Why is my flat roof skylight leaking?

A flat rooflight is not set into the roof surface. Instead it stands on a built-up plinth, the upstand or kerb, which lifts the glass clear of any water lying on the roof. The waterproof membrane (felt, EPDM or GRP) is turned up the outside of that kerb, and the rooflight frame is fixed and sealed over the top of it.

That turned-up edge is where most leaks begin. The membrane can shrink and pull away from the kerb, the edge trim can lift, the sealant can crack, or the corners, where the membrane has to fold neatly, can open. Water then tracks behind the upstand and appears on the ceiling beside or below the rooflight. Our page on the rooflight upstand covers the construction in more detail.

Between the glazed unit and its frame sits a rubber or silicone seal. Sunlight, heat and frost wear it out over the years, and a hardened or shrunken seal lets rain past. Mastic around the outside of the frame ages in the same way. These are cheap parts, and re-sealing is sometimes enough.

The kerb itself can be the problem. Timber upstands can rot where water has sat against them, and concrete or brick kerbs can crack and let water through behind the membrane. A new seal on a failing kerb is wasted money, so we look at the base before we recommend anything.

Fixings matter as well. If the frame is screwed down through the membrane without proper sealing, each fixing becomes a tiny entry point. A rooflight fitted without care over this detail can show the fault.

Flat roofs are meant to have a fall so water drains away. When the roof is designed for a fall of 1:40 it should end up no flatter than the 1:80 minimum in BS 6229. Should the deck sag, or the outlet clog with leaves, water collects against the upstand. A kerb that was fine in dry weather is then standing in water for days, and every weak spot is tested.

Clearing the outlet or gutter takes minutes and costs little, so check it first. If water keeps ponding after the outlet is clear, the roof surface itself may need attention, and that is a conversation about the deck rather than the rooflight.

The deck underneath is worth a thought. A flat roof that moves with temperature, or timber that has swelled after years of damp, can stress the joint between the membrane and the kerb until it opens. In that case the cause sits below the rooflight and the repair needs to reach it, or the same crack returns next winter.

Moss and debris in the corners do the same job in a smaller way, holding moisture against the seals. A quick look after leaf fall is worthwhile.

Many reports of a leaking rooflight turn out to be condensation, which is why we ask when the water appears and whether it had rained. A cold glass surface above a warm, humid room collects moisture which runs down the inside and drips from the frame, especially in kitchens and bathrooms. It looks like a leak, yet it turns up on cold mornings, fades by afternoon and has nothing to do with rain.

Misting between the two panes of a double-glazed unit is a different thing: the sealed unit itself has failed, and that glass unit normally has to be swapped. A damp patch that grows only after rain points to a real leak, and a damp patch that shows up on dry cold days points to condensation. For the pitched-roof equivalent see condensation on roof windows.

If the upstand is sound and the problem is a seal, a loose trim or a lifted membrane edge, a repair can be a small job. The aim is to renew exactly what has failed and nothing more.

Replacement makes sense when the kerb is rotten or cracked, when the glazing is old and misting, or when the rooflight is poorly insulated. For new or replacement rooflights in existing homes, Approved Document L sets a limit of 2.2 W/m²K, and an upstand built on site should achieve no more than 0.35 W/m²K. Both limits matter for any rebuilt kerb.

Older properties may have extra constraints. For rooflights on period buildings, read our guide to conservation rooflights before choosing a replacement.

Put a bowl under the drip and move anything valuable, and switch off any light fitting that is wet. Photograph the stain and the rooflight from inside and out, with dates, because pictures help with a repair and with any conversation with your insurer. We prioritise leaks on our visits.

Please do not walk across a flat roof to inspect a rooflight unless it is rated for foot traffic, and never lean on the glass. Our roof window rain noise page explains why some glass is louder than others, which is worth reading if you are choosing a replacement unit. Send us your photographs and we will arrange a survey, take our own pictures of the fault, and set out the fix in a written quote. See our skylights and roof windows page for more.