Of the two materials - slates and tiles - generally used in towns for roof-coverings, slates are the more common, being cheaper and better suited to the flatter kind of pitched roofs (of about 30 degrees)which are less expensive than the steep ones. Slates are the production of an argillaceous or clay-formed rock, so fine-grained and compact that it splits readily into extraordinarily thin slabs of various sizes, which are formed into slates for roof-coverings, while the greater thicknesses are converted into shelving and slabbing for other purposes.
A good slate should be hard and yet tough, of very fine grain and uniform colour, thin and very non-absorbent, so much so that a good dry slate, after standing 14 hours in water halfway up its height, should not have drawn up and absorbed any moisture above the water-line. Another sign of good quality is to be able to breathe on the slate without bringing forth any clayey odour, which would be a sign that it will not weather.
Wales and Westmoreland produce most of the slates used in this country, the latter being of a green colour - thicker, rougher, and much heavier than the Welsh. The latter are mostly of the same good quality, and remarkable for their thinness and smoothness, the best being from Penrhyn and Port Dinworic, the colours of which are alike. Of the many varieties the chief are distinguished by their colour. A Bangor slate is of a purple colour, while a Penrhyn slate may be either blue, purple, or green. Portmadoc produces a blue slate, and Whitland Abbey those of a green colour, which are thicker and softer than the other kinds.
Stone slates are found at Colley Weston and Naunton, while many of the Yorkshire stones can be split fine enough for roofing purposes. A stone state is 3 non-conductor of heat, while the ordinary kind of slate is a good conductor of heat.
The chief sizes in use (with their names) are as follows: -
30 inches long by
26 " "
24 " "
34 " "
22 „ „
11 or 12 "
20 " "
16 or 14 " "
10 or 8 "
13 " "
10 or 7 "
below the latter.
Slates are fixed, by two nails each, to the roof boardings, or to the longitudinal strips of wood (called laths or battens) ninning along the roof across the rafters. Of the two the latter is preferable to the boarding. The nails are either of galvanized iron, zinc, or copper.
The back of a slate is its upper surface or face, while the underside is called its bed. The head is, of course, its upper and the tail its lower edge. That part of the slate which is visible, when fixed on the roof, is called the margin, its depth being called the gauge.
The lap of a slate is the distance that the top slate projects or laps over the second one beneath it (as Fig.. 504); and this word is used in conjunction with the name of the slates, of course, to denote the kind of work which is required to be done.
Thus we specify that Duchess or Countess slates are to be used, laid to a 2 1/2 or 3-inch "lap"
Sometimes, however, the word gauge is used for the same object; as laid to such and such a gauge, which depends on the lap, though more chiefly on the size or kind of slate to be used. Given the lap and the name or size of the slates, the gauge can be found by calculation - i.e., from the length of the slate deduct the lap, and half the remainder (because the slates are in two thicknesses) will be the gauge. In laying slates to a specification calling for " Countess slates laid to a 2 1/2-inch lap," for instance, the first thing a practical man - not having the gauge at his fingers' end, as it were - would do, would be to calculate the "gauge." Thus: From 20 inches (the length of a Countess slate) deduct 2 1/2 inches = 17 1/2; half of that difference will be the gauge - i.e., 8 3/4 inches. This last-named Fig.ure is the guide by which the slater " holes " his slates for the nails, and fixes his battens to the rafters, or marks his line for the nailing along the boarding. Good work, in slating, should have the tails of all slates perfectly horizontal, and the vertical alternate joints ranging in straight lines from eaves to ridge.
The first proceeding of a slater is to square three sides of the slates, the head being left rough, and then to hole them for the nails, which may be done either at the head or in the centre. When nailed at the head, the two nail-holes are pierced at a distance of about 1 inch downwards from the head, so that the tail of the second slate, when laid on above it, covers their position by 2 or 3 inches, which is the difference between two gauges and the length of the slate, less the distance the nail-holes are pierced from the head of the slate.
Centre-nailed slates do not have their holes exactly in the centre, but at the distance of the sum of the gauge and lap above the tail. Thus, the holes of Countess slates laid to a 3-inch lap would be: 8 1/2 inches (the gauge) plus 3 inches (the lap) = 11 1/2 inches, above the tail, or thereabouts; so that the nails may clear or miss the head of the slate below. Centre-nailed slates resist high winds very much better.
Fig. 505 gives a section and plan of Countess slates laid to a 3-inch lap and centre-nailed; while Fig.. 506 gives the same particulars of Duchess slates "laid to a 2 1/2-inch lap and top-nailed"; and Fig. 507 those of 16 x 8 inches Ladies' laid to a 6 1/2-inch gauge and nailed at head.
From the above sketches it will be noticed that there is a different-sized slate at the bottom (at the eaves), shown by dotted lines; this is called double eaves, and its length is a little more than the sum of the lap and gauge, so as to allow for the nail-hold above. It should also be pointed out that, in centre-nailed slates, this involves the placing of two battens together at the bottom for eaves, whence, it should be noted, all slating starts, and is worked or laid in courses upwards.
Slates are bonded by half-bond, as the plans indicate, this necessitating a half-slate at the gables, etc.
Ridges have also a double course at the top, as Fig.. 508, surmounted by either a plain ridge, as Fig.. 509; slate ridge and roll, as Fig.. 508; or ridge-roll and crest, as Fig.. 510.
Torching is the pointing of the underside of slates with hair mortar, used to stop the driving of snow.
Full torching is the filling up of the space between battens with mortar on the underside.
Shouldering is bedding the heads in mortar to tighten the nail hold and prevent the wind penetrating.
Rendering is the filling up of the space between battens or boarding to bed the slates down.
Open slating is used for open sheds and inferior work, to save slates, each slate being from 1 1/2 to 4 inches apart, instead of butting up to each other in the courses.
Bedding is the setting in mortar of the top of the bed of one slate or tile on the back of the one beneath it.
Cement-filleting an inferior substitute for lead or zinc soaker flashings, is the filling up with cement-mortar of the angles on roofs where they abut up to vertical parts, such as dormers, gables, chimneys, and other roofs, so as to prevent the damp running down the walls at the edge of the slates.
Verge-pointing is the pointing up of the sides of the slates running up gables over which they project.
Slates may be cut or laid to patterns, in order to give the roof a more ornamental appearance; but such alterations must not interfere with either lap or bond.