The short answer
Cotswold District Council's technical guidance names two oolitic limestones as the source of the region's roofing slates, Stonesfield Slate and Forest Marble. Forest Marble can be worked into slates by hand shortly after extraction, while Stonesfield Slate is frost-split, left outdoors until winter weather opens the bedding. Both are limestone, so repairs should match whichever stone is already on the roof.
Two limestones, one roofing tradition
Ask most people what a Cotswold roof is made of and they will say "stone" and leave it there. The council's technical guidance for owners is more exact. It names two rocks and calls both oolitic limestones. Oolitic means the rock is built from tiny rounded grains of lime that gathered on the floor of a warm shallow sea, long before any village was built from it.
Neither is a slate in the geologist's sense. True slate is a metamorphic rock that splits along planes created by heat and pressure deep underground. These limestones part along their natural bedding, and only certain beds are thin and even enough to roof a building. A quarry may hold metres of rock and yield very few beds that will ever sit on a batten.
The same guidance records how far the tradition runs, northward to Chipping Campden and southward almost to Bath. Across that long belt the roofs differ in colour, thickness and surface. Some of that comes from which of the two stones was used, and some from the individual bed in the individual quarry.
Forest Marble: the one that splits without frosting
Forest Marble is the stone worked into slates by hand soon after it leaves the ground. The council's guidance describes the quarryman following the natural lines in the block while it is fresh, and the slates open out without any further treatment.
The Stone Roofing Association's geology notes say the Forest Marble furnished great quantities of coarse roofing tiles. Unlike Stonesfield Slate, it does not require frosting before splitting. The same notes admit a drawback that every slater remembers: peg holes are difficult to make in it. A hole drilled with too much weight behind it can take a corner off the slate, and a slate lost at the peg hole is a slate lost.
On a roof, a Forest Marble covering reads as rougher and more irregular. The face varies from slate to slate, and the shadow lines between courses are deeper. When we look at an old roof close up, that texture helps us tell the two stones apart at a glance.
Stonesfield Slate: the frost-split stone
Stonesfield Slate is frost-split. The block is left out so winter weather can reach the bedding: water works between the layers, freezes, expands and prises them apart into thin sheets that can then be dressed to size. The method depends on the season as much as on skill, which goes a long way to explain why the stone was always a scarcer and more prized material.
Roofs laid in it tend to look finer. The slates are thin, the face is more even, and the pattern of courses has a quieter rhythm than a Forest Marble roof. Searches for the name "stonesfield slate" often come from owners who have been told that is what they have, or who want to be sure.
Thin does not mean weak, and thick does not mean tough. A thin slate is lighter on the timbers but can chip at the edges when it is handled. A thicker one weighs more on the frame and holds water for longer. Neither stone is the better choice in the abstract. What counts is which of them your roof already carries.
Why the stone matters when something needs repair
For an owner the practical point is plain. Where a roof has lost slates, the replacements have to come from the same family, in a matching thickness and colour. A mismatched patch is obvious from the road and can alter how water runs off the slope.
Guidance reproduced in the council's document goes further. Its advice is to buy newly cut stone where that can be done, and to refit old slates only to the roof they were lifted from. For the decision between mending and relaying, see our page on repairing a stone roof or relaying it.
The fixings are part of the same story. Stone that resists drilling asks for a different approach from stone that takes a hole easily, and a slater who has not met the material before can lose a lot of slates while finding that out. We check how the slates are held and what the battens look like before we say what can be saved.
What each stone means for weather and age
Frost helps Stonesfield Slate in the quarry and threatens every stone roof once it is up. A slate that soaks up water and then freezes can flake or lift along the bedding. Our page on frost damage on stone roofs describes what that looks like on a roof and what to do next.
On lifespan, the council's guidance puts a well-maintained Cotswold roof at 100 years easily, with 200 or 300 years possible. That figure dates from 2000 and is not a promise. It depends on the stone, the bed it came from and the upkeep it has had over the years. Our page on how long a stone roof lasts goes through the figures.
A listed house, or one inside a designated conservation area, brings the council into the decision about the roof covering too. Our page on listed stone roofs covers when consent comes into it.
Finding out which stone you have
At a survey we study the thickness and weathering of the slates, the shape of the peg holes and the way the courses step down from eaves to ridge. Our written quote comes with pictures of your own slope, so the evidence is yours to keep.
Not certain what covers your house? Send a few pictures taken from the garden and through the loft hatch. Our Cotswold stone roofing page sets out the wider service.