107. Zinc, one of the useful metals, generally extracted from mountain limestone and magnesium limestone in conjunction with galena, is met with abundantly in the form of a sulphuret. The ore is roasted, mixed with charcoal, and heated in peculiar retorts. The zinc is converted into vapor, condensed, and fused.

Cast zinc is brittle when cold. Heated to 200°, it is ductile as well as malleable, and may be rolled into sheets retaining these properties at that temperature; while, if the temperature be allowed to exceed 400°, the zinc returns to its original condition of brittleness. When exposed to the air, at a high temperature, it will burn and be consumed.

Zinc is easily acted upon by moist air, but a film of oxide is soon formed, protecting the metal from further action. Its expansion and contraction are greater than that of any other metal. Zinc is nearly as durable as lead or copper, and is not as liable to be affected by the sun's heat. In its metallic form, zinc is largely used for roofing and cornices, and also for galvanizing iron.

Good zinc should be free from iron, and zinc containing more than 1 per cent. of lead should be rejected, the lead making it brittle. Good sheet zinc is uniform in color, tough, and easily bent backwards and forwards without cracking. Inferior zinc is dark in color and of a blotchy appearance, caused by the presence of other metals. The conjunction of the dissimilar metals is liable to set up galvanic action, which soon destroys the zinc.

Zinc should not be secured by, or connected with, iron, copper, or lead, for, in either case, voltaic action is induced, which destroys the zinc.

108. Zinc is described by the weight and thickness of the sheets to the superficial foot. The gauges used for roofing run from 13 to 16. The weights of these gauges run from 1 pound to 1 pound 8 ounces per square foot.

The size of sheets varies from 24 to 52 inches wide by any length ordered. The ordinary length of sheet varies from 6 to 10 feet, but zinc may be obtained in long roll lengths.

109. Solder should never be used in zinc roofing except for the small vertical joints of flashings. Before soldering, the joints are fluxed with muriatic acid.

Paper or felt should always be used under zinc where it comes in contact with woodwork containing acids, as, for instance, oak; otherwise, these acids cause the zinc to corrode.

110. The methods employed in laying zinc roofing are hollow roll, wood roll, trough and cap, and standing seam. Where there is no boarding, corrugated roofing is generally employed; the corrugations give the plates strength. Soldered or rigid connections are to be particularly avoided, and the joints must be so arranged as to be weather-tight, but allowing free movement for expansion and contraction.

Hollow-roll zinc roofing is constructed in the same manner as with copper, as shown in Fig. 63.

Wood Roll

Same as copper roofing, Fig. 65.