A leaking Velux (roof window) feels like a small problem—until the plaster starts bubbling, the insulation gets soaked, and you’re suddenly pricing up a bigger job.
The good news: a lot of “Velux leaks” aren’t actually the glass failing. The bad news: many are caused by the bit around the window (flashing, felt, tiles/slates), which needs a roofer who understands roof windows.
This guide covers:
- the most common reasons Velux windows leak
- safe checks you can do yourself
- typical UK repair costs
- when it’s smarter to replace the whole unit
First: is it a leak… or condensation?
Before you assume the worst, work out what kind of “water” you’re seeing:
Likely condensation (not a roof leak)
- Water droplets on the inside of the glass in the mornings
- Damp only in cold weather
- Worse in bedrooms/bathrooms
- No obvious “run line” from the top corners
Condensation is usually a ventilation/humidity issue rather than a failed window.
Likely an external leak
- Drips appear during rain (especially wind-driven rain)
- Wet patches at the top corners of the frame or ceiling reveal
- Water marks that spread over time
- Dripping continues for a while after rain stops (insulation holding water)
VELUX themselves note that internal water often points to issues with the roofing materials and weathering around the window rather than the window itself.
The 6 most common causes of a leaking Velux window
1) Flashing problems (the #1 culprit)
The flashing kit is the metal system that seals the window to tiles/slates. If it’s:
- incorrectly fitted
- damaged
- the wrong type for the roof covering
- lifted by wind
…water can track straight in.
Typical flashing kit prices vary by type, often roughly £90–£170 for common kits (before labour).
2) Roofing felt or underlay issues around the window
Even if the window looks fine, water can get under tiles and then travel along underlay until it finds the opening around the roof window.
This is why some leaks only show up in heavy rain or specific wind directions.
3) Cracked/missing tiles or slipped slates near the window
A single slipped tile above a roof window can send water right into the flashing area.
4) Worn seals / gaskets / minor frame issues
Seals can perish over time, especially on older units. This can cause draughts and occasional ingress.
(Seals are typically among the cheaper fixes, compared to flashing or a full replacement.)
5) Drainage channels blocked (leaves, moss, roof grit)
Roof windows have drainage paths. If these are blocked, water can back up in the wrong place.
6) The unit is simply old (and modern replacements are better)
If the window is very old, it may be more cost-effective to replace a Velux window rather than repeatedly repair—especially if you’re also improving insulation and airtightness.
VELUX states a like-for-like replacement can often be done in about four hours, and gives a labour-cost ballpark of ~£550–£650 (plus the new window).
7 safe checks you can do before calling a roofer
You don’t need to climb onto a roof to gather useful info.
- Check exactly when it leaks (heavy rain only? wind from one direction? constant?)
- Look for staining patterns around the top corners and ceiling line
- Inspect internal trims for damp timber or bubbling paint
- Open the window and check seals for obvious splits or gaps
- Check for condensation patterns (droplets vs run marks)
- Look outside from ground level with binoculars: any slipped tiles near the window?
- Take photos/video during the leak (this helps roofers diagnose quickly)
Typical Velux repair costs in the UK
Real pricing depends on access (scaffold vs roof ladder), roof type, and how far water has travelled.
A useful ballpark from a UK cost guide:
- Average repair costs: £150–£1,000
- Flashing or frame repair/replacement: £175–£490
- Seal replacement: £60–£150
- Remove and replace: £1,000–£2,500
If the job needs scaffolding or difficult access, it can rise quickly.
Repair vs replace: how to decide
Choose Velux repair if:
- the window opens/closes well
- the issue is clearly flashing/tiles/underlay
- the unit is relatively modern
- there’s no recurring history of leaks
Choose replacement if:
- the unit is very old and parts are becoming a pain
- repeated leaks keep returning
- the frame is tired, warped, or you want better thermal performance
- you’re renovating the loft/room anyway (best time to upgrade)
VELUX also publishes budgeting guidance for roof window replacement and notes labour for like-for-like replacement in the £550–£650 range, plus the window price.
What to ask a roofer (so you don’t get fobbed off)
When you ring around, ask:
- “Will you check flashing type matches my roof covering (tile/slate/profile)?”
- “Will you inspect underlay/felt around the window, not just re-seal the frame?”
- “If you recommend replacement, is it like-for-like or does it need structural changes?”
- “Will you replace flashing at the same time if needed?”
- “What access do you need — ladder or scaffold?”
If the answer is basically “we’ll just silicone it”, be cautious. A proper fix is usually about the system, not a bead of sealant.
FAQs (good for Google snippets)
Can a Velux window leak even if the glass is fine?
Yes. Many “Velux leaks” come from flashing/tiles/underlay around the window rather than the glazing.
How much does it cost to replace a Velux window in the UK?
Like-for-like labour is often quoted around £550–£650, plus the cost of the new window (and potentially flashing/access).
Can I just replace the flashing kit?
Sometimes, yes—especially if the window itself is sound and the leak is clearly around the perimeter. Flashing kits themselves are commonly under a couple of hundred pounds depending on type.
How do I know if it’s condensation, not a leak?
Condensation tends to be droplets on the inside of the glass, worst in cold weather and humid rooms. Leaks usually appear during rain and leave staining around trims/plaster.
