Why do roof lanterns leak?

First decide whether it is rainwater or condensation

Record when the moisture appears. A mark that develops during or shortly after rain points towards an external route. Widespread droplets during cold weather, especially without rainfall, may be condensation on a cold surface or within the surrounding build-up. The site’s condensation guidance distinguishes a defined rain-linked stain from moisture spread across colder surfaces. That distinction prevents repeated external sealing when ventilation or vapour control is the real issue. Dry the area, photograph it, and compare the next occurrence with weather rather than assuming every drop beneath glazing has crossed the roof covering.

Check the glazing and its seals

Water must move from the lantern panes into the designed drainage path. A displaced cap, failed seal or blocked channel can interrupt that path. Look from a safe internal position for a consistent trail from one pane or bar, but do not dismantle caps without knowing the system. Some visible moisture can also be internal condensation running down the glazing. A useful inspection follows the trail back to its highest wet point and compares all similar bars. If only one junction behaves differently, that evidence is more useful than covering every external seam with a general-purpose sealant.

The kerb is part of the waterproofing

The lantern normally sits on a raised kerb so water on the flat roof stays below the opening. The membrane or GRP finish should turn up that kerb and be completed neatly at corners. A low, interrupted or poorly bonded upstand gives wind-driven water a route behind the covering. Inspect the full perimeter rather than only the side nearest the ceiling stain because water can travel along timber before dropping. The site’s flat-roof service specifically checks upstands, trims and the deck as part of one specification; a lantern diagnosis should not separate the frame from that surrounding construction.

Falls and outlets can make a sound lantern look faulty

If water ponds beside the kerb, the lantern detail is being asked to resist standing water rather than shed rainfall promptly. Clear the nearest outlet and observe whether the area drains. If the depression remains, the issue may be the roof fall or deck. Water can also back up when leaves or moss restrict an outlet, then reach an upstand that performs perfectly in normal conditions. Repairing a lantern seal without restoring drainage leaves the same pressure around the kerb. The surrounding roof should therefore be assessed before the lantern is blamed or replaced.

Movement reveals weak corner details

Flat-roof decks, kerbs and lantern frames do not all move in exactly the same way. Corners and material changes accommodate that movement, so rushed detailing tends to show there first. Look for a crack, lifted edge or repeated line of previous mastic at one corner. On GRP, a stable and correctly fixed deck is particularly important because the finished surface is rigid. On EPDM, the membrane is flexible but the bonded edges and penetrations still need correct preparation. The repair method should match the actual roof system and substrate, not simply bridge every joint with one product.

Why the ceiling stain can mislead you

Water follows the easiest route across a deck, along a timber or around the lantern lining before becoming visible. A drip at the lowest corner may have entered higher up the roof. The same principle appears throughout the site’s leak guidance: trace the source rather than patching where the water shows. Internal inspection can reveal the direction of staining, while a controlled external check narrows down the detail. Broad, uncontrolled hosing can create water routes that normal rainfall never uses, so testing should be careful and carried out by somebody able to interpret the result.

What a proper repair decision looks like

A local failed detail can often be repaired while the lantern and surrounding roof remain in place. A soft kerb, damaged deck, widespread failed upstands or a flat-roof covering already at the end of its useful service may justify a broader rebuild. Ask the roofer to identify the entry point, the affected construction and the proposed tie-in to sound material. The quote should say whether glazing work, kerb work, deck repair or flat-roof waterproofing is included. Clear boundaries stop a narrow seal repair being mistaken for a guarantee over an older surrounding roof.

What to send when asking for help

Take one photograph of the full lantern and surrounding roof from a safe place, close-ups of the suspect corner and outlet, and an internal picture showing the whole opening and stain. Note the rainfall direction, whether the leak starts immediately or after prolonged rain, and whether it also appears in cold dry weather. Do not climb onto the flat roof if it is not intended for access. HRD’s service covers rooflights, flat-roof systems and leak tracing within the stated area, allowing the lantern and the roof around it to be considered together rather than passed between separate trades.

FAQs

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