Roof windows — Velux is the brand most people mean, though Keylite, Fakro and others work the same way — are one of the most common leak sources we are called to. The window itself is rarely the problem. In our experience, nine out of ten leaking roof windows are leaking because of how they were installed, not because the unit has failed.

Here are the real causes, how to identify them, and what each fix involves.

Cause 1: failed or incorrect flashing kit

This is the number one cause, by a distance. The flashing kit is the metal (usually aluminium) collar that seals the junction between the roof window and the surrounding tiles or slates. It is a precision component designed for a specific window size and roof covering profile.

Flashing leaks happen because:

The wrong flashing kit was used. Tile-profile flashing on a slate roof (or vice versa) will never seal properly. We see this on DIY installations and cheap jobs where the installer used whatever kit was to hand.
The flashing was never installed properly. The apron at the bottom must dress correctly over the tiles below; the side flashings must interleave with the tile courses. Rushed installations get this wrong.
The flashing has aged. On older installations (15+ years), the flashing can corrode, lift in wind, or work loose as surrounding mortar and tiles move.

The fix: replace the flashing kit with the correct one for the window and roof covering. A roofer can usually do this without replacing the window itself. Expect £250–£500 in London depending on access and roof covering.

Cause 2: perished seals and gaskets

Roof windows rely on rubber gaskets and seals around the sash and frame. After 15–20 years, these harden, shrink and crack — particularly on south-facing windows that take the full brunt of UV. Once the seals go, wind-driven rain finds its way in around the opening sash.

The fix: replacement gasket sets are available for most Velux models (you will need the window’s size code, printed on the data plate). A seal replacement service typically costs £120–£250. If the window is over 20 years old and multiple components are failing, replacement may be better value — see our Velux window replacement cost guide.

Cause 3: blocked drainage channels

This one surprises people. Modern roof windows have built-in drainage channels in the frame that carry away any water that gets past the outer seals. These channels are small and they block — with moss, dirt, dead insects and general debris.

When the channels block, water backs up and overflows into the room, often appearing as a leak from the top of the window. The window is not actually leaking; its drainage is just overwhelmed.

The fix: open the window fully (most roof windows have a cleaning position — rotate the sash 180 degrees) and clear the drainage channels with a soft brush and water. This is free and takes ten minutes. Do it once a year and you will prevent a surprising number of “leaks”.

Cause 4: condensation mistaken for a leak

Roof windows are cold surfaces in a warm room — prime condensation territory, especially in bedrooms and bathrooms. Water droplets on the inside of the glass, or water pooling on the internal sill, is often condensation, not a leak.

How to tell: condensation appears on the room-side glass and frame, typically in cold weather, and dries when the room is ventilated. A real leak shows water coming from around the frame edges, the head of the window, or the surrounding plaster — and it correlates with rain, not cold.

If it is condensation, the fix is ventilation and humidity control, not roofing work. Trickle vents (standard on modern roof windows — make sure yours are open), extractor fans, and keeping the room aired all help.

Cause 5: problems with the surrounding roof

Sometimes the window is innocent and the leak is coming from the roof around it — a slipped tile above the window, failed valley or flashing nearby, or perished felt at the eaves. Water travels along timbers and felt before dripping, so the drip point inside is often some distance from the actual entry point.

How to tell: if the water appears away from the window frame — on the surrounding plaster or ceiling — or if the window area is dry but the reveal is wet, suspect the surrounding roof. Valley and flashing failures nearby are other common culprits worth checking.

Cause 6: installation errors on the roof covering

On tiled roofs, the tiles around a roof window need to be cut and dressed correctly. Common installation faults include tiles cut too short (leaving gaps under the flashing), the underfelt not lapped correctly into the window’s insulation collar, and missing or misplaced support battens. These faults may not leak for years — then one bad winter exposes them.

Repair or replace?

Use this rule of thumb:

Under 10 years old: almost always repairable — flashing, seals or drainage. Do not let anyone sell you a new window.
10–20 years old: repairable in most cases, but get the whole unit assessed. If the flashing and seals both need doing, compare the combined repair cost against replacement.
Over 20 years old: replacement often makes more sense. Modern roof windows are significantly better insulated and more weathertight than 1990s units, and you will get a fresh guarantee.

If you are having roof work done anyway, consider the timing — our guide on whether to replace skylights during a re-roof explains why doing both together saves money.

What not to do

Do not seal a leaking roof window with silicone or mastic smeared around the frame. It will not fix a flashing failure, it makes proper repair harder (the roofer has to clean it all off first), and it can trap water and accelerate corrosion. Similarly, do not ignore it — water ingress around a roof window damages the surrounding plaster, insulation and timbers, and the repair bill grows the longer it is left.

Getting it fixed properly

A proper roof window leak repair starts with diagnosis: is it the flashing, the seals, the drainage, condensation, or the surrounding roof? Each has a different fix and a different price. Any roofer who quotes without looking is guessing.

Victoria Roofing Expert Ltd repairs and replaces roof windows across Camden and all of London through our skylight repair and skylight installation services. Family-run since 1982, 5.0 from 338 Google reviews, open 24 hours, fully guaranteed workmanship.

Get a free quote — we will diagnose the actual cause and give you an honest price for the right fix.