Cotswold District Council's technical guidance on stone slate roofing names two limestones used for the purpose, Forest Marble and Stonesfield Slate, and observes that the rock breaks into many more small pieces than big ones. That is the reason for the graded pattern on a traditional roof: the longest stones, normally about 600 mm, form a double course along the eaves, and the shortest, which can be about 15 cm, sit just below the ridge.
Q2.1What holds a Tetbury stone slate in place?
Every slate is hung from a batten by a peg of wood passing through a hole drilled close to its head. The guidance mentions oak, and when a peg rots the slate slides. A stone roof is also very heavy, roughly 1000 kg for every 10 square metres in the council's figure, so the timbers underneath must be sound before anyone relays the covering. During the survey we check rafters, purlins and bearing points, and we report what we find.
Q2.2Can a stone valley or ridge in Tetbury be kept?
Traditional stone roofs often have valleys formed from specially cut slates that curve round in a gentle sweep, and ridges made from dressed stone sections. Replacing a swept valley with a plain lead trough alters both the appearance and the way water runs off. Where the existing valley can be kept, we keep it.
A stone-slated outbuilding or barn conversion in the countryside around Tetbury follows the same logic. The roof was built to flex a little, shed rain quickly and last for generations, and a good repair respects all three.
Upkeep counts too. The council's guidance claims that scraping a stone roof clean each year can double how long it lasts, which in practice means clearing moss and debris before it holds damp against the stone. A short visit to rehang the odd slipped slate can spare you a far bigger bill later.