Zinc is a very thin, light metal, used for roof-covering on a cheaper scale, being about one-third of the cost of lead. It is used for gutters, valleys, soakers, and flashings, in common work, in like manner to lead; but it is chiefly as a cheap covering to flat and iron roofs of stations and other large erections that it is valued and mostly used. Zinc is very fusible and tough, and can be easily bent backwards and forwards without cracking. Moist air, when it does not contain acid, forms a coating over it of oxide, which protects it; but sea and town atmospheres, tainted with acid, soon destroy it.
Zinc is sold in sheets, the qualities of which are distinguished by gauges, varying from 1 to 26, and weighing from 1 to 60 ounces per foot, superficial. The qualities most used for building purposes are from 14 to 20 gauge; more generally the former, which weighs about 18 ounces per superficial foot.
The simplest form in which zinc is used as a roof-covering is in corrugated sheets, about 30 inches wide and 7 feet long, the ordinary kind having corrugations every 3 or 4 inches. The sheets are laid from purlin to purlin, placed about 30 inches apart, for the purpose, lapped on to the corrugations at the longitudinal joints, as well as at the transverse joints, up the roof; and they are secured to the woodwork by round-headed screws with washers, the joints being connected by very small bolts.
The Italian corrugated zinc roof has much wider corrugations - namely, 15 inches apart - and the sheets are laid over rafters with rounded tops, appearing in section as Fig. 492, from which it will be seen they lap alternately over the rafters, and are secured by mushroom-headed screws to the same rafters. Fig.. 493 is a representation of Braby's Italian pattern.
Flats are covered by zinc, as illustrated by Fig.. 494, in which the expansion and contraction (which is greater in zinc than in lead) is provided for. No solder nor external fastenings whatever should be used; and, to ensure the best work, the zinc should be laid by experienced men only.
Zinc is, of course, laid on boarded roofs with 2 1/2-inch drips, to joint the zinc lengthwise, which is treated as Figs. 495 and 496, shown in section and elevation; and these drips should be at intervals of every 7 feet 6 inches, or thereabouts. Roofs with falls exceeding 12 inches in 8 feet need no drips, and the sheets are jointed by folds, as Fig.. 497. The wood rolls, not rounded as for lead, are fixed, as in Fig.. 498, 2 feet 10 1/2 inches centre to centre, so that the zinc between them may be in one sheet, the joints being made in the rolls, as in Figs. 499 and 500.
The ends at the bottom and top of capping are treated as Figs. 501 and 502, and the flashings against walls are dealt with as in Fig. 503.
Zinc may be treated in a very ornamental manner, as well as used for ordinary work and flat roofs. The embossed work on the Hotel Metropole,London, was executed in zinc by Messrs. Braby; and it should be pointed out that the system of zinc roofing here explained is as advised by that firm, who are the English agents for the " Vieille Montague," the best and purest zinc in the market.