Allusion has been made in a previous column to the various mate. riala 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 them, keeping out alike the extreme heat of summer and cold of winter. They are objectionable on account of their harbouring vermin, being easily damaged by wind, and danger. ously combustible. The frequent repairs required make thatch also an expensive material. Besides straw, reeds and heath arc sometimes used for thatching, aud possess the advantage of greater durability. Tiles admit heat and moisture more than good slates. Pantiles, having nn boles for nailing through, are simply hung, by ledges, upon lathe nailed to the rafters. Plain tiles, laid in mortar, and over-lapping, se as to be of double thickness everywhere, make a very good though heavy covering. Tiles of a peculiar form, called hip-tiles, are used for covering salient angled ; and gutter-tiles, which aro similar to them, but placed with the concave side upwards, in the valleys or recediug angles. Slates are laid in various ways. They are sometimes nailed down on a close 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 occasionally 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 beet large slates are selected, and fixed without either boarding or battening, the common rafter 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 lamb= of stone are used in lieu of slates or tiles. Shingles, which are like elates, but made of wood, were formerly much used in covering pyramidal steeples, and in roofs of steep pitch. They are still used in the United States, and are usually laid on boarding, in a similar manner to common slates.
Sheets of metal are very convenient for covering domes, and curved or angular surfaces generally ' • and also for flat roofs, or such as have too little slope for elating. 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 contraction. A fall of a quarter of an inch in a foot is sufficient for surfaces covered with sheet metal.
Cements of various kinds have been applied to the formation of roofs, and in some eases with success, though they have often been found to crack, and thereby become permeable to water. Mixtures of tar with lime, sand, gravel, ashes, &c., have been recommended; and asphalte has been applied to this purpose, apparently with great advantage. Compositions of tar, resin, and similar substances, spread upon sheets of coarse paper, have also been used.
(Nicholson, Architectural Dictionary, Practical Builder, &c., &c. ; Tredgold, Principles of Carpentry ; Robison, Mechanical Philosophy; Rondelet, I 'Art de bdttir.)