Matching stone slates on a Stroud extension

Matching is not only about colour. English Heritage policy, printed in Cotswold District Council's 2000 stone slate guidance, names five points of comparison for any new slate: the rock it was cut from, its colour, its texture, and its length and thickness. Limestone and sandstone are not interchangeable in either direction. In practice that means finding out what your existing roof is made of before anyone prices a replacement.

The Cotswold guidance names two limestones behind the traditional slates: Forest Marble, which is hand-split, and Stonesfield Slate, which is frost-split. Each ages in its own way and sits on the roof in its own way. A sample held against the old slope in daylight tells you more than a brochure, and a wet slate and a dry one can look quite unlike each other.

Thickness matters because it changes how the slate lies on the batten and how the courses shadow one another. A slab that is too thick looks heavy at the verge and eaves; one that is too thin may not hang flat. We look at the old slopes from the ground and at close range, then note the sizes and thicknesses on the survey.

Reclaimed stone looks right immediately, because it has already weathered. That is why it is tempting. The same guidance leans the other way. It asks for new stone wherever that is possible, and keeps salvaged slates for the building, or group of buildings, that they were taken from. Salvage trade also strips other roofs, the guidance warns, and occasionally that happens through theft.

There is a practical middle course. If you are re-laying part of the old house at the same time, slates taken from that slope can be sorted and reused, and the extension then takes new stone. Or the sound old stone can go on the most visible slope, while the new stone covers the part the street sees less. Our page on the roof timbers under heavy stone slates covers what carries all of this.

Substitutes are a different matter. On historic buildings the policy counts resin-based slates, concrete and cast imitation stone among the unsuitable replacements for the real thing. On a modern house with no listing, the choice is yours, but the match will not be as close.

Even perfect stone looks wrong if the courses are wrong. Cotswold slates go on in diminishing courses, which means the rows get shorter as they climb from the eaves towards the ridge. The biggest slates, usually around 600 mm and sometimes 750 mm, are doubled up at the eaves according to the guidance, while the smallest, only about 150 mm long, are saved for the top of the slope.

An extension creates a puzzle here, because its slope has its own height. The courses on the new roof should read as the same family as the old, with the same rhythm from big to small, rather than a block of uniform slates that stops at the join. We plan the sizes slope by slope, so that a stone merchant is asked for the right sorted lengths and does not simply send a pallet.

The pitch needs the same attention. For the plainest traditional Cotswold roof, the guidance describes a straight pitch of 47 to 55 degrees. That is traditional practice, not a minimum, so the new roof's pitch is set by the match to the old and by the structure beneath.

Stone is heavy: the Cotswold guidance gives about 1,000 kg for every 10 m² of roof. A frame built for something lighter may not cope. Approved Document A calls a rise in loading of more than 15% significant, and the Planning Portal says a covering that raises the weight by 15 per cent or more needs approval. Ask your building control body before you commit to stone on a frame built for tiles.

Planning is a separate question. A new extension is development, and its roof material is usually part of the application, so the council may specify stone. In a conservation area, in the Cotswolds National Landscape or on a listed house, the planning team will care about the match. Check with Stroud District Council's planning team early, and keep the decision about stone in writing. We talk you through which consents apply and deal with the Building Regulations notification where the job calls for one.

Checkatrade's slate roof guide, updated in 2026, puts stone slate at roughly £160 to £210 per m², with the stone's origin, its age (fresh from the quarry or salvaged) and your postcode all shifting the number. It is a national range, nothing Cotswold-specific about it, so treat it as a yardstick and not as our price. Scaffold, stripping, waste and edge details are extra, and the quote covers them when we put it in writing after surveying.

Finally, the join. Where the new slope butts up against the older masonry, a mortar fillet or lead does the weatherproofing, and our guide to verge details on stone roofs and the answer on fillets against walls and chimneys cover those details. For re-laying, repairs and new work, see Cotswold stone roofing. We give a 10-year workmanship guarantee in writing and carry off the rubbish. To begin, send photographs of the old roof along with the planned footprint.