The short answer
A stone-roof dormer is a timber-framed box with its own small roof, joined to the main slope by lead soakers, flashings and a valley. The surrounding slates are lifted, kept and relaid in their own courses. Dormers are not permitted development in a conservation area or National Landscape, so planning permission is likely to be needed.
What goes into a stone dormer
A dormer is a small structure poking up from a sloping roof to carry an upright window. It has a front face and two side walls called cheeks, and above them sits a small roof that belongs to the dormer alone. On a stone house each part needs thought, because the main roof is no flat, uniform sheet. It is a field of heavy limestone slates in courses that dwindle towards the ridge, and every course has to be cut or lifted where the dormer breaks through.
The frame is normally timber, tied into the existing rafters. Its roof may be stone, lead or sometimes slate, to suit the house. Stone looks most natural but adds load, which the structure must carry. Cotswold District Council's 2000 guidance reckons a stone roof at about 1000 kg per 10 square metres, so the existing rafters and the plates that carry them get examined before we agree to any addition. A dormer on a roof with tired timbers is asking for trouble, however neat the new stonework looks on the day it is finished.
Our Cotswold stone roofing page sets out the wider range of stone work we carry out.
How water is kept out
Water hunts for junctions, and a dormer has three sets: the cheeks, where the side walls meet the slope; the front, where the face sits on the roof; and the valleys, where the dormer roof meets the main roof. Each is sealed by its own method.
At the cheeks, soakers (small pieces of lead folded to tuck under the slates and turn up the wall) go in course by course, with a flashing over the top of the upstand. Done properly, each slate sheds its water onto the soaker below. At the valleys, the traditional Cotswold answer is a swept valley, where specially cut slates curve around the corner. Our page on swept valleys explains it. The council's 2000 booklet allows that lead valleys, closed or open, are cheaper to make, yet judges them poorer in appearance.
Lead thickness is chosen to suit each detail. Royston's published guide, for instance, marks Code 3 as suitable only for soakers, while Code 4 and Code 5 serve flashings, and the heavier codes 6, 7 and 8 suit valleys and gutters. We choose the sheet for every junction when the dormer is designed, so that nothing is left to be improvised once the scaffold is up and the stone is already lifted.
Keeping the stone looking undisturbed
A dormer only looks right if the stone around it looks as though nobody touched it. So the slates lifted for the opening are kept and put back in their original positions wherever possible. The original roofer sorted them by size, longest at the eaves and shortest near the top, and mixing the sizes up shows from the ground.
Extra stone for the dormer's own roof should resemble the old in kind, thickness and colour, and sandstone and limestone should not share a roof. English Heritage's sourcing policy leans towards fresh stone when stone has to be added, and keeps reclaimed slates for the building they came off. Our page on stone against Welsh slate shows why a foreign slate jars on a dormer.
The dormer's courses should step down in the same rhythm as the main slope, which takes patience and a roofer who understands the pattern. If you are unsure whether a tiny dormer roof deserves stone at all, lead can be a cheaper covering that still sits quietly on the house. We will tell you which suits yours.
Planning, Building Regulations and heritage
Dormers are the roof change most likely to need consent. Under Class B of the permitted development rules, no part of a dormer may rise above the ridge of the original roof, it must not extend beyond a slope facing a highway, and must not add more than 40 cubic metres to a terrace house or 50 cubic metres to any other. None of those allowances is available on article 2(3) land such as a conservation area or a National Landscape, so across much of the Cotswold edge a planning application is needed. A listed building needs listed building consent as well.
Building Regulations are a separate hurdle. A new dormer is structural work, and a room behind it raises questions of insulation and escape. We explain where consent applies and handle the Building Regulations notification where work is notifiable. Because we cannot give legal advice, confirm your own position with the council's planning team.
Heritage matters too. In a conservation area the dormer's proportions, its position on the slope and the material of its cheeks all come under the planning officer's eye. A narrow, modest dormer set well back from the eaves is far more likely to be accepted than a wide one that dominates the roof.
Our page on buying a house with a stone roof may help if a dormer is part of why you are buying.
What shapes the price
A stone dormer is bespoke, so its price follows the size, the covering, the quantity of lead, the condition of the rafters and how the site is reached. Scaffolding is nearly always needed because ladders are no way to work a stone roof. When quotes arrive, ask whether each includes VAT and the scaffold.
We never price from a drawing alone. A visit lets us look at the roof, test the rafters, examine the stone and take pictures of the position, and a written quote follows. Where stone near the opening is already fragile, you hear about it before work begins, since cutting into tired slates can break more than you expect.
If you are weighing a dormer against another way to gain space, our page on Cotswold stone roof costs shows how stone work is priced. Send pictures of the roof and the room you want to light, and we will arrange to come and see.