slate is a dense rock which is formed from fine grained clays or muds which have been subjected to a metamorphic process involving heat and pressure during which cleavage planes are formed which differ from the original bedding. It readily splits into thin smooth plates. The word slate has now become something of a generic term used to describe any slab material capable of forming a flat stone plate. This might include fissible sandstone which is a sedimentary rock and schists which have been subjected to some metamorphic action but will still split along their bedding planes. These tend to be named after their most conspicuous material e.g. mica schists.
Quality of slate varied depending on its geological background. “Best,” while implying durability related mainly to thinness which was a prized quality, because it meant the slates were easier to transport and would require a lighter roof construction. “Seconds” or “standards” were just as durable but were thicker slates. While the terms are indicative of a common standard this did not arrive until the mid 1930s. In fact, the quality of slate could vary considerably between quarries, reputation was important and many architects, rather than simply asking for “Scots” or “Welsh” slate or even simply “natural slate,” had a preferred source which they would use for classification, Ballachulish in Scotland, Delabole in England or Penryhn in Wales for example. The old slaters had a variety of personal methods of testing a slate. Some would simply strike it and judge its quality by its ring, others would go to the length of standing a sample of slate on its edge in a depth of water which covered half the slate. If over night, capillary action absorbed over say, quarter of an inch of water, they would be discarded.
(Illustration - Craiglea Quarry near Perth)
In most parts of Britain, there were around twenty sizes of slates recognized as standard sizes, most of which had picturesque titles such as “Queens”, “Duchesses” and “Countesses”. Most quarries also produced smaller sizes which tended to be given slightly derogatory names such as “rags” or “peggies.” The Scottish quarries were capable of producing large standard sized slates but tended to concentrate on producing random sizes which were classified into two main groups, “sizeables” and “undersized.” Whatever size was produced the overall tradition throughout Britain was to produce slates in which the width was not less than half the length, a Duchess for example was 24x12 ins a Countess 20x10 (not many trades can boast of laying 250 countesses in an afternoon). Scottish sizable slates would average 15x8ins while undersized slates averaged 10x6.
An adequate definition of slating is difficult. Definitions, if they are to be remembered should be kept simple and the purpose of slating is simple. It is to lay a network of flat stone plates, calculating the manner in which they overlap, in such a way that water will run under the force of gravity from the ridge to the wallhead without penetrating into the roofspace. However, people rightly expect their roofs to last and because roofs can be large, accounting for a surprising volume of a building, and can also be highly visible with eye-catching features such as turrets and dormers, to this basic definition should be added some recognition to the ideal of achieving a degree of permanence and aesthetic quality.
Slates should be laid double lapped i.e. each slate covers part of the slates in two courses below. They can be single or double nailed. Colours are sometimes mixed for decorative effect, and for similar reasons, can be shaped differently from the rectangle, i.e. fish-scale slating. Smaller slates require a steeper pitch of roof, bigger slates can be laid to a shallower pitch but this should never go below 22 degrees. Slates were traditionally sold by number or by weight, a tally, hence tally slating, which was sometimes referred to as “ton” slates.