Zinc in sheets is a material largely used as a roof covering, and if care be taken to ensure metal of good quality, it is strong and durable, as well as light and inexpensive. Zinc is stronger weight for weight than lead, slate, tile or glass, but weaker than copper, wrought-iron or steel, although with the exception of the latter two it is not so durable under normal roofing conditions. It is not liable to easy breakage as are slates, tiles and glass. It is usually supplied in flat sheets, although it can also be had in the corrugated form similar to corrugated sheet-iron. When exposed to air, a thin coating of oxide is formed on the surface which protects the metal beneath from any further change, and obviates the necessity of painting. In laying the sheets, the use of solder and nails should be avoided entirely except for fixing clips and tacks which do not interfere with the free expansion and contrac tion of the sheets. Zinc expands freely, and sheets laid with soldered seams or fixed with nails are liable to buckle and break away owing to movements caused by changes of temperature. The usual sizes of zinc sheets are 7 ft. or 8 ft. long by 3 ft. wide and weighing from I II to 25 oz. per sq.f t. The thickness varies from 25 to 19 Standard Wire Gauge. A dependable method of laying zinc on flat roofs is with the aid of wood "rolls," about 2 in. X 2 in. in section, splayed at the sides, spaced 2 ft. 8 in. apart and fixed to the roof boarding with zinc nails. Iron nails should not be used as this metal affects the zinc. The sheets of zinc are laid between the rolls with their sides bent up II in. or 2 in. against them, and held firmly in position by clips of zinc attached to the rolls. A cap of the same metal is then slipped over each roll and fastened down by tacks about 3 in. long sol dered inside it so as to hook under the same clips that anchor the sheet. Drips of about 24 in. are made in the slope at intervals of 6 ft. or 7 ft.—that is, the length of a sheet—and care must be taken at these points to keep the work waterproof. The lower sheet is bent up the face of the drip and under the projecting portion of the upper sheet, which is finished with a roll edge to turn off the water. The end of the roll has a specially folded cap which also finishes with a curved or beaded water check, and this in conjunction with the saddle piece of the roll beneath forms a weather-proof joint (figs. 15 and 16). The fall between the drips is usually about 12 in. deep, but where necessary it may be less, the least permissible fall being about 1 in 80. Felt laid beneath zinc has the effect of lengthening the life of the roof and should always be used, as the edges of the boarding upon which it is laid are, when the latter warps, apt to cut the sheets. It also forms a cushion protecting the zinc if there is traffic across the roof.
Sheet-lead forms a much heavier roof covering than zinc, but it lasts a great deal longer and more easily withstands the attacks of impure air. Lead must be laid on a close boarding, for its great ductility prevents it from spanning even the smallest spaces without bending and giving way. This characteristic of the metal, however, conduces largely to its usefulness, and enables it to be dressed and bossed into awkward corners without the necessity of jointing. The coefficient of expansion for lead is nearly as great as that for zinc and much higher than for iron ; precautions to allow free expansion and contraction must be taken when laying the lead covering. The manner of laying is with rolls and drips as in the case of zinc, the details of the work differing some what to suit the character of the material (see figs. 17, 18 and 19) ; the use of nails and solder should be avoided as far as possible. Contact with iron sets up corrosion in lead, and when nails are necessary they should be of copper ; screws should be of brass. Lead is supplied in rolls of 25 to 35 ft. long and 6 ft. to
7 ft. 6 in. wide. That in general use varies from one-fourteenth to one-seventh of an inch in thickness. The weights most suitable for employment in roofing work are 7 or 8 lb. per square foot for flats and gutters, 6 lb. for ridges and hips, and 5 lb. for flashings.
As a roof covering copper is lighter, stronger and more durable than either zinc or lead. It expands and contracts much less than these metals, and although not so strong as wrought-iron and steel it is much more durable. From a structural point of view these qualities enable it to be classed as the best available metal for roof covering, although its heat-conducting properties require it to be well insulated by layers of felt and other non-conducting material placed beneath the metal. On exposure to the air cop per develops a feature of great beauty in the coating of green carbonate which forms upon its surface and protects it from further decomposition. Perhaps the chief disadvantage in the use of copper has been in its first cost, but it is now comparatively cheap and account must also be taken of the almost imperishable nature of the metal and that its light weight requires less sub stantial framework for its support. Copper roofing should be laid in a similar manner to zinc, with wood rolls at intervals of about 2 ft. 4 in. It is, however, often laid with welted seams. The general stock sizes of sheets are from 4 ft. to 5 ft. 3 in. long and 2 ft. to 3 ft. 6 in. wide. The thickness almost invariably used is known as 24 S.W.G. and weighs 16 oz. per square foot. Thinner metal would suffice, but owing to the increased cost of rolling very little would be gained by adopting the thinner gauges.
In the United States of America "tin" roofs are quite com monly used. Sheets of wrought iron coated either with tin or zinc are used of a size usually in. by 20 in., though they may be had double this size. Prepara tion for laying is made by fixing an insulating foundation of some what stout paper or felt ; this must be dry, else it is apt to spoil the impermeable covering laid upon it by causing it to rust.
Junctions between the sheets are made by welted seams in which the four edges of the sheets are turned over so as to lock togeth er, thus forming one large sheet of tin covering the roof. In high-class work of a permanent nature the seams in addition are soldered, rosin only being used as a flux. Each sheet also is se cured to the roof with two or three tin cleats. The life of such a roof may be practically doubled by the application of a coat of good paint, which, however, adds considerably to the cost.
The greatest use of bituminous materials in modern roof cover ings is in the form of tar and gravel or tar and slag roofs. These consist of three or more layers or plies glued to each other and covered by heavy coats of coal tar pitch. Onto the upper layer of pitch, before it has hardened, there is sprinkled a bed of gravel or slag, which is held in place by the pitch, and at the same time protects it. Flat tile roofs, ter races or promenades are fre quently built in a somewhat similar manner, with an under layer of several plies of pitch and felt, covered with an inch thick bed of cement, on which the tile is laid. Adequate expansion joints, filled with some elastic cement, are usually installed in the tiled surface. Such roofs, either tile covered or gravel and slag covered, are perfectly water tight, do not crack under ex tremes of heat and cold, and have become the most common type for the better class of flat roof, such as those of large modern office and apartment buildings, etc. If adequately flashed at the intersections with walls and parapets, they can be built perfectly level without fall or slope.