Slate double roofing on iron laths shown in Figure 8a is accomplished in an ingenious manner by means of a specially-formed hook. An enlarged view of the latter is given in Figure 8b. The hook liang-s upon the lath above, and grasps the slate below in the middle of the lower edge; so that the hook lies in the joint between two slates of the row next below. To prevent the suspended slate from being lifted by the wind, a small cross plate is fastened to each hook, and is made to lie tinder the two slates that meet at the hook. Each hook is 43/I inches long by of an inch broad, and should be made of copper or g,alvanized iron. In nailing, slates, any straining or bending must be carefully avoided. By a method used in France, slates are fixed in place by a wire clip which holds the bot tom of each slate; for the same purpose there has also been introduced a lead clip which, it is claimed, diminishes the quantity of slate required.
Bedding for Slates.—In covering a roof with projecting eaves, a broad board is placed over the end of the rafters; but when the eaves tail into the rafters, the gutter-board should be of sufficient width to receive the eaves-course. In light slating it is considered best to cover the roof en tirely with rough boarding, but for heavy slates fillets, laths, or battens are used, and applied to suit the length of the slates, at the convenience of workmen. A feather-edged board—styled a " tilting fillet "—is laid against gable- or party-walls, to turn the water. In case of a hipped roof, every course is completed up to the angle by shaping- the slates to fit the slope, and these are covered by an overlap of sheet lead screwed or nailed to the hip rafter. It is usual to employ slate ridging with a roll, or with a groove fitted to receive the ornamental crcstings which are so popular. Elastic cement is also preferred as a bedding for the top course on the ridge and for a space around the hips, valleys, and gutters. Tarred-felt roofing paper is also placed between the roof boards and the slates.
Siacs and Names of Slates.—In the English market the popular sizes for slates have received the following singular names: " ladies," measur ing 13 by S inches; " countesses," 2o by io inches; " duchesses," 24 by r 2 inches; and " queens," 36 by 24 inches—increasing in value as they ascend in rank, with the singular inconsistency that a thick queen is known as a "Welsh rag" and a smaller lady is called a " double." The thickness of the slates varies from —136 to of an inch. Recent authorities in the United States give the pitch of a slated roof at about r in height to 4 in length, the laps being about 3 inches:or a little more. Slate has usually
been classed among the incombustible materials, and as such is especially adapted to use in building; but it cannot be depended upon as a resistant against fire, as it cracks on the application of heat.
Metallic Roofing.—The defence of roofs from fire and moisture by the application of complete coverings of thin sheets of metal has long been practised and is very general, the choice of the material depending mainly upon cost and the peculiarities and variations of climate. As a rule, the sheets should be laid in such a manner as to require the minimum amount of soldering, and to permit the unavoidable expansion and contraction of the metal to take place without injury to the roof. An under course of boards usually supports the metal, but by certain methods this support is rendered unnecessary. To insure the security of the work strict attention must be paid to the points where the sheets are joined, and a clear distinc tion must be preserved between the horizontal joints running in the long direction of the roof and those running from the ridge to the glitter. In the horizontal joinings, folds are usually so arranged that the ends of the sheets are bent either upon or over one another, and are afterward ham mered flat. By this means the water, as it runs down, is kept from pene trating- beneath the sheets, while the necessary freedom of action in the metal is insured.
Instead of these foldings, the sheets are sometimes merely overlapped for about 4 inches. The joints running down toward the glitters are secured in different ways; the most usual methods are seeu in Figures 9 to 12 GO/. 7). Figure 9 exhibits what is known as the "standing- seam," in which the junction of the ends of the sheets is accomplished in a manner similar to that in the horizontal joint, except that it is not hammered flat. The attachment to the roof boarding is secured by a tongue of iron nailed on one side to the under course and soldered on the other to the sheets where they meet. Figure ro, which shows the rolled joint to which the tongue is also applied, requires no further explanation. In the lath joint, shown in Figure r 1, the ends of the sheet are bent up over a wooden fillet and nailed fast on one side over the ends, a special metal cap being placed and also nailed fast to the fillet, the heads of the nails being soldered. Figure 12 exhibits the covered fold, where the sheets, after being rolled together and held by the tongues, are covered by a special cap of metal which grasps the rolls.