Do roof lanterns leak?

The sealed glazing units in a modern lantern are the least likely part to fail. They are bonded and gasketed in a factory to a tolerance no site seal can match, and the aluminium frame that carries them is drained and vented so that any moisture reaching the rebate runs back out rather than pooling. If you are watching a lantern in heavy rain and worrying about the panes, you are usually watching the wrong thing.

The water almost always comes from lower down. A lantern sits on an upstand, a raised timber or built-up kerb that lifts the frame clear of the roof surface. That kerb is wrapped in the roof covering and then linked to the frame by flashing or a purpose-made trim. Three junctions matter, and they are all site work: covering to kerb, kerb to frame, and the corners where two runs of flashing meet. Get any of those wrong and rain finds it, often weeks or months later, and often a long way from where it eventually drips through.

This is why a leak can be so hard to trace. Water runs downhill along a joist or a plasterboard seam before it shows, so the damp stain on the ceiling can sit a metre from the actual breach. The lantern gets the blame because it is the thing everyone can see, when the breach is a folded corner of flashing nobody can see at all.

So it is worth understanding what that upstand has to do. If it is too low, driving rain and standing water rise above the point where the covering turns up the kerb, and the frame is bridged from the outside. As a working figure the finished kerb wants to lift the underside of the frame at least 150mm above the roof surface, more on an exposed roof where wind pushes water uphill. Estuary-facing extensions around Southend catch a lot of wind-driven rain off the water, and a kerb that would be fine on a sheltered inland roof can be marginal here.

Height is not the only thing that goes wrong with a kerb. It has to be square, level and rigid. A kerb that is out of level leaves one corner of the frame proud and one pinched, so the compression that a lantern relies on to seal is uneven before a drop of rain has fallen. A kerb built from the wrong timber, or one that is not properly fixed down, flexes under wind load and works the seals loose over a season or two. None of that is visible once the trims are on, which is exactly why it survives to become a leak.

Insulation and a cold-bridge break belong in the kerb too. A bare timber upstand with glass on top is a cold ridge running round a warm room, and in a south Essex winter that ridge collects condensation on the inside face. People report that as a leak because they find water below the lantern, but it is the room’s own moisture, not the weather, and it is a build detail rather than a fault in the glass.

Because the risky junctions are all made on site, the fitter matters more than the brand on the box. A well-detailed installation dresses the roof covering up and over the kerb, laps the flashing so that every joint sheds downhill, and seals the frame to the kerb with the manufacturer’s own gaskets and tapes rather than a bead of silicone squeezed on as an afterthought. Silicone has its place, but a lantern that depends on a smear of sealant to stay dry is a lantern that will leak the day that sealant ages.

The order of work counts as well. The covering and the flashing have to go on in the right sequence, each layer overlapping the one below like tiles, so that gravity is doing the sealing and the mastic is only a backup. Reverse two of those laps and the joint will look perfect and leak in the first storm. This is close, unglamorous work, and it is where a considered install earns its keep. When we handle a roof lantern installation the kerb detailing and the flashing sequence get as much attention as the lantern going on top, because that is the part that keeps the room dry.

If your lantern already leaks, the right first call is a roofer who can open up the flashing and find the breach. We install and replace, we do not carry out that kind of investigation. Where the fault turns out to be a failed or badly specified unit, though, taking the old lantern out and fitting a new one on a corrected kerb is a replacement, and that is our work. The opening and much of the structure are already there, so it is often a cleaner job than the original.

Most lanterns sit on flat roofs over rear extensions, and a flat roof is never truly flat. It is laid to a slight fall so that water runs to an outlet rather than sitting still. If that fall is missing or points the wrong way, rain ponds around the base of the kerb, and standing water finds weaknesses that running water would pass straight over. A puddle held against a flashing lap for a day has all the time it needs to wick through a fault that a shower would never expose.

The lantern frame itself is pitched too, and for the same reason. The glazing bars slope so that rain sheds off the glass and away, and any condensation forming inside the frame drains to the gutter detail at the eaves rather than collecting in the corners. A quality lantern is engineered around this drainage path. Blocking it, by bedding the frame in mastic in the wrong place or by fitting internal blinds that trap moist air against the glass, is a common way to turn a dry lantern into a weeping one. Our flat roof skylight installation work starts with checking the fall of the roof deck, because a lantern can only be as watertight as the surface it lands on.

Here is the part most people miss. A lantern that never lets in a drop of water can still make the room beneath it hard to use, and around here that is the more common regret. A great many Southend, Westcliff and Thorpe Bay extensions face south across the estuary, with an open horizon and light bouncing off the water, so a lantern over a rear kitchen collects sun from mid-morning to late afternoon through the whole warm season. The room does not leak. It overheats.

That is a glass question, not a sealing one. Clear double glazing sits at a solar factor, or G-value, of around 0.6, which lets roughly 60 per cent of the sun’s heat straight through. Solar-control glazing at around 0.3 halves that while keeping the room bright. On a south-facing lantern the two decisions travel together: specify the kerb and flashing so it stays dry, and specify the glass so the room stays usable in July. Getting the first right and the second wrong is how you end up with a watertight lantern you cannot sit under at four in the afternoon. That balance is the whole basis of our energy-efficient skylight installation work.

If you are choosing a lantern, spend your attention on the upstand and the fitter, not the marketing. Ask how high the finished kerb will stand above the roof, how the covering is dressed up it, how the flashing laps are sequenced at the corners, and whether the kerb is insulated against condensation. A supplier who can answer those plainly is telling you they know where lanterns actually leak.

If yours is already letting water in, get a roofer to trace the breach before anything else, because guessing wastes money. And if the honest answer is that the unit or the kerb was never right and needs to come out, tell us the orientation and the rough size and we will come back with a specification and a price. Ring 01702 898232 or request a quote, and we will look at how the whole thing sheds water and handles the sun, not just the glass on top.