Roof windows such as Velux windows and rooflights bring light into thousands of London loft conversions, top-floor flats and offices. They are also one of the most common places for a roof to leak. Water around a roof window is not always the window itself: it is often the flashing around it, the roof next to it, or condensation forming inside. This guide explains the usual causes, how leaks are found and fixed, and when a roof window should be replaced.
Why roof windows leak
Flashing problems
The flashing kit seals the gap between the window frame and the roof covering. If it was fitted badly, has been disturbed by slipped slates, or has corroded, rain can run behind it. This is the most common cause of leaks we find.
Worn seals and gaskets
Rubber seals around the opening sash harden and shrink with age, letting wind-driven rain in when the window is closed.
Blocked channels and debris
Leaves and moss build up around the frame and in the drainage channels, so water backs up instead of running off.
Fitted at the wrong pitch
Every roof window has a minimum roof pitch. Windows fitted on roofs that are too shallow, which is common on some dormers and extensions, are much more likely to leak.
Condensation
Warm, damp air from showers, cooking and drying clothes condenses on cold glass and runs down the frame. It can look like a leak, but the fix is better ventilation rather than roof work.
Signs of a roof window leak
- Damp patches or staining on the ceiling or wall around the window.
- Water dripping from the frame during or after heavy rain.
- Peeling paint or swollen timber on the inside lining.
- Daylight or draughts around the frame when it is closed.
- Slipped or missing slates or tiles next to the window.
How roof window leaks are fixed
- Inspection: we check the window, the flashing and the roof around it, and rule out condensation.
- Clearing: debris is removed from the frame, flashing and channels.
- Flashing repair or renewal: slates or tiles around the window are lifted and the flashing is refitted or replaced.
- Seals: worn seals are replaced where parts are available.
- Replacement: where the frame or glazing has failed, a new window and flashing kit are fitted to suit the roof pitch.
Repair or replace?
If the window itself is sound, renewing the flashing and seals usually solves the problem. Windows that are very old, rotten, cracked or fitted at the wrong pitch are normally better replaced, and a modern window with the correct flashing will be warmer and more weathertight.
On a recent job we fitted a new rooflight in a warm deck flat roof and detailed it to the slate-clad dormer and its roof window. See the project.
Roof windows in conservation areas
Many London streets are conservation areas. Replacing a roof window like for like does not normally need permission, but adding new rooflights, especially on front roof slopes, may. Councils often ask for slim “conservation” rooflights that sit flush with the roof. Our listed building guide covers the extra rules for listed buildings.
Where we work in London
Roof window leak FAQs
Leaks that appear only in heavy or wind-driven rain usually point to the flashing or worn seals, rather than the glass.
Usually not. Water on the inside on cold mornings is normally condensation. A leak tends to show as staining around the frame after rain.
Often, yes. Renewing the flashing and seals solves many leaks. Replacement is needed when the frame or glazing has failed.
Yes. We work on homes, flats and commercial buildings and can provide a written quote for the freeholder or managing agent.
Like-for-like replacement does not normally need permission. New rooflights, particularly on front slopes in conservation areas, may.
