The short answer
A leaking Velux window is most often a flashing, seal or drainage fault, and some drips turn out to be condensation. Look at the glass for moisture on the room side, check the gutter and valley above the window for blockage, and note when the water appears. A roofer then inspects the flashing and seals from outside. In the meantime, catch the water in a bucket.
How to tell condensation from a real leak, where roof windows let water in, and how to cope until the repair is done.
Is it a leak or condensation?
Many suspected leaks are condensation. Warm, damp air from a bathroom, a bedroom or a kitchen meets cold glass and turns to water, which runs down the pane, collects on the frame and drips from the lining. It is worst on cold mornings, after a shower, or when a room is shut up, and it eases when the air dries out. A real leak follows the rain instead of the room.
A quick test helps. Wipe the room side of the glass and watch whether moisture returns when the weather is dry. If it does, you have condensation, or a failed sealed unit if the mist is between the panes. If the drips only come during or after rain, or stains spread from the top or the sides of the frame, the water is coming in from the roof. Compare the dates in a notebook with the weather.
Condensation has its own causes and cures, covered in our answer on condensation on a Velux window. It is worth settling this first, since a roofer sent to find a leak that is really condensation spends time and money on the wrong fix.
Flashing and the kit around the frame
The flashing is the set of metal pieces joining the window to the roof covering. It carries rain round the frame and onto the tiles or slates below, so a fault here sends water in at the corners, at the sides or under the head. Kits are made for particular coverings, so a mismatch is a classic cause when a window was fitted into a different roof type. Cracked or lifted pieces, a missing apron, and gaps that were filled with sealant are all warning signs.
The water often shows up away from the fault. The water tracks along a timber or the underlay before it drips lower down, so the stain on your ceiling may not sit under the problem. A roofer checks the flashing from the roof, looking for gaps, lifted tiles at the edge and signs that the underlay has not been dressed into the frame. A small repair may be enough: refixing a piece, replacing a tile or dressing the lead. If the kit is the wrong one, it must be replaced. Our page on Velux flashing kit types explains what each kit does.
Seals, the sash and the glazing
Inside the flashing, the window has its own defences. A rubber or foam seal runs round the sash and presses against the frame when it closes. Over time, rubber hardens, shrinks or splits, and the window stops sealing in wind-driven rain. A sash that has dropped slightly, a handle that does not pull the window tight, or a hinge that has loosened will have the same effect. Check whether the sash closes snugly and whether the seal runs unbroken.
Then the glass. A cracked outer pane lets rain into the gap, and the water then drips from the lower edge. A failed seal around the glass unit does the same. If the leak comes in only when the window is open or ajar, check whether it was left slightly open, and look at the hinges.
Many of these are fixable without a new window. A fresh seal, an adjusted catch or a swapped pane often cures the drip, but a rotted frame or a sash that has stopped meeting its seal may tip the balance towards replacement. Our answer on replacing a Velux window sets out how that works. Photos of the seals and the corners, taken in daylight, help us judge it before a visit.
Blocked gutters above, and what to do now
A window can be blameless and still leak if water arrives faster than the roof can clear it. A gutter or a valley above the window, clogged with leaves, moss or tile grit, overflows and sends a sheet of water over the head of the frame, where flashing is least able to cope. Moss building up on the tiles above can also dam the flow. If the leak starts in heavy rain and the drip falls near the top of the window, look first at what is above it.
For now, keep the room safe. Put a bucket under the drip, lay towels or a tray to protect the floor, move furniture and keep electrics well away. Stay off the roof. If the water is heavy or near wiring, switch off that circuit and call a roofer to secure the area. A leaking roof gets made safe first here, and the cause is photographed for you.
As a national guide, Checkatrade's July 2026 table gives £165 to £1,050 for a Velux repair, and £1,000 to £2,500 for a like-for-like replacement. The cause decides where your job falls. Our roof windows and skylights page has more, with a single written total once the survey is done.
More on this in Velux Roof Windows & Skylights in Crawley
Two more leak questions
Can I seal a leaking roof window with mastic?
Mastic is a stop-gap. It can slow a drip for a short while, but it hides the fault, traps moisture and cracks as the roof moves. The lasting fix is a repaired flashing, a new seal or a swapped pane. If you must use something before the roofer arrives, use a tray inside, not sealant outside.
Why does the leak only show after heavy rain?
That pattern points at flashing, drainage or a window that does not seal under wind-driven rain. Light showers run off, while heavy rain overwhelms a blocked gutter or finds a small gap. Write down the dates, the wind direction and where the water appears, and send it with photos. It helps a roofer find the point of entry.