The short answer
English Heritage policy, quoted in Cotswold District Council's roofing guide, says one of these slates must never stand in for the other and the two must not share a roof. Cotswold slates are oolitic limestones, so a patch of sandstone would be the wrong rock. They differ in grain, weight, colour and weathering, so a mixed roof looks wrong and can wear unevenly.
The rule, and where it comes from
The roofing guide that Cotswold District Council wrote for owners sets out English Heritage's policy on where stone slates should come from. One line in it settles this question: a sandstone slate must never be used in place of a limestone one, or the other way round, and the two kinds should not appear together on a building.
It is a short rule with a long reason behind it. A stone roof is a single material laid by a slater who knew exactly how it would behave. Introduce a second rock and the covering stops acting as one thing. Some slates take up water differently, some weather to a different colour, and some hang from the battens at a different weight.
The same policy prefers new stone to second-hand wherever possible and holds that old slates should return only to the roof they were lifted from. It also rules out substitutes for historic buildings, naming concrete tiles, fibre resin slates and reconstituted stone. Our page on reconstituted stone slates deals with that last point.
Why Cotswold roofs sit firmly on the limestone side
The council's guide is clear about the source: Cotswold slates are split from two oolitic limestones, one called Forest Marble and the other Stonesfield Slate. Both are limestone, so they are compatible with each other in a way that neither is with a sandstone.
Elsewhere in England some roofs are covered in sandstone slate, and a reader who has seen them might wonder why the difference matters so much. The answer is that a roof is judged against its own region. In the Cotswolds the colour, texture and grain of the limestone give a village its look, and a sandstone patch would stand out against it even from the road.
For a fuller account of the two Cotswold stones, see our page on the geology of Cotswold slate.
What goes wrong when the two are combined
Appearance comes first. Stones from different beds differ in tone, grain and surface, and a patch of the wrong rock shows as a flaw in an otherwise uniform roof. Where the building is listed, or the covering helps define the street, that matters a great deal.
Performance comes second. Thickness and weight vary from rock to rock, and a slate heavier or thinner than its neighbours will not settle onto the course beneath it in the same manner. The lap between courses can open, and water finds a path into the gap. Stones also take up moisture at different rates and respond to frost in their own ways, so a mixed roof ages unevenly, and failures appear in the weaker material first.
Fixing is the third issue. Each slate hangs from a peg set in a hole at its head, and a different stone may call for a different hole, a different peg or a different batten spacing. We cover the principle on our page about oak pegs.
None of this is a matter of snobbery about stone. It is a matter of the whole slope behaving the way it was designed to behave, season after season, with nothing on it that wants to move, flake or drain differently from the rest.
Ridge, underlay and the rest of the roof
The rule reaches beyond the slates in the main field of the roof. Traditionally the ridge was dressed from stone in a plain angled section, and the stone chosen should agree with the stone beneath it. Our page on stone ridge tiles explains the detail.
Underlay is a separate question because it is hidden. Whatever lies beneath the slates should suit a roof of this weight and character, and our guide to underlay for stone roofs goes through the options.
All of this adds up to a simple instruction for owners. Wherever a repair is planned, whether it is a few slates, a new ridge or a hip, the stone should match the stone already on your building. If anyone proposes something different, ask for the reason in writing, and ask where the stone has come from.
Theft, salvage and the temptation to patch
Mixing often starts with a shortage. Slates go missing in a storm or are taken, the owner wants a quick fix, and whatever is available gets laid. A pallet of salvaged stone of unknown origin may turn out to be the wrong rock, and reclaimed slates properly stay with the building they were taken from.
If slates have been taken from your roof, our page on stolen stone slates explains what to do and why replacement stone needs choosing with care.
Looking after the roof you have reduces the pressure to patch. Our page on stone roof maintenance shows what a yearly check involves, and catching one loose slate early is far cheaper than hunting for matching stone after several have gone.
How we handle it on your roof
At a survey we identify the stone already on the roof and report which areas, if any, are a different material from the rest, since earlier repairs sometimes introduced one. Photographs of the slope accompany the written quote, and where the fault is small we propose a small repair in matching stone.
If you are planning work on a Cotswold building and want the stone question settled before anyone climbs a ladder, see our Cotswold stone roofing page and send us a few photographs.