Why is my flat rooflight leaking?
Water appearing around a flat rooflight almost never comes through the glass. The sealed unit is a factory made component and it fails in one specific, recognisable way. Everything else that produces water on a ceiling comes from the two or three hundred millimetres of roof around the frame: the height of the upstand, the falls, the flashing laps, and where the covering terminates.
Working out which of five or six causes is at work is a diagnostic exercise, and it is worth doing properly before anybody touches the roof. This page runs through them in the order a survey checks them. The installation detail behind them is on flat roof rooflight installation.
First, establish whether it is water from outside at all
The single most common misdiagnosis is condensation. It appears on the room side of the glass, on the frame, or as a damp line down the internal reveal, and it looks exactly like a leak to anybody standing underneath it.
The test is timing. Water from outside tracks rainfall: it appears during or shortly after heavy or driven rain and stops in dry spells. Condensation tracks temperature and humidity: it appears on cold mornings, after a shower or a long spell of cooking, and it is worst in still, cold weather with no rain at all. The mechanism is set out on whether flat rooflights cause condensation.
An upstand that is too low
The most frequent genuine cause, and the hardest to see, because it is a decision made at installation rather than a fault that developed.
A flat rooflight relies on height. The covering runs up the outer face of the kerb and terminates near the top, and the unit lands over that termination. If the kerb is short, or if the finished roof surface has been built up since with a ballast layer or a second covering, the termination sits close to the level water can reach. In driven rain the water climbs the vertical face, gets over the top, and arrives behind the covering. It then travels along the deck and appears somewhere else entirely.
The industry figure and the reasoning behind it are covered on how high a rooflight kerb should be.
Standing water sitting against the kerb
A flat roof is laid to a fall so water leaves. Where the fall is inadequate, where the deck has deflected, or where an outlet has silted up with leaf litter and moss, water stands. Standing water against an upstand is a sustained hydrostatic condition rather than a passing shower, and it finds any weakness in the termination given time.
This is common on older Essex flat roofs where a deck has sagged between joists over decades, and on parapet roofs with a single internal outlet, where one blocked gully turns the whole roof into a shallow tank. Clearing the outlets is the first thing to check and the cheapest thing to change.
Flashing laps running the wrong way
Every overlap in a weatherproof surface has a direction. Upper layers lap over lower layers so water passing over the joint keeps travelling downhill. Reverse a lap and the joint becomes an open funnel pointing uphill.
It happens most often on the high side of a rooflight, where the covering coming down the roof meets the covering coming up the kerb. Get that sequence the wrong way round and the roof is watertight in light rain and lets water in whenever the flow is heavy enough to run rather than trickle. It is invisible from below and obvious once the detail is opened up.
Where the covering terminates under the frame
The termination is the top edge of the covering as it comes up the kerb, and it is meant to be mechanically held and dressed neatly so the frame lands over it cleanly.
Two failures recur. The covering was trimmed short, so it stops below the frame flange and leaves a strip of bare timber or plywood exposed to anything that gets under the unit. Or it was left loose and unfixed, so it has slumped away from the kerb face over a few summers of thermal movement, opening a gap behind it. Both put water into the deck rather than onto it.
Fixings through the flange
The unit is mechanically fixed down onto the kerb, and each fixing is a deliberate hole through a weatherproof line. Placed correctly they sit high on the upstand, above any water that could ever reach them, with the sealing washer compressed evenly.
Placed low, or overtightened so the washer is distorted, or driven at an angle so it does not seat, they become entry points. This shows up as water arriving in a specific spot at the perimeter rather than generally, and it correlates with wind direction because it needs pressure behind it to drive water into a small opening.
A sealed unit that has reached the end of its life
This is the one genuine glass failure, and it does not produce water in the room. The perimeter seal of a double glazed unit eventually admits moisture into the cavity, the desiccant in the spacer bar saturates, and moisture then condenses between the panes where nothing can reach it.
The signs are consistent: misting inside the cavity that comes and goes with the sun, a permanent haze, or fine deposit lines on the inner faces of the glass. Twenty to twenty five years is a typical working life, and the domed polycarbonate units fitted across Essex in the seventies and eighties are well past it. Sunspire replaces units that have got to that point.
Tracing where water enters against where it appears
The last complication is that water travels. It arrives at a defect in the perimeter, runs along the top of the vapour control layer or along a joist, and drops through the ceiling at the first place it finds a route: a downlight, a plasterboard joint, or the far corner of the reveal.
So the wet mark on the ceiling is evidence of where water left the build-up, not where it entered it. A proper diagnosis works uphill from the stain, checks the whole perimeter detail rather than the nearest suspicious point, and where the picture is unclear uses a hose test working systematically from the low side upwards. That is slower than guessing and it is the only method that ends the question for good.
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