A leak at a flat rooflight kerb almost always comes from one of four things: an upstand built too low for the exposure, a roof covering that was stopped short of the top edge and sealed instead of wrapped over it, a termination bar or adhesive lap that has let go, or kerb timber that has been wet long enough to go soft. It is very rarely the glass. The kerb is the lowest weathered detail on the whole unit, so when it fails it is the first place water finds its way to a ceiling. The honest fix is to strip back, rebuild the kerb detail correctly and reset the unit. Running a bead of sealant round the frame buys a season and hides the timber decay underneath.
This page is the diagnosis. It walks through each of the four causes, how to read which one you have without going anywhere near the roof, and what the repair actually involves. The full construction specification for a kerb that does not do this is on our rooflight kerb construction page, and the wider job it sits inside is our flat roof skylight installation page.
Cause one: the upstand is too low for where the house is
Water does not only run down a flat roof. In a driven wind it travels sideways and climbs a vertical face. The upstand is the vertical face that keeps that climbing water out, and its height is what decides how much of it can be resisted before the water tops the covering and gets behind the frame.
We build to a minimum of 150mm above the finished roof surface, measured at the lowest point of the fall, and to 200mm within a mile of open sea at Felixstowe, Harwich and Frinton. The common failure on other people’s work is not that they ignored the number. It is that they measured it at the high side of a falling roof. A roof that drops 40mm across an opening, measured at 150mm on the high corner, is barely 110mm at the low corner, and the low corner is precisely where the water collects and where the wind pushes it up. That corner leaks first, and it leaks in the storms that come from one particular direction.
A second version of the same fault is a kerb that was tall enough when built and has since been overtaken. Every time a roof is recovered, the build-up rises a little. A 1980s dome kerb that started at 100mm can be down to 70mm of effective height after two recovers, and the unit sits half-drowned in the roof.
Cause two: the covering was stopped short and sealed
The principle that keeps a kerb watertight is simple. The covering comes up the full height of the upstand and is dressed over the top edge, so the rooflight frame sits on top of the covering. Water then has to climb the face, cross the wrapped head and pass under a compressed bedding line before it reaches anything it can damage.
Where that principle is broken, the covering is stopped somewhere on the face and the gap between it and the frame is filled with sealant. Sealant is a gasket. It excludes air and levels a joint. It is not a waterproofing layer, and any detail that relies on it has a service life of about five years, after which it shrinks, cracks and lets water track straight down behind the covering. This is the single most common defect we find on flat rooflights fitted by general builders, because wrapping a covering over a kerb head properly is a roofing skill, not a carpentry one.
Cause three: the termination or the lap has failed
Even where the covering was dressed over the head, the detail can still let go at the point it is held down. Each membrane is terminated differently, and each fails differently.
| Covering | Where it lets go | What shows on the ceiling |
|---|---|---|
| EPDM rubber | Site-folded corners, adhesive starved on the vertical face | Staining at one corner, worse after wind from that side |
| GRP fibreglass | A crack at the sharp internal angle where no fillet was laminated in | A fine line of staining down one face, appears after a hard frost |
| Single-ply PVC or TPO | A cold weld at a corner, or an incompatible patch | Intermittent staining that tracks along one edge |
| Felt | Mastic-filled corners, stacked laps, cap sheet not turned over the head | Spreading brown patches, granule debris in the gutter below |
How each of these is dressed and terminated correctly is set out on our rooflight flashing and membrane page.
A mechanical termination bar with sealant behind it holds longer than an adhesive-only detail, but it still relies on the sealant behind it and on the fixings staying tight. On an exposed coastal roof a bar fixed with the wrong grade of screw corrodes at the fixing heads, loosens, and the whole termination lifts in a gale.
Cause four: the timber has gone, and it went first
This is the one that decides most jobs, and it is invisible until the unit comes off. By the time water shows on a ceiling, the kerb timber below the covering has usually been damp for years. Treated softwood held at high moisture content in still air, with no drying cycle, goes soft on the top edge and at the corners where the end grain is exposed. A soft kerb head will not hold a fixing, so even a perfect new covering detail has nothing sound to be fixed to.
On a dome replacement we survey on the expectation that the kerb may need rebuilding, quote the rebuild as a provisional item, and photograph what we find once the old unit is off so you can see the reason before we carry on. Roughly a third of the 1980s and 1990s dome kerbs we take off need rebuilding. The rest are sound enough to keep, and we keep them.
