112. Slate used for roofing purposes, commonly known as clay slate, is obtained from an argillaceous or clayey rock, of a compact and fine-grained texture. This rock is formed through the deposition of matter by the action of water, and belongs to a class geologically termed sedimentary rock, which split very readily into thin laminae or slabs, along parallel planes, called the cleavage planes - a characteristic that renders slate very valuable as a roofing material.

The cleavage planes do not run in any fixed direction with respect to the beds, but incline at various angles, from perpendicular to horizontal. The slate whose cleavage planes are most nearly horizontal or parallel to the natural bed, is unfit as a roofing material, as the slabs will have a rough and irregular surface, which will prevent them from being fitted closely when laid.

113. Good slate should possess both toughness and hardness, and a very fine, but easily distinguished grain. The slates should be tough enough to be easily punched for nailing, and should cut to standard sizes, without splintering or becoming friable at the edges; and they should be hard enough not to absorb much moisture, as the action of frost upon the moisture will cause the edges to crumble and will also tend to enlarge the nail holes, and thus cause the slate to loosen from the roof.

The grain should run lengthwise of the slate. Veins or ribbons are objectionable markings, especially when parallel with the grain. Crystals of pyrites are sometimes found in slate, the yellow variety of which is not very injurious, but still objectionable, except in those qualities classed as second rate. Slate containing white pyrites, however, should always be rejected. The color of slate varies considerably, and is in no way indicative of the quality of the material. Blue, blue-black, purple, gray, green, and red are the commonest tints, though cream color is occasionally but rarely found.

114. Qualities of slate vary with the characteristics of the rock, as above described, as do also the straightness, smoothness of surface, and uniformity of thickness of the quarried product. All qualities are obtainable in various market sizes from 6 in. X 12 in. to 24 in. x 44 in., all 1/8 to 1/4 in. thick, the common average being 3/16 in. Tests to determine the quality of slate are sometimes made, but cannot be relied upon, though they are of value in some instances where certain characteristics are to be determined.

The porosity and amount of absorption may be obtained by weighing a dry slate, as delivered from- the quarry, and reweighing it after soaking for 24 or 36 hours in water; the difference in weight shows the amount of water absorbed. Or a slate may be permitted to stand on edge, in a shallow dish of water, the height to which the water rises in the pores being noted; if the absorption is great, the slate is not suited for roofing, as the frost would tend to splinter the edges.

Slate submitted to the action of dilute sulphuric acid, will, if of a proper degree of hardness, remain unchanged for several weeks, but if of soft quality, it will decompose and crumble in a few days. Powdered slate, submitted to the action of muriatic acid, will effervesce strongly when it contains carbonate of lime, and should not then be used for roofing purposes. Powdered slate submitted to a high temperature, will give off a yellow sublimate of sulphur, when it contains pyrites, and should not be used, as it is not of a durable quality.

Though the above tests tend to show some of the characteristics of different qualities of slate, they are not entirely reliable as determining the value of the slate as a roof covering, as some of the hardest slates will undergo decomposition on the roof, even after passing all of these preliminary tests.

A good slate should present a bright, silk-like luster, and should emit a clear metallic ring when struck with the knuckles, showing that it is hard; if it is soft, it will have a dull lead-like surface, and will give out a muffled sound. When cut, the edges should show a fibrous-like texture, free from splinters, and the material should not show signs of being either brittle or crumbly.

No better test of the wearing or weathering qualities can be contrived than the simple and effective one of examining roofs where the slate has been in service for several years.