Why do dormers leak?

A dormer is a small structure built out of the roof slope to give headroom and a window. It is made of a flat or pitched roof on top, two side walls called cheeks, a front with the window, and valleys where it meets the main roof. Every one of those edges needs its own waterproofing, and each is a joint between materials that expand and settle at different rates.

A plain hipped roof has few such joints. A dormer adds many, and they sit close together at the point where a roof is hardest to reach. Chalet bungalows and many interwar and post-war houses along this coast have them, and the older the dormer, the more likely its joints have aged. The checklist on our causes of roof leaks page covers faults in general, and a dormer can collect several of them in one small area.

The cheeks are the side panels of the dormer, often clad in tile, slate, lead or boarding. Where a cheek meets the main roof, a soaker or flashing should run up the side and under the tiles. Soakers are little folded sheets of lead or other metal slipped between the courses of tiles, and they fail when the metal splits, slides down or was cut too short in the first place. Lead is the usual material, and the right thickness is a point of detail settled at the survey.

The valleys either side take the water from both the main roof and the dormer, so they carry more than a plain valley would. A valley lining that has cracked, or one clogged with moss and leaves, can overflow into the roof void. Our page on valley repair and the answer on moss and leaks explain how blockage ends with water inside the house.

Many dormers have a flat roof, covered in felt, EPDM, GRP or lead. It is small, so it is easy to overlook, but it takes all the rain that falls on it without a slope to help. The common faults are a split or blistered covering, a lifted edge trim, and water that sits on the surface instead of running off. Flat roofs should be laid to a fall, and on a small dormer that fall is easy to get wrong.

BS 6229:2025 keeps a minimum finished fall of 1:80, and a careful roofer lays it steeper, so water still clears after the covering settles. A dormer that ponds is a candidate for relaying with the right fall, not patching. Our flat roofing page explains the coverings, and the answer on standing water on a flat roof shows why the fall matters.

Not every dormer leak starts on the roof. The window frame can let water in at its joint with the cladding, especially where an old sealant line has cracked. The front face may be timber boarding, tile hanging or render, and each has its own weak points where it meets the roof edge. A leak that appears round the window is easy to blame on the roof, and sometimes the fault is in the frame and its seal.

A survey looks at the whole structure, not just the flat roof. We check the frame, the cladding, the roof covering, the cheeks, the valleys and the gutter, and photograph each so the finding is clear to you. Where the fault is the glazing or frame, we say that plainly and suggest the right trade for it. This keeps you from paying for a new roof covering when the window seal is the culprit, and it spares the dormer a repair that cannot work. Water that enters at the frame often runs down the inside of the cheek before it shows, so the stain can mislead.

The repair depends on the fault. A lifted flashing is re-dressed, a split valley lining is renewed, and a worn flat roof is re-covered. Many dormer leaks come down to repairs, not rebuilds, and the survey ends with a verdict on whether to mend or renew, with reasons. If the structure is rotten, the cheeks and frame may need rebuilding, which is bigger work and may bring Building Regulations into play, since Approved Document L treats the roof parts of a dormer as roof.

If you are thinking of changing the dormer, remember that planning rules differ in conservation areas and the National Park, and our guidance on that sits with the skylights and roof windows pages. The price follows the survey as a single written figure. For the overall approach to a leaking roof, start with the roof repairs page.