The short answer
Stone slate in the Cotswolds is a limestone of the oolitic kind, split thin and pegged to battens in wood. Sizes shrink up the slope, from broad slabs at the eaves to little pieces at the ridge. The result is a very heavy roof, traditionally laid steep, which asks a great deal of the timbers below.
In April 2000 the district council issued a technical guide for people who own and occupy buildings with stone slate roofs. It is dated, which makes it a record of traditional method and no use as a price list, but the way these roofs go together has barely changed.
Q1.1Which stone is it made from?
The slates come from two limestones: Forest Marble, and the stone that gives Stonesfield Slate its name. Forest Marble comes off the quarry face and is split by hand quite soon afterwards. Stonesfield Slate takes longer, because it has to be frost-split: left outside for the winter until the cold prises it apart along its beds.
Q1.2How do the courses diminish?
Nature supplies few large slates and plenty of small ones, and the roof follows what the quarry gives. Down at the eaves go the longest pieces, typically around 600 mm, laid in a double course called cussoms and topped by followers. Climbing the slope, each course is a little shorter than the one below, and the shortest of all, the short cocks, sit right at the top. Each slate is holed near its top, and an oak peg through the hole hangs it over the batten. Traditional pitches are steep, in the range of 47 to 55 degrees.
Q1.3How heavy is it?
The guide gives a figure of about 1000 kg for every 10 square metres of roof. That one number explains why stone slate asks so much of the frame and the walls, why scaffolding and handling matter on these roofs, and why anyone proposing a change of covering has to think about the load first. Traditional ridges are dressed stone, cut to an angle, rather than clay or concrete sections. If you would like to read more about the work, our Cotswold stone roofing page covers it.
