitness for thatch, and oat-straw the lowest of the four. The reed is a very durable material for thatch, but is generally too expensive. It has been stated that, in Norfolk, where the reed is a favourite material for thatch, a reed roof will lie fifty years without wanting repair, and that, with very slight attention, it will last for a whole century. Viewed in this light, a reed roof may probably be considered economical.