A dormer is the part of a loft conversion most exposed to weather and least visible for inspection. It has a small flat or pitched roof, two vertical cheeks, a face, and four junctions with the main roof covering. Every one of those junctions is a place water can get in, and the dormers that leak almost always leak at a junction rather than through a surface.
Flat roof dormer construction: the roof on top of the box
Most rear dormers carry a small flat roof, and it is a flat roof like any other: it needs a designed fall to an outlet, a covering suited to a small area with upstands on several sides, and insulation arranged so the deck stays warm. A warm deck build up, with the insulation above the structural deck and below the waterproofing, is the usual modern answer because it removes the ventilated void and the condensation risk that goes with it in a space nobody will ever inspect.
Dormer cladding options for the cheeks
The vertical sides of the dormer are the cheeks. Tile hanging matches the roof and suits a house whose covering is a plain tile or slate. Lead or a lead substitute is the traditional answer and still the most durable. Zinc and other standing seam metals give a crisp modern face. Timber and fibre cement boards are lighter and cheaper. Whichever is chosen, the cheek needs a drained and ventilated cavity behind it rather than a cladding fixed flat to the sheathing, because the cheek is a wall in a very exposed position.
The junctions decide everything
Where the cheeks meet the main roof slope there is a soaker and flashing detail that has to shed water from the tiles onto the covering below. Where the dormer roof meets its upstands the waterproofing has to turn up far enough to stay above driven rain and splash. Where the face carries a window, the window has to be weathered into the cladding. These are trade details rather than product choices, and they are the reason the same dormer specification can be watertight on one house and not on another.
Insulation and the thin wall problem
Every millimetre of insulation in a cheek or a dormer roof eats internal space, which creates constant pressure to make the build up thinner than it should be. Approved Document L sets what a new element has to achieve, and hitting it in a shallow cheek usually means a high performance board rather than mineral wool. Skimping here shows up as a cold room at the ends and, where the vapour control is also compromised, as condensation inside the construction.
Add dormer to existing loft conversion: not simply an add on
It is possible and it is not simply an add on. The existing roof has to be opened, the structure around the new opening trimmed, and the new box tied into a floor and a ceiling that are already finished. Planning and building control both apply again. Where a rooflight conversion has proved too cramped it can be the right answer, but price it as a structural alteration rather than an extension of the previous job.
Questions people ask about dormer roof
What is the best cladding for dormer cheeks?
Lead or a lead substitute is the most durable and the traditional choice, tile hanging matches the existing roof, and zinc or a fibre cement board suits a modern design. The detail behind the cladding matters more than the material: it needs a drained, ventilated cavity.
Why does my dormer leak where it meets the roof?
That junction is weathered with soakers and flashings that carry water from the tiles onto the roof below, and it is the most commonly botched detail on a dormer. Mortar fillets used in place of proper flashings crack and are a frequent culprit.
Should a dormer roof be warm or cold construction?
Warm, in almost every case. A small flat roof over a finished room has no realistic way to ventilate a cold void and nobody will ever see inside it, so keeping the deck warm and the vapour control continuous is the reliable approach.
Can a dormer have a pitched roof?
Yes, and on a visible elevation it is often what the planning authority prefers. It costs more and gives less head height at the edges than a flat roof of the same footprint.