The resulting wide variety in quality and colour meant that slating practice can vary from region to region. In Scotland, the industry never properly mechanised, slate remained a hand produced product and to avoid wastage a vast range of sizes were produced. This meant that slates were laid in diminishing courses , where the size of the slate decreases towards the ridge. They can also vary between 6-15mm in thickness giving roofs a wonderful texture. Elsewhere in Britain the industry mechanised and slating became less random in nature as the range of sizes produced became standardised.
Slates can be single or double nailed and are sized in a variety of ways.
In Scotland, they are single nailed, round headed or shouldered, and laid on sarking , and they are numbered depending on the distance between the nail hole and the leading edge. Colours are sometimes mixed for decorative effect, and for similar reasons, can be shaped differently from the rectangle, ie fish-scale slating. Smaller slates require a steeper pitch of roof, bigger slates can be laid to a shallower pitch but this shouldn't go below 22 degreees. In certain of the scottish quarries, every 1000th slate was stamped with the mark of the quarry, something which producers in England, Ireland and Wales never did. Glossary - Slate Terminology Diagam Back the upper surface of a slate as laid.