A dormer pushes a vertical window out through a pitched roof so the room behind gains head height. Seen from the lane, it is a little gabled or flat-topped box with its own roof, two side walls and a front face. Seen from the roofer's side, it is a cluster of junctions: the top where the dormer roof meets the main slope, the sides where its walls meet the tiles, the window head and sill, and the flat or pitched covering of the dormer itself.
That count of junctions explains why dormers leak more often than plain roof slopes. This guide follows the water: through the dormer roof, down the cheeks (the side walls), and along the abutments where the dormer meets the main roof. It is about repair and maintenance. If you are asking whether a new dormer needs planning permission, read our separate answer on dormers and permitted development. The wider service is on our pitched roofing page.
How a dormer is put together
Most dormers share a basic frame. Two cheek frames, each a short timber stud wall, stand on the main roof structure. A front frame holds the window. Across the top, either a small pair of rafters forms a pitched roof, or joists form a flat one. The frame is covered with sheathing, a waterproof layer and a finish: tiles, slates, lead, render or boarding on the cheeks, and a membrane or sheet metal on a flat top.
The two common shapes need different care.
Pitched-roof dormers
A gabled or hipped dormer roof is tiled like the main slope, usually to match it. The risk sits at the valleys where the small roof meets the large one. Water from both runs into a narrow channel, which makes those valleys prone to the same faults we describe in our guide to valleys on tiled roofs.
Flat-roof dormers
A flat-topped dormer has a roof covering of felt, lead, GRP (glass-reinforced plastic) or EPDM, a synthetic rubber sheet. Flat roofs are rarely truly flat: BS 6229 guidance says a roof should be designed with a 1:40 fall to give a finished fall of 1:80, according to Rockwool citing the 2018 edition. A small dormer top with too little fall holds a pool of water against the upstand (the lip where the covering turns up), and ponding is a main cause of early failure.
Cold or warm
Behind the coverings, dormer cheeks and roofs are insulated. If the insulation or the air gap is wrong, condensation forms inside, and the stain it leaves can look like a leak. That is a separate fault, worth ruling out before money is spent on the outside.
Cheeks: where tiles, lead and timber meet
The cheeks are the side walls of the dormer. They are covered in a number of ways, and each fails differently.
- Tile-hung cheeks. Small plain tiles are hung vertically on battens. They are attractive and breathable, but they crack and slip, and nail corrosion lets rows sag.
- Lead-covered cheeks. Lead sheet dressed over the frame. Its weak points are laps that have opened, fixings that have pulled through, and splits where the metal has been stretched at corners.
- Render or board-clad cheeks. They need sound paint or finish, and the base of the cheek, where it meets the main roof, needs a protected junction.
The cheek-to-roof junction
At the bottom of each cheek, the vertical wall meets the sloping tiles. The classic detail is a stepped flashing: soakers (small folded pieces of lead, one per tile course) interleaved with the tiles, with a lead flashing dressed over the cheek above. Lead for this kind of work is usually Code 4 or 5. A correctly fitted lead flashing can last more than 80 years, according to MyJobQuote (updated September 2026), but only if the lap is deep and the lead has room to move.
On older dormers, the flashing is often replaced by a bead of mastic or a mortar fillet. Both crack as the materials move at different rates. A mastic repair on a cheek that already leaks usually buys a short time.
Flat tops: felt, EPDM, GRP and lead
On a flat-topped dormer the covering is the whole waterproofing, and a small area makes for a quick job, but also leaves little room for error. Typical life spans from national guides, which depend on workmanship and upkeep, are:
| Covering | Typical life | Where it tends to fail |
|---|---|---|
| Built-up felt | 10 to 20 years (Less, updated March 2026) | Laps and upstands, blistering |
| EPDM | 20 to 30 years or more (GreenSpec) | Edge trims, corners, punctures |
| GRP | 25 to 40 years (Less) | Cracks at movement joints, edges |
| Lead | 50 to 100 years on flat roofs (Less) | Splits at laps, long untreated runs |
What goes wrong on a small flat top
Because the area is small, a single detail such as a dormer top upstand carries a great deal of weight. Common faults are an upstand that is too low where it meets the main slope, an edge trim that has lifted, a roof outlet or gutter that has blocked so water ponds, and old felt that has cracked at the corners.
Repair or recover?
If the deck below is dry and sound, a worn covering can be replaced with a new one over a fresh underlay. If the boards are soft or the joists dark, the deck needs renewing first. MyBuilder (updated August 2026) gives flat roof replacement at £70 to £100 per m² for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP. A dormer top is small, so scaffold and set-up costs often outweigh the material.
