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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.

Glossary - slate terminology thumbnailSlate Terminology Diagam
Back the upper surface of a slate as laid.
Bed the underside of a slate as laid.
Gauge - the distance between the nail hole of a slate and the nail hole of the slate in the preceeding course.
Head - top edge of a slate.
Head lap - the distance the leading edge of a slate overlaps the nail hole of the slate two courses below.
In band - also referred to as batchelor slates, are narrow slates which sometimes have to be inserted into courses to maintain the proper width of side lap.
Lap or cover - the distance between the tail of a slate and the nail hole of the slates it covers in the preceeding course.
Margin - tail area or tway (Scottish), the area of slate left exposed when laid on a roof.
Nail sickness - failure of the nails holding slates in place due to rusting, hence, copper nails are now widely specified.
Patent slating - is large slates laid with no lap, where the joints are mortared and covered with slate fillets. More usually seen where stone slates are used.
Pin rule - a length of timber marked with half-inch gradations which is used by scots slaters to grade their slates.
Sage - (Scottish) a rectangular metal bar with a spike at right angles at each end, which can be hammered into sarking boards. Used by slaters to dress slates while on a roof. Also referred to as an "edge" or "top edge".
Saixe - a slaters knife, used for dressing slates. Usually a heavy blade, off set from its handle, with a 2"-3" spike on its top edge, used for putting nail holes in the slates. The blade is sharpened only on one side, as a result of which this is a tool in which both right and left handed versions are available.
Side lap - the distance between the edge of a slate and the edge of the slate it part covers in the preceeding course.
Slate-and-a-half - a slate, positioned at the end of a course where it meets a hip or valley, that is at least 1.5 time the with of other slates in the course. Used to prevent water ingress.
Shouldering – the practice of rounding the top corners of a slate which meant that an uneven slate would sit better but, more importantly, shouldering and single nailing also allows a slate to be swung aside to expose the nail head of the slate below so that it can easily be re-nailed in the event of it having worked loose.
Tail - the leading edge of a slate.
Tifting - the laying of thicker slates on the wallheads or close to skews so that water runs into the middle of, and down the pitch of the roof, rather tan spilling down the face of gables, or penetrating under mortar fillets.
Tingle - a metal strip, one end of which can be hooked over a batten or nailed to a sarking board, after which the other end is bent to hold in place a flashing or a slate that has slipped. Sometimes referred to as a "latchet" a "tab" or a "tack".
Water fan - this is the fan-shaped area that water will cover having penetrated the joint between slates. Clearly the calculation of side lap and head lap becomes very important otherwise water can penetrate through to the sarking or battens.(Illustration)
Wrestlers slates which are notched at each side allowing them to interlock to form ridges.