Roof Windows & Skylights Explained: Leaks, Installation and Common Problems

Skylights and roof windows make up a tiny fraction of most roofs by surface area, and yet they show up again and again as a source of leaks — often before anything else on the roof has given any trouble at all. That's not a coincidence, and it's not usually a sign of a badly built roof either. It's simply what happens when you cut a hole in a continuous waterproof surface and rely on a much smaller, more complex junction to keep doing the same job.

This page covers what actually makes skylights and roof windows different from the rest of the roof, why they fail in specific, predictable ways, how to tell a skylight leak apart from ordinary condensation, and what's worth knowing if you're considering installing one — whether as part of a loft conversion or simply to bring more light into a room.

Every skylight or roof window installation involves cutting a gap into what would otherwise be a continuous waterproof surface, and then relying on a much smaller system — flashing, seals, and drainage channels built into a relatively small frame — to do the job that the roof covering does over every other square metre. That system has far less margin for error, and far more individual components that can each fail on their own.

• A tile or slate roof sheds water in a simple, repeating pattern across a huge area; a skylight interrupts that pattern and needs its own dedicated system to redirect water around it instead.
• The flashing kit around a skylight is doing concentrated work that a much larger area of ordinary roof covering shares between many overlapping tiles or slates.
• Rubber seals and gaskets — present on most skylights in some form — degrade with UV exposure and age faster than the tiles or slates around them, which don't rely on rubber at all.
• A skylight is also a common point where two different trades' work meets (the roofer and whoever fitted or specified the window), which occasionally means a gap in exactly whose responsibility the junction was during the original installation.

Exposure plays a part too. A skylight on a slope that faces the prevailing wind and rain — commonly a south-westerly aspect in most of the UK — is doing that concentrated work under harder conditions than an identical unit on a more sheltered slope of the same roof. This is one reason two skylights on the same house, fitted at the same time by the same installer, can age at noticeably different rates; it's rarely a sign that one was fitted badly and the other wasn't.

• Staining or a damp patch on the ceiling or wall directly around the skylight frame, particularly worse during or shortly after rain
• Visible daylight or a draught around the edge of the frame from inside
• Water pooling on the internal sill, especially after heavy or wind-driven rain
• Cracked, perished or missing sections of the rubber seal around the glazing
• Staining or a green tinge on the external flashing, or flashing that's visibly lifted or damaged
• Condensation that seems unusually heavy or persistent compared with other windows in the property

As with the rest of the roof, our general leak guide covers what to do about active water ingress. The main thing worth doing differently with a skylight is checking whether the water is actually coming from around the frame itself, or tracking down from higher up the roof and simply becoming visible at the skylight — the two point towards very different fixes.

This distinction matters even more for skylights than it does for the rest of the roof, because a skylight is exactly the kind of cold surface — set into an otherwise warm ceiling, often above a bathroom or bedroom — where warm, moist household air is most likely to condense. Our loft ventilation guide covers the general mechanics of this in more depth, and the same principle applies directly here.

It's genuinely common for a homeowner to notice water around a skylight, assume the worst, and book an expensive repair — only to find the glazing and flashing are both sound, and the actual issue is condensation from a bathroom extractor fan or simply poor airflow in that part of the loft or room. Worth ruling this out before assuming the skylight itself has failed.

• A failed or incorrectly fitted flashing kit — by far the most common cause, and one that can develop years after an otherwise fine installation as materials age and settle
• Perished rubber seals or gaskets, particularly on units more than 10–15 years old, or on units that used a generic rather than manufacturer-specific flashing kit
• A blocked drainage channel at the base of the frame, where the skylight's own small drainage system is meant to direct any incidental water back outside rather than let it pool
• Debris, moss or leaves collecting around the upper edge of the skylight and disrupting how water flows around it — worth checking alongside any wider moss build-up on the roof
• The original installation itself, if the wrong flashing kit was used for the roof covering, or the unit wasn't correctly sized and sealed into the opening cut for it

A single failed seal or a localised flashing problem on an otherwise sound, reasonably modern skylight is usually a straightforward repair — resealing or replacing the affected section rather than the whole unit. This is broadly the same logic as repairing a single guttering fault rather than replacing an entire system over one localised issue.

Older single-glazed skylights, particularly ones nearing or past the 15-year mark, are a different conversation. Seals and gaskets don't age gracefully on their own — once one has perished, others fitted at the same time are usually not far behind, and a pattern of repeated small leaks across a few years often ends up costing more than a single replacement with a modern, better-sealed unit would have. Double-glazed roof windows with integrated flashing kits also tend to perform noticeably better on both weatherproofing and heat loss than the units they're commonly replacing.

If a roofing professional identifies the flashing kit itself as the problem rather than the glazing, that's generally good news — flashing can usually be replaced without disturbing or replacing the window unit itself, provided the frame and seals around the glass are otherwise sound.

It's worth asking specifically whether any replacement uses a manufacturer-matched flashing kit for your roof covering, rather than a generic universal version — the difference in cost is usually modest, but the difference in how reliably it seals against your specific tile or slate profile is not, and it's a detail that's far easier to get right the first time than to revisit a year later.

Adding a skylight, whether as a standalone project or as part of a wider loft conversion or re-roof, involves a few considerations beyond simply choosing a window and a spot for it.

• Structural work — cutting an opening in a roof usually means adding trimmers around the gap to support the rafters either side of it, which is structural work rather than a simple fit-and-seal job.
• Matching the flashing kit to the roof covering — flashing kits are generally designed for a specific tile or slate profile, and using the wrong one for your roof covering is one of the more common causes of a leak developing later.
• Planning permission — many roof window installations fall under permitted development, but this can depend on the property, its height, and whether it's in a conservation area, so it's worth checking with your local authority before committing, similar to the planning considerations covered in our roof replacement guide.
• Position and purpose — a skylight fitted primarily for light does a different job to one intended to open for ventilation, and the two aren't always interchangeable once installed.

Roof covering also matters more than it might seem. A flashing kit designed for a flat, thin material like slate sits and seals differently to one designed for a thicker, profiled interlocking tile, and manufacturers generally produce distinct kits for each rather than one generic version that fits everything reasonably well. Reusing or improvising a flashing kit designed for a different covering than the one actually on your roof is a recognised cause of leaks that can take months or years to show up, since the mismatch is often subtle enough not to fail immediately.

It's also worth thinking about extras at the same time as the main installation rather than as an afterthought — blinds for glare and heat, trickle vents for background airflow, or an electric opening mechanism are all far simpler and cheaper to specify upfront than to retrofit once the unit is in and sealed.

A roof lantern sitting on a flat or low-pitch roof shares the general logic of this page — a cut in the roof surface, relying on a dedicated seal to keep it watertight — but the specific risks lean more towards the issues covered in our flat roof guide. Ponding water sitting against the base of the upstand for extended periods is a particularly common contributor, since a lantern's flat surroundings don't shed water away as readily as a sloped tile roof does around a pitched skylight.

If you have a roof lantern and you're seeing damp around its base rather than the glazing itself, it's worth having the surrounding flat roof falls and drainage checked alongside the lantern seal — the two are often connected rather than separate faults.

Whether you're dealing with a leak, trying to work out if what you're seeing is condensation, or planning a new installation, the details that help most are which type of skylight or roof window it is, roughly how old it is if known, where exactly any staining or damp is appearing, and any photos taken safely from inside the property or ground level. Our guide on what information to give a roofer covers this in more general detail, and the same approach applies here.