This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
A good slate should present a bright, silk-like luster, and should emit a clear metallic ring when tapped; if it is soft, it will present a dull, lead-like surface, and emit 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. It is this element in slate, which may be called its temper, that largely defines its value; if brittle or too hard, the slate will be liable to fly to pieces when being squared and punched, or to be readily broken when being nailed in place. If the slate is soft, it will, by absorbing moisture, be liable to be attacked by frost, so that the edges will crumble away, and the slates will work loose, owing to the nail holes becoming enlarged.
There are many varieties of color in slate, due to the presence of iron and mineralized vegetable substances. The pronounced colors are bluish black, blue, red. and green, while many tints of these shades exist, blending into grays and purples.
The commercial classification is based on straightness, freedom from curled and warped surfaces, smoothness of surface, and uniformity of color and thickness. The best way of judging the classification is to examine samples of the various grades. First-class slate should be hard and tough, non-absorbent, unfading, straight-grained, free from ribbons and other imperfections, and of uniform color throughout. No better test of the weathering qualities of slate can be contrived than the simple one of examining roofs where it has been in service for several years.
The sizes of slate range from 6 in. X 12 in. to 16 in. X 24 in., there being about 25 different sizes. The size to select depends on the character of the edifice and its location; for ordinary dwellings, a common size is 8 in. X 16 in. The thickness of slate is about 3/16 in., or 5 to the inch; for extra-strong roofs, slates 1/4 in. thick are used, while with the larger-sized slates, the thickness is 3/8 in. or more. Slate weighs 175 lb. per cu. ft., and if 3/16 slates are used, the weight of the material on the roof will average 6 1/2 lb. per sq. ft. of surface covered. Slates are sold by the square, meaning that this quantity will cover 100 sq. ft. of roof surface; an ordinary railroad car has a capacity of between 40 and 50 squares.
The roof being devised for protection against the elements, particularly rain and snow, the steeper the pitch, the more effective will be its power to shed them; while the wind will not so readily blow rain under the courses, nor strip them off so easily. Where slates are small and light, and exposed to violent winds, the greater the necessity for increasing the pitch. Experience shows that the minimum pitch for slate roofs varies with the size of the slate. The pitch may be expressed either by the angle which the roof makes with the horizontal, or by the ratio of the height of the ridge to the span; the latter expression is the one generally employed. (SeeTableVII,page68.) For large sizes of slate the pitch should not be less than 21° 50', or one-fifth; for medium sizes, 26° 33', or one-fourth; and for the smaller sizes, 33° 41', or one-third. Slates are laid on either strips or boarding; the latter costs the most, but the results justify the extra cost. Close boarding makes the roof a better non-conductor of heat, and is required for strength where thin slates are used. In Fig. 1 is shown a sectional view of a roof on a frame building, where theslates are nailed to strips a. These strips are usually from 1 to 1 1/4 in. thick and 2 1/2 or 3 in. wide, and are well nailed to the rafters; the distance between centers of the strips should be equal to the gauge or posed portion of the slate. The lowest strip is thicker than the others, so as to tilt the first course enough to insure a close-fitting joint between the first two thicknesses. In commencing to slate, the first course is laid double, the lower course being laid with the back or upper surface of the slate downwards. The length of this course will be equal to the gauge plus the lap. The lap is the distance that the upper slate overlaps the head, or upper end of the second slate below it, and should not be less than 3 in., although slaters sometimes use only a 2" lap. The gauge or exposed length of the slate is equal to half its length after deducting the lap.
There are several ways of nailing the slate, either near the head or at the shoulder, in which the distance of the holes from the head will be slightly less than the gauge of the slate, to enable them to clear the head of the one underneath.
Another method of laying slate, where economy is desired, is that known as half slating, in which a space is left between the edges of the slate in each course equal to one-half the width of the slate. By this method, only two-thirds of the usual number are required to cover a given surface. This class of work is adapted for covering sheds, and while serviceable under ordinary conditions, will not be watertight under the action of driving storms of rain and snow.
Slate nails have large flat heads, so as to have a good hold on the slate; their lengths vary with the thickness of the slate, and are usually called 3-penny or 4-penny*, having lengths of 1 1/8 in. and 1 3/8 in., respectively; the proper length should be twice the thickness of the slate plus the thickness of the boarding. To prevent rust, slate nails are usually galvanized iron or steel; sometimes they are tar-coated. For extra-good work, copper nails are used.
For other data on Slating, see page 351.