There are also several areas in Britain where the traditional roofing material is locally sourced flagstones; such as Caithness flagstone in the north of Scotland or Northumberland stone in the north of England. Flagstones are sedimentary rocks which are capable of being split along primary bedding planes. They are also referred to as “grey slates” to distinguish them from blue or real slate.
The type of sedimentary rock varies; sandstone, limestone and sandy shales are all used as roofing materials, for example limestone of the Great Oolite in the Cotswolds, and carboniferous sandstone is common in the north of England. Only those in which the original bedding planes are spaced between 15 and 25mm are suitable for roofing; closer spaced bedding produces a material which is too friable. Conversely, flagstones produced from seams in which bedding is more widely spaced are too thick and heavy to be widely used as roofing.
Flagstone, being thicker and therefore heavier than real slate, were not normally transported far from source. As a result, the type of roofing material and hence the vernacular architecture was influenced by the local geology providing variation so important to the built heritage.