The structural arrangement of a building frequently renders it impossible to form a sloping roof over all parts of it, and hence flats are necessary. When this is the case, metallic coverings are the best that can be adopted.

Formerly it was not uncommon in buildings where cost was not a consideration to use sheet copper for covering flats or slight slopes. Copper forms a very light covering, as it may be safely used in sheets not more than ' 03 in. thick, which would weigh about 20 oz. per ft. super. Copper slowly oxidizes when exposed to the air, but the oxide does not eat into the substance of the metal as is the case with iron; it seems rather to form a protective coat. The cost of copper renders its use very limited, and zinc has to a large extent taken its place.

Zinc is also a very light covering; in fact its specific gravity is slightly less than that of copper, but it has not a good reputation, owing to the fact that on its first introduction it was used in very thin sheets, and sufficient care was not taken in laying it. Its expansion is greater than that of any other metal, and therefore it should always be laid with ample play, or it will soon buckle and crack. The Vieille Montagne Company have greatly improved the methods of laying zinc, and they have also introduced thicker sheets than could previously be obtained; if zinc is used at all, it should never be less than No. 16 gauge, which weighs about 24 oz. to the ft. super, and is as nearly as possible 1/24 in. thick. Zinc resembles copper in the fact that it oxidizes on the surface only; but in smoky districts it will not last at all, as sulphuric acid completely destroys it.

The surface oxidation only of zinc when exposed to ordinary atmospheric influences, suggested the attempt to prevent the rusting of iron by giving it a thin coating of zinc. This led to the production of "galvanized" iron for roofing purposes. This "galvanizing" process consists in first precipitating tin upon sheets of iron by means of weak galvanic action, and then placing the plates in a bath of liquid zinc. Iron thus treated will last, under favourable circumstances, for a long time; but when used for roofing, it is almost impossible to avoid nailing the sheets in some places, and where the nail holes occur, moisture invariably makes its way to the iron itself, which rusts internally, and the thin zinc coating then comes off in flakes. What was previously stated as to the action of sulphuric acid upon zinc will show the utter uselessness of galvanized iron in smoky districts.

The most durable metal covering for roofs is milled lead. This is lead which, after being cast, is passed through a mill between rollers adjusted so as to give the requisite thickness to the sheets which are rolled out. This thickness varies from .075 in. to .236 in., the weight of these qualities being 4 lb. and 14 lb. respectively per superficial ft. The qualities chiefly used for roofing are the 5-, 6-, and 7-lb. lead, the latter being the lightest that should be used for flats or gutters. In laying lead on flats, the latter should be close boarded, and care must be taken to allow for expansion and contraction of the metal; consequently the joints of 2 adjacent sheets must not be soldered together. In order to prevent the water from penetrating at the joints, fillets of wood, 2 1/2 in. by 2 in., rounded at the top, called rolls, are nailed to the boards, and one sheet of lead is dressed close up to and half-way over the roll, while the next sheet is brought up to the opposite side of the roll, and lapped completely over the roll and the turned-up portion of the first sheet. If the lead is closely hammered down with wooden mallets, no nails are required, and they are better omitted.

When it is necessary to nail the lead round skylights or in other positions, copper nails should always be used.

Thin flat plates of iron rendered "rustless" by the Bower-Barff process have lately been introduced for roofing purposes by Horn, Black & Co., and many advantages are claimed for it on the score of cheapness, durability, and appearance, as compared with other metallic roofings. There are several modifications in the pattern and mode of fixing, but the simplest is illustrated in Fig. 1360. Here plain flat plates, 24 in. by 12 in., are laid like ordinary slating, except that oxidized hoop iron is laid over each line of plates, through holes in which, as well as in the slates, are driven the nails which fasten both to the boards beneath. The use of this iron lath allows any accidentally damaged plates to be removed at any time, when specially-made plates can be substituted. These special "mending-plates" are so formed as to hook easily and securely upon the iron lath, and to be so fixed without the necessity of cutting the nails which fasten the adjoining plates. All these plates, both ordinary and special, are supplied ready bored with the holes for the nails; and suitable nails, similarly protected with the oxide, are furnished at a reasonable price.

It is only fair to say that much of the preceding information has been gathered from a paper on Building Materials by John Slater, B.A., in the Sanitary Record.