Slate (q.v.) is a strong and very impermeable material, and these qualities and the fact that it is easily split into thin plates suitable for laying, as well as its low cost, for many years caused it to be by far the most generally used of all materials for roof covering.
Slates are cut to many differ ent sizes varying in length from 10 in. to 36 in. and in width from 5 in. to 24 in. There are per haps thirty or more recognized sizes, each distinguished by a dif ferent name. In common prac tice those generally used are "large ladies," 16 in. by 8 in.; "countesses," 20 in. by io in.; and "duchesses," 24 in. by 12 in. Generally speaking, the rule gov erning the use of the different sizes is that the steeper the pitch the smaller the slate, and vice versa. Buildings in very exposed positions naturally require steeply pitched roofs, if they are to be covered and rendered weather-tight by small lapped units of covering.
Slates may be fixed by nailing at the head or at about the middle. The latter method is the stronger, as the levering effect of the wind cannot attain so great a strength. There is a small economy effected by centre nailing, as the margin is slightly larger and fewer slates are required to cover a given space; longer nails, however, are required, for as slates are laid at an angle with the pitch of the roof their centres cannot be made to approach so near to the slating battens or boarding as the head, which lies close on the surface to which it is fixed. Another important point is that the nail-holes in the centre-nailed slating are only covered, by 3 in. of the tail (the amount of the "lap") of the course of slates above, and rain is very liable to be forced under by the wind and cause the wood battens or other woodwork to rot. Head-nailed slates, on the other hand, have their holes covered by two layers of slate, and are removed from exposure by the length of the gauge plus the lap, which in the case of "countess" slating equals II in.
A point in favour of centre-nailing is that the slates are more securely held to the battens or boards and offer much more resistance to being lifted by the wind.
"Open slating" is an economical method of laying slates that is often adopted for the roofs of sheds, foundries and temporary buildings. The slates in the same course are not laid edge to edge as in close slating, but at a distance of two or more inches apart. This forms a roof covering light in weight and inexpensive, which, although not strictly weather-proof, is sufficiently so for the buildings upon which it is used.
Slates are laid upon open battens fixed upon the rafters or upon close boarding or upon battens fixed upon boarding. The battens are 4 in. or i in. thick and Li in. to 3 in. wide, and are spaced to suit the gauge of the slates. When close boarding is used it is often covered with inodorous asphalted felt, and to allow of ven tilation of the materials, under-battens are sometimes fixed ver tically, ridge ventilators introduced and air inlets arranged at the eaves. The beds of slates laid without provision for the admission of air have been found occasionally to have rotted so as to scale and crumble easily.
The nails used in slating are important and the durability of the work depends on a good selection. They should have large flat heads. The most satisfactory are those made of a composi tion of copper and zinc, but others of copper, zinc, galvanized iron and plain iron are used. Those of copper are most durable, but are soft and expensive while zinc nails are soft and not very durable ; they will last for about twenty years. Iron nails even if galvanized are only employed in cheap and temporary work; they may be preserved by being heated and plunged in boiled linseed oil. The pitch of a roof intended for slating should not incline less than 25° with the horizontal, while for the smaller sizes 3o° is a safer angle to adopt.
Modern slate roofs are frequently laid with varying courses and of varying thicknesses, usually with the heavier slate from three-quarters to an inch and a half thick at the eaves, with thinner and smaller slate in the upper part of the roof. Marked variation of colour is often sought by combining green and purple slate, or fading and unfading slate.