The short answer
The parts that hold the stone up, far more often than the stone. Rotting pegs, softening battens and failed felt or lime beneath let sound stones slip. Verges, ridges and the junctions at chimneys take the weather first, and are where leaks usually begin on a roof of this kind.
Shilton's older houses draw on local Cotswold stone, and where the roof matches the walls, the result is heavy, durable and demanding to look after. Most problems come from age acting on the fixings, not from the stone itself.
Q1.1Why do pegs, battens and bedding give way?
Traditional stone slates are held to a batten by a wooden peg, often oak, threaded through a small hole near the head of each stone. As the decades pass the pegs decay and the battens lose their grip, and stones start to drop. Gaps between courses were once dressed with moss or lime, and when that goes, wind-driven rain gets in and the felt or boarding beneath takes the damage. Fixing one stone at a time rarely holds if the surrounding fixings are equally tired, so we test a wider area before quoting.
Q1.2Why do verges, ridges and chimneys leak first?
The gable ends and ridge take the worst of the weather. Ridges on old Cotswold roofs were traditionally dressed stone, bedded in mortar rather than clipped, and that bedding eventually perishes. Our roof repairs page describes how a failed ridge or verge is made good. Chimney flashings are the other usual failure, where lead has cracked or mortar fillets have fallen away. A chimney in an old Cotswold house is also heavy, so we check that the stack is not leaning before any new lead is dressed against it.
Q1.3How is the character of a stone roof kept?
Matching stone is chosen by size, thickness and tone, and new stone is cut to continue the diminishing run rather than sitting out of step with it. Our Cotswold stone roofing page explains how courses diminish and why substitutes are rarely a good idea on historic roofs. Concrete products made to imitate stone slates are heavier and look different, and conservation guidance treats them as unsuitable for historic buildings. Reclaimed stone is best kept for use on the building it came from.
Photographs from the survey show which of these faults your roof actually has, and which can safely wait for another year. Written notes on the loft side of the roof, such as sooty staining, soft battens or pegs lying in the insulation, often explain what the outside view cannot. A roof of this weight rewards getting the diagnosis right before any stone is touched, because every stone lifted is a stone that must go back exactly as it was.