Why is my roof lantern leaking?

A lantern is a big expanse of cold glass and frame above a heated room, and moisture settles on it readily. Wet bars on a cold morning, or a drip that turns up with no rain outside, point to condensation. Water that arrives during or after rain, follows the same line each time and marks the plaster beside the kerb points to a leak.

Watch the timing and the place. A stain that grows after heavy or wind-driven rain suggests water getting in from outside. Staining on the ceiling some distance from the lantern may still be the lantern, because water travels along the deck before it drops. Photograph it whenever it happens, with the date, and note the weather that day. Pictures help far more than a description when someone comes to look. If you suspect condensation, our answer on condensation in roof windows explains why it forms and what clears it.

The most frequent way in is the junction between the roof covering and the upstand, the short wall the lantern stands on. The covering has to turn up that wall high enough for splashes to stay below the join, and finish beneath the lantern's own cill flashing. If the membrane has pulled away, split at a corner, or was cut short when the lantern went in, water gets behind it and travels inward.

Look at the flat roof itself too. Water that sits against the kerb after rain tells you the roof slopes too little there or the deck has sagged, and water left standing finds any weak point eventually. Our answer on standing water on a flat roof covers that side of the problem. The cure at the kerb is usually to renew the upturn and corners in the same covering, which repairs the roof and leaves the lantern alone.

Between the glass and the frame sit gaskets or sealant, plus a capping that holds each pane down. Over the years the rubber hardens and shrinks, sealant cracks and capping screws can work loose. Water then runs down the glazing bars into the frame and out along the lantern's inside edge, which looks like a leak from the middle of the roof even though the entry point is higher up.

Ridge and hips are the other soft spots, because several frame members meet there and every junction has to be sealed. A hairline gap on a sunny day can open wide in a gale with rain driving behind it. Re-sealing the bars and renewing gaskets is a reasonable repair if the frame is sound. A lantern with rotten timber, corroded fixings or a frame that has moved is another matter, and it is cheaper to hear that at the start than to keep patching.

Put a container under the drip and move furniture and electrics out of the way. Should water get near wiring, such as a ceiling light or a socket, isolate that circuit at the board and get an electrician in. Mop up standing water promptly, because a wet floor is a slipping risk and soaked plaster dries slowly. Stay off the roof, and never stand on a lantern: rooflights are fragile surfaces unless the maker rates them for foot traffic, and glass gives way without warning.

Keep the evidence together. Dated photographs of stains, and of the lantern from inside, help whoever surveys it. If you plan to claim on your home insurance, tell the insurer first, file the receipts for any stop-gap work along with every photograph. Our answer on whether to call a roofer or your insurer first sets out the order. We prioritise leaks, and keeping the weather out comes before any longer-term plan.

Our approach is a verdict before a price. We examine the upstand, the covering, the bars, the ridge and the roof drainage, photograph every fault, and say whether mending it will last or whether the lantern is worn out beyond sensible repair. If one corner of the upturn has failed, we say that and price that. If the whole frame has moved, we say that too. Whichever it is, you see the pictures behind the recommendation, so the decision rests on evidence rather than pressure.

A repair that removes the cause beats silicone smeared over the symptom, which tends to trap water behind it. Where replacement is the better spend, our guide to roof lanterns covers fitting and the Building Regulations, and the skylights and roof windows page shows the other work we do. If the lantern is only a few years old, check any guarantee that came with it before work starts. After the visit, a written quote gives you one total price.