Where a lantern lets water in
A roof lantern is a glass box standing in a flat roof. It rests on an upstand, which is a raised kerb of timber or insulated board built up from the roof deck, and the flat roof covering is dressed up the outside of that kerb. The lantern frame is fixed to the top and sealed to it.
That gives three places to look:
- The base: where the lantern frame meets the upstand and the covering turns up the outside of it.
- The glazing bars: the ribs between the panes, which carry a cap seal along the top, and the corners where the bars meet the frame.
- The flat roof around it: a membrane that holds water against the kerb will find every weakness.
The place a stain shows inside does not always match the entry point outside. Water runs along a bar or down a kerb before it drips, so trace it upwards before you decide where to start. Our guide to roof lantern installation sets out how the upstand and the flat roof should be built.
Ponding and the flat roof round the lantern
A lantern blocks the path of water running across a flat roof. If the roof does not fall cleanly towards an outlet, water collects against the upstand and sits there. Standing water puts constant pressure on the lap between the covering and the kerb, and any pinhole or open joint there will let it through.
BS 6229:2025 deals with membrane flat roofs of the continuously supported flexible kind, and lead sheet roofs are outside it. Trade guidance on the 2025 update keeps a minimum 1:80 finished fall over the general areas of roof, and the maker IKO advises a 1:40 design fall for single-ply. A roof that has settled since it was laid may no longer drain to either figure.
Check the roof after rain. Pools that are still there a day or two later, a silt line round the kerb, or moss growing on the membrane all say the water is not clearing. If so, sealing the lantern alone only treats a symptom, and the falls or the outlet need attention too. A roofer can lay a straight edge across the roof to see where it dips, and can show you the result in a photo.
Condensation or a leak: telling them apart
A lantern is a large glazed surface in a heated room, and glass is the coldest surface in the space. Moist air condenses on it, and the droplets run down the bars and drip. This looks like a leak and is not one.
A simple comparison helps:
- Condensation forms on the inner face of the glass, is worst on cold mornings or after cooking, bathing or drying clothes, and clears as the room warms up.
- A leak shows up during or after rain, often in the same spot, and it can appear when the room is dry and warm.
- Damp on the kerb lining or ceiling plaster beside the lantern points to the base. Drips from a bar point to the cap seal or a corner.
If you are not sure, wipe the inside of the glass dry and watch where the next wet appears. The same test works on a roof window, and our page on roof window condensation covers the ventilation side of it. Keep a note of the weather each time you see drips, since a pattern of dry cold mornings points to condensation rather than the roof.
Re-sealing or re-flashing the base
The choice depends on what has failed. A split cap seal along a glazing bar, or a corner that has opened up, can often be cut out and re-sealed with the right compatible sealant. That is a repair to one joint and does not need the lantern to be lifted.
A failed base is a different job. If the covering has lifted from the kerb, the upstand is soft or the flashing has pulled away, sealant only buys a short delay. The covering has to be taken back, the kerb checked, and a new upturn and flashing formed. On a larger lantern this may mean lifting the lantern off its kerb first.
Ask a roofer to say which of the two it is, and why. A fair answer names the joint, shows you the photo, and explains whether the repair is local or the whole base. The written price follows the survey, not the first guess. Water behind a lantern also spreads into the ceiling, so finding the entry point early limits what has to be put right inside. For the wider picture, see roof windows and skylights.
