Leaking roof windows & skylights
In short
Most leaking roof windows are not leaking at all — they are condensing. Where there genuinely is water getting in, it is nearly always the flashing kit rather than the unit, and the failure is usually at the top corners where the wrong kit or a lead bodge was used.
Also called: Velux leaking · Skylight leak · Water round roof window · Rooflight leaking
Signs you need this
- Water appearing at the top corners of the window
- A stain on the plaster or lining around the reveal
- Water running off the glass onto the sill
- Damp only when the wind is from a particular direction
- Lead dressed over the window instead of a proper kit
- Rot in the frame or the timber around the opening
The first thing to establish with a 'leaking' roof window is whether water is coming in from outside or forming on the inside. Glass is the coldest surface in the room, and humid air condenses on it and runs down onto the sill — which produces a wet frame, a wet sill and eventually a stain on the plaster, exactly as a leak would.
The tells are reliable. Condensation appears on cold nights regardless of rain, it is worst in bathrooms, kitchens and bedrooms with the door shut, it forms on the glass itself rather than arriving at the frame, and it improves the moment you use the background ventilation flap. A genuine leak follows rain, and it typically appears at a corner or along the head rather than evenly.
Where it is a real leak, the flashing is the culprit far more often than the window. The top corners are the classic failure point, and the usual reason is that the wrong flashing kit was used — or none was, and somebody dressed lead over the top instead. Manufacturers make covering-specific kits for good reasons, and improvised weathering around a roof window rarely survives two winters.
The second commonest is the underlay. The roofing membrane has to be dressed and sealed correctly around the opening, forming a gutter above the window that takes water arriving from the slope above and diverts it round each side. Where that has been cut and left, water running down under the covering goes straight into the opening, and no amount of work to the external flashing will stop it.
Failed seals are the third, and they belong to older units. Gaskets harden with age and stop sealing against the frame, which lets driven rain past when the wind is in the right direction — a leak that only appears in certain conditions. On a unit of any age, that is usually the point at which replacement makes more sense than repair.
Finally, it is worth checking that the water is coming from the window at all. Water travels along battens and rafters before it drops, so a stain near a roof window can easily originate from a flashing or a valley several metres away, and the window is simply the nearest thing to blame. That is why the diagnosis starts in the loft rather than at the frame.
What the job covers
- Condensation ruled in or out before anything is quoted
- Loft inspection to confirm the water is from the window
- Flashing kit checked for type and correct installation
- Underlay above the opening checked for a proper diversion
- Correct manufacturer kit fitted where the existing is wrong
- Seal condition assessed and replacement advised where sensible
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
How do I know if it is a leak or condensation?
Timing and location. Condensation appears on cold nights whether or not it is raining, forms on the glass and runs onto the sill, and is worst in bathrooms and in rooms with the door shut. A leak follows rain and usually shows at a corner or along the head. Using the ventilation flap for a week is a good test.
Why do they always leak at the top corners?
Because that is where the flashing kit works hardest — it is taking water arriving from the whole slope above and diverting it round the unit. If the wrong kit was used, or lead was improvised over the top, or the underlay above the window was not formed into a proper diversion, the top corners are where it shows first.
Can it be resealed with silicone?
Almost never usefully. Silicone around the outside of a roof window bridges the gap the flashing is supposed to shed water over, and it holds water against the frame. It buys a season and makes the proper repair messier. The fix is the right flashing kit correctly installed.
Is it worth replacing the whole window?
If the flashing is the problem, no — fix the flashing. If the seals have hardened, the glazing has misted or the frame has softened, then yes, and a straight swap into the existing opening is usually far less work than people expect because frame sizes have been stable for decades.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.