1. Slate stone is valuable for the property of splitting in one
direction, so as to afford fragments of a sufficient size and thinness
to answer several purposes, but especially for covering houses and for
writing slates. The best slates are those which are even and compact,
and which absorb the least water.
2. The slates used in the United States, are obtained either from our
own quarries, of which there are several, or from those of Wales, in
the county of Caernarvonshire. The stone is quarried in masses, which
are afterwards split into pieces of suitable thinness. These are
trimmed to an oblong figure by means of a knife and a steel edge,
which act upon the slate much in the manner of a large pair of shears.
3. As it is impossible to dress all the slates to the same size
without much waste of material, those engaged in their manufacture
have introduced several sizes, the smallest of which are made of the
fragments of the larger kinds. These are designated by names known to
the trade, and to those practically conversant with the art of
building.
4. The slates, when brought to market, especially those from Wales,
require additional dressing to fit them for use. The manner of
applying them to roofs differs but little from that employed in
putting on shingles, as they are lapped over each other in the same
way, and confined to their place by means of nails of a similar kind.
The nails, however, have a broader head, and are somewhat larger,
varying in size to suit the dimensions of the slate. The holes in the
slate for the nails are made with a steel point attached to the
slater's hammer, or to his knife, technically called a _saix_.
5. Slates are preferable to shingles on account of their durability,
and, in a majority of situations, for their fire-proof quality. They,
however, are objectionable on account of their weight and
expensiveness, and are therefore beginning to be superseded in this
country by sheets of zinc, and by those of iron coated with tin.
Copper and lead are also used for roofs, but the metals just mentioned
are beginning to exclude them altogether.
6. A serious objection to metal roofs has been their liability to
crack, caused by the contraction and expansion of the material, in
consequence of variations in the temperature of the weather; but a
particular method of putting the sheets together has been lately
devised, which appears to obviate the difficulty. Tiles are not used
in this country, although in Europe they are very common.
[Illustration: PAINTER & GLAZIER.]