A loft conversion is supposed to be the best room in the house. Lots of light, a bit of quiet, maybe the only en suite you’ve got. So it’s particularly grim when a damp patch appears in the corner above the bed, or the paint starts bubbling under the Velux after the first wet October.
We see a lot of this. Hounslow has thousands of loft conversions, most of them rear box dormers on 1930s semis and Edwardian terraces, many built in a rush during the conversion boom of the 2000s and 2010s. Some were done beautifully. Some were done in three weeks by a firm that no longer exists. Ten or fifteen years on, the weak points are starting to show. Here’s where dormers usually leak, how to narrow it down, and what a proper fix costs.
Why Dormers Leak More Than the Rest of the Roof
A dormer is basically a box stuck onto a pitched roof, and every edge of that box is a junction where two materials meet. The flat roof on top. The cheeks (the side walls of the dormer, usually tile-hung or clad). The face with the windows. And the joints where all of those meet the original roof slope.
Each junction relies on flashing or a careful detail to keep water out. Lead soakers tucked under the tiles at the cheeks, an upstand where the flat roof meets the main roof, a drip edge along the front. Skimp on any of those and water finds its way in, often tracking along a joist and appearing a metre or two from where it entered.
That’s why a lot of the jobs that come through as general roof repairs in Hounslow turn out to be dormer details rather than anything wrong with the main roof. The tiles are fine. It’s the bits in between.
The Usual Suspects
You can rule some things in or out from the garden or an upstairs window. Please don’t climb out onto a dormer roof. They’re higher than they look and the edges are often fragile.
The dormer’s flat roof
Most box dormers have a flat roof, typically felt on older conversions and GRP or EPDM on newer ones. Look for ponding (water sitting for a day or two after rain), cracks along the edges, or trims lifting at the front. Felt dormer roofs from the early 2000s are now reaching the end of their life.
The cheeks and soakers
Where the dormer sides meet the main roof, there should be lead soakers interleaved with each course of tiles, plus a cover flashing. On rushed jobs these are sometimes missing, replaced with a smear of mortar or silicone. Leaks here usually show as damp running down the inside of the dormer side wall.
The back edge
The joint where the dormer’s flat roof meets the original roof slope at the back. It’s hard to see from the ground and it’s one of the most common failure points, because water running down the main roof hits that junction directly.
Roof windows
Velux and similar windows on the front slope of a loft conversion leak at the flashing kit, not usually through the glass. A fitted kit that was cut or bodged to suit the tiles will let water in during driving rain. Condensation can also look like a leak, which we’ll come to.
Is It Actually a Leak?
Worth checking before you spend money. Loft rooms, especially ones with poor ventilation and an en suite, get a lot of condensation. If the damp is worse in cold weather rather than wet weather, appears as droplets on the window frames, or forms a black spotty mould in corners, it may be moisture from inside the room rather than rain coming in.
A proper leak tends to follow the weather. It’s worse after heavy or wind-driven rain, it often has a tide-mark edge, and it sits in a specific place rather than all over. If you’re not sure, note down which days it gets worse and what the weather was doing. That information saves a roofer a lot of guesswork.
Repair or Rebuild: Our Rule of Thumb
If the dormer roof is under about 15 years old and the leak is at one junction, repair that junction. A new soaker run, a properly dressed lead flashing or a new upstand detail is often all it needs.
If the dormer’s felt roof is cracked and blistered all over, re-cover it. On most dormers we’d strip the old felt, check the deck, and lay EPDM or GRP with proper edge trims.
If the deck is soft, the timber at the cheeks is rotten, or the leak has been going for more than a winter, expect to open things up and replace some structure. It’s worth getting that done properly rather than patching, because a dormer that’s been wet for years will keep failing.
For more guidance on planning property improvements and renovation projects, explore PAD Magazine.
What Dormer Repairs Cost Locally
Typical 2026 figures for a rear box dormer on a two-storey semi or terrace in West London. Access is usually scaffold, and it’s often the biggest single item.
- Resealing or replacing a Velux flashing kit: £250 to £600
- Replacing lead soakers and cover flashing on one dormer cheek: £500 to £1,100
- New upstand detail at the back edge: £400 to £900
- Re-covering a dormer flat roof (around 10 to 15m²) in EPDM or GRP: £1,200 to £2,500
- Strip and replace dormer roof including new deck: £2,000 to £3,800
- Re-tiling or re-cladding dormer cheeks: £800 to £2,000 per side
- Scaffold for rear dormer access: £700 to £1,500
If you’re having one junction fixed and the scaffold is already up, have the rest of the dormer checked at the same time. Paying for access twice is how small dormer leaks end up costing more than they should.
Local Factors
The housing round here makes dormers especially common. Rear box dormers on interwar semis in Hounslow West, Heston and Cranford. L-shaped dormers over back additions on the Edwardian terraces in Isleworth and Lampton. Hip-to-gable conversions across Feltham. A lot of these went in under permitted development, which means there was building control involvement but no planning officer looking at the detail.
The area isn’t particularly wet, but it’s very open to the south-westerly wind, and the flat land around Heathrow means driving rain comes in sideways more often than people expect. That’s tough on dormer cheeks and roof windows.
If your house is in one of the borough’s conservation areas, check before changing the look of a front dormer or roof window. Like-for-like repairs are usually fine.
Finding Someone Who Knows Dormers
Not every roofer is good at dormers. You need someone comfortable with leadwork, flat roofing and tiling, because a dormer combines all three.
Ask for public liability insurance of at least £2 million and check the expiry date. Membership of the NFRC or a TrustMark registration is a good sign. Get a written quote that says exactly which junctions are being fixed, what materials are going on, and what guarantee you’ll get.
Ask for photos from the scaffold before any extra work is added to the bill. A deposit of 20 to 30% is fair. Anyone who knocks offering to fix your dormer because they “noticed it from the street” deserves a polite no.
If your loft room has a damp patch you can’t pin down, get Watertight Roof Repairs Hounslow to take a look at the dormer details before winter really sets in.
Frequently Asked Questions
How much does it cost to fix a leaking dormer in Hounslow?
Small junction repairs like new soakers or a resealed flashing usually cost £400 to £1,100. Re-covering the dormer’s flat roof is typically £1,200 to £2,500, and scaffold adds around £700 to £1,500.
Why is my loft conversion leaking when the roof looks fine?
Most loft conversion leaks come from the junctions around the dormer rather than the main roof tiles. Missing or poorly fitted lead soakers, a failed upstand at the back edge or a worn felt dormer roof are the usual causes.
How long should a dormer roof last?
An older felt dormer roof typically lasts 10 to 20 years. EPDM and GRP coverings, properly installed with good edge details, can last 25 to 30 years or more.
Is it condensation or a roof leak?
Condensation tends to be worse in cold weather and shows as droplets or black mould in corners. A leak usually follows the rain, especially wind-driven rain, and appears in one specific spot with a tide-mark edge.
Do I need permission to repair a dormer in Hounslow?
Like-for-like repairs to an existing dormer don’t normally need planning permission. If you’re in a conservation area or changing the appearance of a front-facing dormer, check with the council first.
