Replacing the flat roof on a dormer

A dormer is a box built out of a sloping roof, usually to make a loft room usable. Its own roof is often almost flat, with a cheek wall at each side and a face at the front, normally holding the window. The roof is small, but nearly all of it is edge.

That is the problem. Each cheek is a junction between the flat roof and a wall clad in tiles, slate, lead or render. The rear of the dormer meets the main slope, where water running down the tiles lands on the dormer roof's top edge. The front often has a drip trim or a short lead apron over the face.

Leaks on dormers rarely start in the middle of the roof. They start where a flashing has lifted, a cheek has opened up or a corner has been folded badly and cracked. The same pattern shows on larger roofs, but a dormer has so little open area that the edges decide everything.

Dormers sit on the roof slope, often at first-floor height or higher, with a steep pitch below them. Standing a ladder on the tiles is not a safe way to work, and carrying rolls of material up it is worse. For most dormers we expect scaffolding, usually with a working platform at eaves level and a way to reach the dormer without stepping on the roof covering.

That adds to the cost, and the scaffold may be one of the larger items in a small job. MyBuilder's scaffolding guide, updated in August 2026, gives about £850 to £1,200 as the typical cost of scaffolding a two-storey house. Ask each firm whether scaffolding and VAT are in their price.

Good access also protects the roof you are not replacing. Tiles around a dormer crack easily under boots, and a broken tile beside the new work can undo the repair.

Upstand is the name for the section of membrane that rises up a wall to stop water running behind it. On a dormer there are at least three: both cheeks and the back wall where the roof meets the main slope. The membrane must run up each one to a sensible height, then be covered by a flashing or tucked into the cladding so rain cannot run behind the top edge.

Lead is common here, often as soakers at the point the cheeks run into the main slope, or as a flashing that dresses over the upstand. A soaker is a little sheet of lead worked in under tiles or slates at an angle in the roof, and a flashing is a strip that seals the junction with a wall. Both need to be sized for the exposure.

At the front edge the covering finishes over a drip trim, so water drops clear of the face and into the gutter or onto the slope. Short or loose trims are a regular cause of damp at the window head below. See torch-on felt roofing for how built-up systems are finished at edges.

Small, awkward roofs often suit EPDM or GRP, because both can be shaped around corners with few joints. EPDM comes as a single sheet that is folded into corners and glued, and GRP is laid as a continuous fibreglass shell with trims at the edge. Single-ply PVC is also used, and our page on single-ply membranes for houses sets out where it fits.

Built-up felt can work well, but felt is laid in layers with a flame or bitumen, and fire safety on a timber dormer needs real care. Many roofers prefer a cold-applied method near cladding and timber. The method should be agreed at the survey, not left to the day.

Published lifespans give a feel for the choices: GreenSpec quotes 20 to 30 years for EPDM, and Less gives 25 to 40 years for GRP. These are typical ranges, and the quality of the edge detail will affect them more than the badge on the roll.

When the old covering comes off, the deck underneath shows what has really been happening. Boards that stay damp for years go soft, and the timber frame of the cheeks can rot where water has run down inside the walls. The deck and cheeks are probed at the survey, and you hear what turns up.

A dormer is also part of a heated room, so insulation matters. In Approved Document L, putting a fresh waterproofing layer on a flat roof is classed as renovating a thermal element, and once over half the roof is renovated, the whole roof should reach 0.16 W/(m²·K) under Table 4.3. That can mean adding rigid insulation above the deck, which raises the roof level slightly. We cover the height issue in insulating a flat roof from above.

On a dormer, the extra thickness can bring the roof close to the window head or the main slope, so it needs planning with the upstand heights before work starts.

A dormer visit means getting up to it safely, either from scaffold height or with a roof ladder. We photograph both cheeks, the front edge and the back junction with the main slope, then go inside and look at the timber from the loft room. Those pictures sit alongside the written quote. On a young dormer the fix may be a resealed upstand, not a new roof, and we will say so.

Where replacement is right, the 10-year workmanship guarantee comes as a written document, and the scaffold and rubbish go when we do. Notifiable work means we file the Building Regulations notification for you. Our number is 01793 626096, and WhatsApp or the contact form also work, to fix a time to see your flat roofing job.