The short answer
A stone slate roof is repaired by refixing slipped stones, renewing the pegs and battens that hold them and keeping the stones in diminishing courses, biggest at the eaves, smallest below the ridge. Stripping a whole slope is a last step, taken only when the fixings have failed across it.
The village has a stone slate roof that anyone can look at: the Great Barn. The county history describes its walls as rubble with ashlar-faced buttresses, under a stone slate roof, and it has stood since the late thirteenth century. Smaller roofs in the village work on the same principle, scaled down to a cottage, an outbuilding or a porch.
Q1.1How do the courses work?
Stone breaks into far more small slates than large ones, so the biggest go along the eaves and the smallest are saved for the top. Cotswold District Council's technical guidance, written in 2000, calls the long eaves stones cussoms and the tiny ones at the top short cocks. Every stone is pegged in oak through a hole at its upper end and hung over a batten.
Q1.2What goes wrong with stone slate?
The stones outlast what holds them. Pegs rot, battens soften and a stone slips, leaving a gap that water enters. On many cottages the repair is to refix the stones that have moved, with new pegs, and to re-bed the ridge, and the roof carries on for years. A stone ridge is a simple angled piece of dressed stone, and we reset it rather than replace it where we can.
Q1.3When is a slope stripped?
Where the fixings have gone right across a slope, we strip it carefully, grade the stones by length, replace battens and lay the sound stones again. Broken ones are made up from matching stone, and the guidance prefers new stone to second-hand. Our Cotswold stone roofing page sets out the process.
Stone slate is also very heavy, about 1,000 kg for every 10 square metres in the council's guidance, so we check the roof structure before any plan that changes the covering, and we photograph the slope before it is lifted. On completion we sweep the site and carry the waste off.