The short answer
Weight, pattern and age. Cotswold stone slates are heavy, laid in rows that get shorter as they climb, and hung on oak pegs. They suit the roof timbers beneath, which were built for them. That is why swapping to another covering is a bigger decision than it looks, and why repair is usually the first idea.
Bledington lies in the Evenlode valley about four miles south-east of Stow-on-the-Wold, and the river marks the eastern boundary of the parish and the county line. Its older roofs belong to the Cotswold tradition: stone slates split from oolitic limestone, laid in graded rows, large stones at the foot of the slope and the tiniest near the top.
Q1.1How does a stone roof actually hold together?
Cotswold District Council's technical guidance of 2000 describes slates hung over the batten by one, sometimes two, oak pegs pushed through a small hole cut at the top of each stone. Nothing is glued or nailed flat. The roof depends on gravity, overlap and the pitch, which traditionally ran at 47 to 55 degrees. Once a peg decays or a batten goes soft, a stone drops out of line, and the gap lets water run straight on to the timbers beneath.
Q1.2Why not swap to a lighter covering?
A stone roof is a heavy thing, about 1000 kg per 10 square metres in the guidance's words, and the timbers beneath were sized for it. Changing the covering alters more than the look: it changes the weight, the pitch the roof needs, and how the edges and valleys are formed. Our Cotswold stone roofing page sets out the details of the work.
The guidance also notes that, in the south of the district, people speak of stone tiles and in the north of stone slates. In Bledington you will hear both. Whatever the name, the stone behaves the same way, and a wise owner learns to read the roof from the lane, looking for dark patches, dropped stones and lines that no longer run straight. The stone itself spends decade after decade in sun and frost, and the repair is almost always to the fixings, not the stone.