Slates are a species of argillaceous rock which, owing to lateral pressure, has become laminated, causing it to split easily in thin sheets along its plane of cleavage. Slates suitable for roofing purposes are found in many parts of England and Wales, but the quality and texture vary a good deal with the locality from which they are obtained.
Irish Bangor slates should be avoided, for they are of an unpleasant and gloomy blue-purple colour, which never tones to an agreeable hue. Irish slates are, as a rule, somewhat thicker and coarser, especially from the quarries of Kilkenny and Killaloe.
English slates are thick and have a rough surface with jagged edges. They are obtained from Westmorland, Lancashire and Cornwall, and are infinitely preferable to any others, as they are of a beautiful grey-green colour, which gives a bright and cheerful appearance and forms a pleasing contrast with redbrick walls. Installation: They should be laid in courses diminishing in size towards the ridge, which helps to give scale to the design. There is also a practical advantage in this, because as the bigger slates are at the lower part of the roof, over which most of the rain-water passes, there are fewer nail-holes for the possible entry of rain.
Slates are in some cases preferable to tiles in exposed situations, as they are less absorbent and can be used with a roof of lower pitch, but, as they are conductors of heat, they make the upper rooms cold in winter and hot in summer.
Increasingly popular today are manmade and artificial or synthetic roof slate tiles, and these imitation and man made slates can also be considered.