Roofs in Ireland are most commonly constructed of slate. This is a natural material deriving from a geological transformation of the sedimentary rock, shale. The principal characteristic of slate is that it can be split into very thin sheets along the bed of the sedimentation and since it is a rock it provides a surface that is impenetrable to water. Slate is naturally a blue black colour. Its shade can vary greatly according to locality and it is widely found throughout Ireland.
Slate of a very high quality is necessary to provide a good roofing material, and from the early nineteenth century it was common for the best slates to be imported to Ireland: either from Ballachulish in Scotland or from Bangor in North Wales. As with the manufacture of glass, the size of individual pieces of slate becomes larger as methods of extraction were more refined. An older building will usually have smaller slates which will vary considerably in size between those at the top or ridge of the roof (which are small) and those at the bottom (which are large). From the middle of the nineteenth century slates are usually uniformly large over the whole roof.
In this period to counteract the dull appearance of large plain roofs, architects, who wished to create a picturesque effect, sometimes introduced patterns of slates using different colours or changes in the cutting of the slate to give a diamond or semi-circular ‘fish scale’ effect.
Artificial ‘slates’ made of a cement compound are commonly used in Irish building today though they lack the life and texture of the true material.
Source : www.askaboutireland.ie