Dormer roofs & dormer cheeks
In short
A dormer is a small roof with a lot of junctions — a flat or shallow top, two cheeks, and a joint back into the main roof — which is why they leak out of proportion to their area. Most dormer problems are at the cheek-to-roof junction or in the cladding, not in the flat top.
Also called: Dormer roof leaking · Dormer cheeks · Flat roof dormer · Loft conversion roof
Signs you need this
- Damp on a bedroom ceiling below a dormer
- Staining down the internal face of a dormer cheek
- Water appearing where the dormer meets the main roof
- Ponding water on the dormer top
- Lifted, split or missing cheek cladding
- Bituminous paint or mortar at the cheek-to-roof junction
A flat-roofed dormer is a box pushed out of a pitched roof, and every one of its edges is a detail that has to be right. The top is a small flat roof. The sides — the cheeks — are near-vertical faces exposed to driven rain. And where the whole thing meets the main roof slope there is a junction that has to shed water in two directions at once.
The commonest leak is not the flat top at all. It is the junction where the cheek meets the main roof, which needs proper soakers or a stepped flashing dressed into the covering and up the cheek. Done badly — or dressed with mortar, or painted with bitumen — it lets water in behind the cladding, where it runs down inside the cheek and appears on the ceiling below in a place that makes no sense.
Cheek cladding is the other half. Traditionally it is lead or tile-hanging; on a modern loft conversion it is more often a proprietary cladding or slate. All of them work; all of them need the right laps and the right fixings, and all of them need a way for any water that gets behind to get out again. A cheek that is sealed at every edge traps water rather than excluding it.
The flat top itself is a straightforward small flat roof and gets treated as one: strip to the deck, check for rot, form real falls, and re-cover in EPDM, GRP or a modern high-performance felt. Because a dormer top is small and often awkwardly shaped, GRP frequently suits it — the upstands and the junction back to the cheeks come out as one seamless laminate.
Falls are easy to get wrong on a dormer because the roof is short and the fall has to be created within very little depth. It still has to be there. A dormer top laid dead flat ponds within a year, and the standing water sits directly above a bedroom.
Loft conversion dormers bring an extra consideration: they are usually insulated as warm roofs, with insulation above the deck rather than below it, and the ventilation and vapour control layer arrangement matters. Getting a warm roof build-up wrong causes interstitial condensation inside the structure, which produces damp with no visible leak at all and rots the deck from the inside.
What the job covers
- The cheek-to-roof junction inspected and detailed properly
- Soakers or stepped flashings dressed correctly into the covering
- Cheek cladding repaired or renewed with correct laps and fixings
- Dormer top stripped, deck checked and falls formed
- EPDM, GRP or high-performance felt to suit the shape
- Warm roof build-up and vapour control checked on conversions
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
Why does my dormer leak when the top looks fine?
Because the top is usually not the problem. The junction where the dormer cheek meets the main roof slope is the commonest failure — it needs proper soakers or stepped flashing, and it is regularly done with mortar or bituminous paint instead. Water goes in there, runs down inside the cheek and appears somewhere that makes no sense from the outside.
What is the best covering for a dormer top?
GRP often suits it best, because a dormer top is small and awkward and GRP forms the upstands and the junctions as one seamless laminate. EPDM works well on a simple rectangular top. Modern high-performance felt is also fine. What matters more than the choice is that the deck is sound and the falls are real.
My dormer is damp but there is no leak. What is going on?
Very likely interstitial condensation, which is common on loft conversion dormers built as warm roofs. If the vapour control layer is missing or breached, moist air from the room gets into the structure, condenses on the cold side and rots it from the inside. It produces damp with no ingress — and no amount of work to the covering will change it.
Can the cheeks be re-clad without touching the top?
Sometimes, but usually not sensibly. The cheek cladding laps under or into the roof covering at the top, so doing one properly generally means disturbing the other. Where both are tired, doing them together is cheaper than doing them a year apart and gives one continuous detail rather than a joint between two ages of work.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.