Stone Slate

A stone slate is a thick, flat roofing piece split from a sedimentary rock, usually limestone or sandstone, rather than from true metamorphic slate. Because they split along their original bedding planes, geologists call such rocks tilestones. Stone slate roofs are heavy, long-lived and strongly tied to local geology, and they give regions such as the Cotswolds and the Pennines in England much of their character. In France, flat roofing stones of this kind are called lauzes, and in the Swiss Alps roofs are made from stone slabs several inches thick.

Main English types

Historic England groups English stone slates by rock type:

• Limestone: Purbeck (Dorset), Stonesfield and other Cotswold slates, Collyweston (Northamptonshire) and Whitwell magnesian limestone (Derbyshire).
• Sandstone: Horsham stone (Sussex, Surrey, west Kent), Pennine sandstones from the Coal Measures and Millstone Grit, and Welsh Marches tilestones.

Sandstone and limestone slates should never be substituted for one another or mixed on a roof.

Production

Most stone slates came from shallow surface workings called delves rather than deep mines; weathering weakens the bedding planes and makes splitting easier. At Collyweston and Stonesfield, blocks were wetted, covered and left over winter so that frost would split them, producing thinner, lighter slates. Slates were then dressed to shape and holed with a pointed tool, giving a tapered hole that grips a peg.

Laying

• Diminishing courses: slates come in random sizes. The largest go at the eaves and the sizes reduce towards the ridge, while the headlap is kept constant.
• Pegs: slates were traditionally hung on laths with dried oak pegs, often bedded in lime mortar at the head; copper nails or pegs are also used.
• Torching: lime mortar mixed with aggregate and hair may be applied to the underside to steady the slates and reduce draughts.
• Swept valleys: specially cut slates turn the roof through a curve in place of a metal valley.

Pitch depends on the slate: small, thin Cotswold slates need about 50°, while large, thick Pennine sandstone flags suit lower pitches.

Weight and life

Stone slate roofs are very heavy; Cotswold guidance gives about 1,000 kg per 10 m², or roughly 100 kg/m² (20 lb/ft²). With regular maintenance, including scraping off moss, a roof can last about 100 years and possibly 200-300 years.

Supply and conservation

Declining production has led to the use of imported, second-hand or artificial substitutes. Historic England's guidance favours new slates matching the original geology over second-hand slates (which should stay on the building they came from), and regards concrete and reconstituted stone as unsuitable for historic buildings. Some quarries have reopened to supply conservation work, including Horsham stone at Nowhurst in 2004 and a Collyweston mine approved in 2015.