The short answer
A roof lantern most often leaks at a joint rather than through the glass: a failed seal along a glazing bar, a split or lifted flashing at the kerb, a blocked drainage channel, or water pooling on the flat roof around its base. Condensation is sometimes mistaken for a leak. Finding which one it is needs an inspection from outside and inside.
Start with where the water appears
A roof lantern is a raised glazed structure on a flat roof, with sloping glass panels meeting at a ridge and a perimeter frame sitting on an upstand. Because it has many joints, the place where water shows inside is a clue to the cause, though it is rarely a precise one.
Drips along the line of a glazing bar point to the bar seals or the caps on top. Staining at the ceiling edge, or the corner where the lantern base meets the plaster, suggests the kerb and its flashing. Water that appears only after prolonged rain often travels a long way along timbers before showing, so the visible spot can be some distance from the real entry.
Write down when it happens. Does it leak in wind-driven rain, after a long wet spell, or only when the roof has been warm and then cooled? Those details narrow the search before anyone goes on the roof.
Seals and glazing bars
Between each glass panel and the bar that carries it sits a gasket or a bead of sealant. Over time sunlight, heat and movement harden or shrink these, and a gap opens. Water then runs along the bar and drips inside.
Aluminium and timber bars fail in different ways. On aluminium, the problem is usually the seal and the cap, the strip that clips over the top of the bar. On timber, moisture can also get into the wood itself if the paint has failed. A loose or missing cap is simple to spot from a ladder, while hidden seal failure may need part of the glazing to be lifted and resealed.
At the ridge, where the glass meets at the top, the detail is more complex. There may be a ridge cap, end caps and a sealed junction, and any one can lose its seal. A common temptation is to smear sealant across the leak. That can hide the problem for a season while trapping moisture, so a proper reseal after cleaning the surfaces is better. Where several panels leak, the whole lantern may need re-glazing rather than patching.
The kerb and the flashing
The lantern sits on a kerb, a raised upstand around the opening, and the roof covering is turned up against it. The weatherproofing at that joint is a flashing, which is metal, usually lead, lapped down over the upstand so water runs clear.
The kerb is a frequent source of trouble. The flashing can lift in wind, crack where the metal has fatigued, or be pulled away by movement of the structure. Lead needs room to move with temperature changes, and a long run fixed hard down has nowhere to go and can split. Our reference figures for lead describe flashing pieces kept to about 1.5 metres in length for this reason. If the upstand is timber, wet rot may have taken hold behind a failed flashing, and the repair then goes beyond the metal.
On some older roofs the lantern was installed with a cement fillet or a sealant bead as its only protection. That does not last. It cracks, and the correct repair is proper leadwork or a fitted upstand system, not another layer of mastic. The hub page on roof windows and skylights sets out the wider options, while our page on the roof window leaking causes covers the same flashing problems on sloping glazing.
Drainage, ponding and the roof around it
Sometimes the lantern is not at fault at all. A flat roof should be laid to a fall, with BS 6229 recommending a design fall of 1:40 which leaves a finished surface draining at 1:80 or better. If the roof around the base of the lantern holds water, that pool sits against the kerb joint for days, and any small weakness will leak.
Leaves and debris make this worse. Many lanterns have a small valley or channel where they meet the roof surface, and when that blocks, rain backs up. The membrane around the kerb, whether felt, EPDM rubber or GRP fibreglass, can also fail in the corners, where the material is folded. A split there will let water in under the flashing and then track along the deck.
This is why a lantern repair often includes the roof edge too. It makes little sense to reseal glass while the surrounding membrane is failing. We check the fall and the corners as part of the inspection, and if the roof needs work, the choice between a patch and a new covering is discussed openly. The roof window slate roof page deals with a different covering, but the logic of water-shedding details is the same.
Condensation, and when to call it a repair
Some apparent leaks are condensation. Large areas of glass cool quickly at night, and warm damp air from the room settles on them as droplets that run down the bars. This is more likely in kitchens and bathrooms or in extensions with little ventilation. If the water appears on the underside of the glass, in the morning, and without rain, condensation is a likely cause. Better ventilation and a warmer glazing specification are the remedies, not sealant.
A real leak will usually appear during or after rain and will be worse in driving weather. A sheet of paper held under the drip, or a photo log with dates, helps. If you cannot tell, a roofer can run water over one section at a time to isolate the source, starting low and working up.
For repair or replacement, the decision depends on the age and the kind of failure. Sealing and capping are usually repairs. A rotten kerb, widespread glazing failure or a collapsed membrane points to rebuilding. If you are weighing a different form of glazing, our comparison of a rooflight vs roof window sets out how they differ. A live leak is dealt with first so the room stays dry, then the survey leads to a fixed price in writing.
