A technical guide for owners and occupiers, published by the district council in April 2000, remains a fair account of traditional practice. Two oolitic limestones supply the slates, Forest Marble and Stonesfield Slate, and the author observes that quarried rock naturally yields far smaller pieces than large. The roofs are graded because the stone is.
Q2.1What does a diminishing course actually mean?
The biggest stones, normally about 600 mm long, go along the eaves in a doubled course, which the guidance names as cussoms. Followers get smaller as they climb, and the smallest, the short cocks, finish the slope near the ridge. It makes good use of what comes out of a quarry, and it gives the roof its graded, weathered look. A new stone in the wrong course shows immediately. Our Cotswold stone roofing page covers the method.
Q2.2Is the stone or the fixing the usual failure?
More often the fixing. Pegs, traditionally oak, carry each slate over a batten, and once a peg or batten rots the stone falls though nothing is wrong with it. The same guide says a roof scraped each year might last twice as long, a hint that steady upkeep beats any single big repair. Weight matters too: about 1000 kg to every 10 square metres, which is why we look at the rafters before any change of covering.
Q2.3Can I use reclaimed stone?
That depends on its source. The guide records English Heritage policy favouring new stone slates, with reclaimed ones kept for the building from which they were taken, so we ask where any stone has come from.