A waterproof material to cover buildings is an universal requirement, not least in a wet climate such as Britain’s. In the past the choice of material usually depended on local availability. Hence, in the absence of suitable stone, thatch, either of straw or reeds, was commonly used throughout the country, while clay tiles were used in areas with suitable clay deposits, such as the southeast of England, . Flagstone was used locally in many parts of Britain, such as Horsham stone in Sussex or Caithness flagstone in the north of Scotland. Slate, which is generally found in mountainous parts of Britain, was used as a building material in the North Wales, parts of Scotland and the Lake District. This use of locally sourced stone led to regional vernacular architecture reflecting the local geology.
Unlike flagstone which was rarely transported far from its source, the distribution of slate gradually increased in the 19th century, spreading out from its source, often in remote areas, along historic trade routes. Improved transport systems coinciding with rapid urban growth resulted in slate being transported to all the major cities in Britain and Ireland, eventually becoming the principal roofing material. The industry expanded rapidly to accommodate this demand, producing over 650,000 tonnes per annum in 1898. In spite of the rationalisation of the industry, and more recently globalisation, it is still possible to recognise the historical trading links. For example, because of the ease of transport across the Irish Sea, Cambrian slate from North Wales was used widely throughout Ireland and is still the preferred slate in that country. Similarly Cumbrian slates were transported northwest along the coast into Ayrshire and are still used extensively in the SW of Scotland. Scotland too had a significant slate industry, producing from four different areas of which Ballachulish is the best known.
Production started to decline soon after 1900 and had already dropped to 111,000 tonnes in 1918. The industry partly recovered in the 1920s, to 297,000 tonnes in 1929, but by then manufactured clay tiles had become a major competitor and were taking an increasing proportion of the roofing market. This decline in production continued to the end of the 20th century reaching a low of 25,000 tonnes in 1993 followed by a modest recovery. Quarries in Wales and England continued to close although production never ceased completely. However, in Scotland the largest quarry at Ballachulish closed in 1955 and the last remaining Scottish quarries closed in the 1960s.