Reading which one you have, from inside
Nobody should be walking a flat roof to diagnose this, least of all a wet or frosty one with an ageing unit in it. You can get most of the way from a window and a ceiling.
- Staining at one corner or along one edge, worse after wind and rain from one direction. That is water ingress at the kerb. A covering or termination failure.
- Brown tide marks rather than clear water. Water that has travelled through timber and plaster, so a leak rather than condensation.
- Staining that appears in cold weather, evenly on all four reveals, and stops in mild weather regardless of rain. That is not a kerb leak at all. It is condensation from an uninsulated kerb, and it is covered on cold bridging and damp around flat rooflights.
- A unit that visibly sits low in the roof. The kerb has been overtaken by later roof build-up and the effective height is gone.
The single most useful thing you can send an installer. A photograph taken from an upstairs window two days after heavy rain, showing where water sits on the roof, plus a photograph of the stain inside. Those two pictures tell us more about a kerb leak than any measurement you can take.
Why the fix is a rebuild, not a reseal
The trade default on a leaking kerb is to run mastic round the frame. It addresses none of the four causes above. The upstand is still too low, the covering is still stopped short, the termination is still failed and the timber is still wet. The sealant closes the visible route for a season or two while the water finds another way and the decay carries on out of sight.
We do not offer resealing or patching on any rooflight. On a kerb that is leaking, a reseal delays the only work that fixes it: stripping the covering back around the opening, rebuilding the kerb where the timber has gone, dressing the covering up and over the head with the correct detail for that membrane, and resetting the unit on 316L stainless fixings where the exposure calls for it. The argument in full is on reseal or replace a dome rooflight.
What the repair job actually looks like
- The unit is lifted off and the kerb is exposed and probed. This is where the timber condition is confirmed, with photographs.
- Where the timber has gone, the kerb is rebuilt in treated softwood, corner-braced, insulated continuous with the roof, and taken to 150mm above the finished roof, or 200mm on the coast.
- The falls around the opening are checked and corrected if the water was sitting rather than running.
- The covering is dressed up the full height and over the head using the right detail, with preformed corners on EPDM and single-ply, a laminated fillet on GRP, stepped layers on felt.
- The unit is bedded onto the dressed covering, levelled and mechanically fixed. The roof is watertight before we leave.
On a sound kerb the whole thing is a one-day job. Where the kerb needs rebuilding it runs into a second day. Prices sit in the published ranges on our flat roof rooflight installation cost page, and the kerb rebuild is a provisional item confirmed once the old unit is off.
Common questions about flat rooflight kerb leaks
Can you just reseal the frame to stop the leak?
No, and we do not offer it. A bead of mastic round the frame closes the visible route for a season while the actual cause, a low upstand, a covering stopped short, a failed termination or wet timber, carries on unaddressed. On a kerb this old the timber is usually the real problem and no surface sealant touches it. The fix is to strip back, rebuild the kerb where needed and dress the covering correctly.
How do I know if it is the kerb or the glass?
It is almost never the glass. Staining that shows at one corner or one edge, worse after wind-driven rain from one direction, is a covering or termination failure at the kerb. A sealed glass unit that had actually failed would fog between the panes rather than drip. When we take a leaking unit off, the glass is nearly always reusable and the kerb detail is the fault.
Why does it only leak in certain storms?
Because a low upstand is topped by wind-driven rain from one direction only. A gale off the North Sea pushes water up the seaward face of the kerb, tops a 110mm effective height that would have been fine on a still day, and drives it behind the covering. Rain falling straight down never reaches that point. This is why the fix on exposed and coastal roofs is a taller upstand, not more sealant.
Is walking on the roof to look safe?
No. A flat roof with an ageing rooflight in it should not be walked, particularly when it is wet or frosty, because an old dome will not carry your weight and a wet membrane is slippery. Diagnose from a window and a ceiling, send us photographs, and leave the roof access to a survey with proper edge protection.
Get a leaking kerb surveyed
Send us a photograph of the stain inside, a photograph of the roof from a window two days after rain, the rough size of the unit and where the property sits. We will survey it, lift the unit if needed, tell you plainly whether the kerb is sound or gone, and give you one written fixed price. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.
Request a fixed quote for a leaking flat rooflight kerb in Ipswich, Suffolk or north Essex. If you want the wider diagnosis of standing water first, that is on ponding water around a flat rooflight.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.