By Charlie the Roofer for Roofline · Published · Checked against the sources listed below
If water appears during or soon after wind-driven rain, often at one lower corner, rain is getting in through the flashing or the way the window was fitted. If drops form on the glass on cold, still mornings and dry up later, it is condensation from humid indoor air. VELUX says condensation is not a product fault.
How can you tell rain from condensation?
Timing is the best clue. A leak shows up in wet, windy weather, while condensation shows up on cold mornings whatever the weather outside.
VELUX explains that condensation forms when warm, humid indoor air meets the cold inner pane, and that it is most common in winter. It tends to cover the glass evenly and run down onto the sill or the rubber seal, so the drips can look like a leak.
Rain usually behaves differently. It appears at one spot, often a lower corner, and only when rain comes from one direction. If it only happens when rain is driven against that side of the roof, the water is almost certainly coming from outside.
VELUX suggests these checks first:
- Make sure the window is fully closed and locked.
- Look for condensation on the inside of the glass, especially early in the day.
- Check the outside of the window for damage or loose covers. Do this from the ground or another window, never from the roof.
Wipe everything dry and note the weather each time it happens. If you also see wet patches in the loft or on a ceiling, our guide on how to find a roof leak explains how to trace them.
What lets rain in around a roof window?
Rain usually gets in around the window rather than through it, and the usual faults are the wrong flashing kit, a frame set too deep, or missing parts under the tiles.
The wrong flashing for the roof
Each VELUX flashing kit suits one kind of roof covering. Roofing Superstore's guide lists the EDW kit for profiled tiles up to 120mm and the EDL kit for slate up to 8mm thick, both for pitches of 15 to 90 degrees. The EDL is not made for interlocking slate. It also says 15 degrees is the usual minimum pitch, with a separate EAW kit for tiled roofs down to 10 degrees. If you have a Yorkshire stone slate roof, ask what was fitted, because the slate kits are made for slates no more than 8mm thick.
A window set too deep
The bottom flashing should lie over the tiles below and follow their shape. If the frame sits too low in the roof, that flashing can end up bent upwards, which forms a dip where water collects. This kind of fault often shows only in strong driving rain.
No drainage gutter above the frame
VELUX's underfelt collar includes a drainage gutter that leads water away from the head of the window if anything leaks above it. Where it was left out, water can track down beside the frame and appear at a lower corner.
Leaves and moss collecting above the window can also hold water against it. Our roof windows service covers re-flashing and refitting.
Why does a Velux get so much condensation, and how do you cut it down?
Roof windows sit high in the room where warm, moist air collects, so they gather condensation first. VELUX says a family of four produces about 10 to 15 litres of water vapour a day.
VELUX recommends opening at least two windows for 5 to 10 minutes about four times a day, including in autumn and winter, and using a dehumidifier if needed. It says condensation is more common in new homes and tends to ease after a few years as the building dries out.
If the plaster lining around the window stays damp or grows mould, the reveals may be poorly insulated. VELUX makes an insulation collar to stop cold bridges around the frame and a vapour barrier collar to protect the roof against damp and condensation. Our guide to a damp patch on the ceiling covers mould and staining in more detail.
Should the window be re-flashed or replaced, and who pays?
If the window still works and seals well, fixing the flashing or the fitting is usually enough. Replacement makes sense when the window itself has failed.
The VELUX GB product guarantee gives 10 years on roof windows, glazing units, flashings and VELUX installation products. It covers manufacturing defects only. It excludes faults caused by installation that goes against VELUX instructions or good workmanship, and water leaks caused by ice or snow blocking the window. A fitting fault is therefore a matter for whoever installed it.
For a straight swap, VELUX puts labour at around £550 to £650 plus the window, taking about four hours, and says most replacements can be done from inside. Trade2base gives £680 to £1,300 fitted for a standard 66 by 118cm centre-pivot window in its June 2026 guide.
Roofline is based in Barnsley and covers Yorkshire. Send a photo of the window and the water on WhatsApp to 07413 797731, or book a free inspection. We send photos of what we find and give a free itemised quote.
Send us a photo on WhatsApp and we will tell you what we think it is and whether it needs a visit. Quotes are free and itemised.
Sources
- VELUX UK support, Windows appear to be leaking
- VELUX UK support, Window condensation
- Roofing Superstore, What VELUX flashing do I need? (dated 11 August 2026)
- VELUX UK, Installation product system
- VELUX GB Product Guarantee (edition effective 1 January 2018)
- VELUX UK magazine, Costs of replacing a roof window (updated 15 September 2025)
- Trade2base, Velux window costs UK (8 June 2026)
General information about roofing in England, checked against the sources above when published. It is not a substitute for advice about your own roof.
