Cotswold Stone Slate

Cotswold stone slates are roofing slates made from the oolitic limestones of the Cotswold hills in south-central England. They are the traditional roof covering of the region's villages, farm buildings and churches and are closely associated with its honey-coloured stone architecture.

Sources

Cotswold District Council guidance describes two source beds:

• Forest Marble: split by hand soon after extraction, while the stone still holds its natural quarry moisture. These slates, called "presents", represent the oldest method, possibly dating from the Roman period.
• Stonesfield Slate: split by frost along its natural bedding planes, giving thinner, more regular slates. These were used on prestigious buildings, including Oxford colleges.

Sizes

Slates are laid in diminishing courses. The largest, about 600 mm (24 in) and occasionally up to 750 mm (30 in) long, form the double eaves course, known locally as "cussoms" and "followers". The smallest, "short cocks", are about 150 mm (6 in) long and were the most numerous, used near the ridge. Slaters traditionally used a range of local names for the intermediate sizes.

Roof form

• Pitch: straight roofs are usually 47-55°.
• Fixing: each slate is hung over a batten with one, sometimes two, cleft oak pegs through a hole near the top. Holes were once made with a pointed hammer and are now drilled.
• Ridges: angled dressed stone, or roll-topped on more elaborate buildings such as churches.
• Valleys: traditional swept valleys use specially cut slates to curve the roof through 90° in plan; a central wedge-shaped "bottomer" is held by side pieces. The guidance regards close-mitred or open lead valleys as cheaper but inferior.

Roofs were deliberately allowed to bow slightly, which tightened the slates and helped shed rain and snow.

Weight and maintenance

A Cotswold stone roof weighs about 1,000 kg per 10 m² (about 20 lb/ft²). Regular scraping to remove moss is recommended; with maintenance a roof can last about 100 years and possibly 200-300 years.

Conservation

Guidance follows English Heritage policy: use new stone slates matching the original where possible, keep reclaimed slates on the building they came from, and avoid artificial, concrete, reconstituted or sawn slates on historic buildings. Imported limestone slates from the Côte d'Or in Burgundy, France, slightly more orange-yellow in colour, have been used successfully on some Cotswold projects.