Abutments, window heads and the junction with the main roof
An abutment is a spot where roof slope runs into a wall that stands higher. A dormer has several: the cheeks, the front face above the window, and the upper edge where the dormer roof tucks into the main slope.
The back gutter
Where a dormer roof rises up the main slope, water running down the main roof meets the dormer's back wall and has to be carried round it. A lead back gutter, a small sloping channel behind the dormer, does this. The roofer sets it into the wall above and dresses it down onto the tiles. If the gutter is blocked with leaf litter or moss, water spills over the lead and into the loft, usually along the line of the main rafters. Lead for gutters is typically Code 5 or above; Royston Lead lists Codes 4 to 8 as suitable for dormers.
Window head and sill
Above the window, a cavity tray or a lead strip stops water running down the wall behind the frame. Below it, the sill must throw water clear. In older dormers these were often omitted or built flat. We check the frame sealant too, but we advise against relying on mastic where a flashing was meant to go.
Chimneys and neighbouring features
Dormers often sit near chimneys and roof windows. The flashings in a crowded area overlap each other, and a poor repair at one can push water to another. We survey the whole junction, not just the stained spot.
Diagnosing a dormer leak
A leak that shows in the room under a dormer may have entered anywhere above it. We work down from the top.
- Flat top or dormer roof. Check for ponding, a cracked covering and blocked outlets.
- Back gutter and upper abutment. Look for debris, lifted lead and a gap between lead and wall.
- Cheeks. Check for cracked tile, loose lead and a failed bottom flashing.
- Window. Look at the head, jambs and sill and whether the frame itself has moved.
- The inside. In the loft, damp timber at a corner of the dormer frame points to the cheek or back gutter.
Why a hose test is not always the answer
Running a hose at one point at a time can pinpoint a leak, but it can also push water into a gap that would not be open in normal rain. We use it sparingly, and we prefer to read the staining, the timber and the condition of each detail together.
Condensation is not a leak
A cold, poorly ventilated dormer room can drip from the ceiling in cold weather without any hole in the roof. A dormer insulated to current standards, with ventilation at the eaves, should avoid it. Our guide to roof ventilation covers what the right gaps are.
Repair options, and how they affect the main roof
Dormer repairs run from small to substantial, and each has an effect on the roof around it.
- Re-dress or renew a flashing. Replace the lead at a cheek or window head. Minor, and often possible without removing many tiles.
- Replace tile hanging. Strip and re-hang a cheek, with new battens and underlay.
- Re-cover the flat top. New membrane, new trims and corrected falls.
- Rebuild the cheeks or roof frame. Needed when timber has rotted. This is structural.
Why the main roof matters
Because a dormer's flashings and back gutter are woven into the surrounding tiles, a dormer repair often means lifting tiles on the main slope. If those tiles are already near the end of their life, it can be sensible to re-lay or renew that area while the scaffold is up. We discuss that option with you, and the written quote covers it. Our guide to tiled roof repair costs shows how the main roof side of the work is priced; and the rule of thumb for roof repairs from MyBuilder is £150 to £1,500.
Building Regulations
Minor repairs do not usually need Building Regulations approval. Altering the structure, or re-covering a flat roof with a different build-up, can. The Planning Portal says approval applies when the work exceeds 25 per cent of the building envelope or 50 per cent of the roof, or when the structure changes. Tell us the plan and we will say what applies, and deal with the notification if one is due.
Winter, insurance and keeping a dormer dry
A dormer is an awkward place to work in bad weather. Open cheeks expose the frame, and a flat top left half-covered ponds quickly. We plan repairs so that each stage is made weather-tight before we leave, with temporary sheeting fixed to the frame. Our guide to tiled roofs in winter explains how temporary cover works.
If a storm has damaged a dormer, the policy wording and evidence matter. Our guide to tiled roof damage and insurance covers how insurers tend to treat storm damage against wear.
Looking after a dormer
Once a year, from the ground or a window, look at the flat top for standing water and the cheeks for slipped tile or lifted lead. Clear the outlet if you can reach it safely; otherwise ask a roofer. Keep climbing plants off the cheeks, as their roots lift tile and trap moisture.
Dormer work carries our 10-year workmanship guarantee in writing, and we photograph the junctions at survey so you see them for yourself. Send us pictures of the stained ceiling and the dormer from outside by WhatsApp, or ring 01453 933613.