Water dripping down the inside of a roof window can feel bigger than it usually is. Before you assume the window itself is failing, it helps to know that most leaks trace back to something around the window, not the window itself. This guide walks you through how to figure out what’s actually going on and which parts you can safely handle yourself.
Start With a Proper Diagnosis
Guessing at the cause before you’ve actually looked wastes time and, in a few cases, money on the wrong repair. Before doing anything else, work through this checklist:
- Note exactly when the leak appears. If it only happens during heavy rain and wind from one direction, it points to flashing. A slow drip, regardless of weather, points more toward condensation or a seal failure than an actual leak.
- Check where the water first appears on the frame or ceiling. Water tracking down from the top of the window usually means the flashing or the area above the window. Water pooling at the bottom sill more often means the drainage gutter.
- Look at the glass itself. If there’s misting trapped between the panes rather than actual liquid water, that’s a failed seal on the glazing unit, not a leak in the traditional sense, and it’s a different repair entirely.
- If you can safely look at the roof from a loft hatch or with binoculars from the ground, check whether the flashing looks intact, whether the tiles around the window sit flush, and whether there’s visible moss or debris built up along the top edge.
- Think about the age of the window. Flashing has a working life of roughly 10 to 15 years depending on exposure, so a window fitted in the 2000s failing now is not unusual at all.
Once you’ve got a rough idea from that list, the cause is usually one of four things.
Failed Flashing
This is the most common cause by a wide margin, and it’s rarely obvious from inside the room. The flashing is the metal collar that sits between the window frame and the surrounding tiles, and its job is to shed water around the window rather than let it track underneath. Over time, sun, frost, and general weathering can cause the metal to corrode or the seal at its edges to lift slightly. A gap you’d struggle to spot from the ground is more than enough for wind-driven rain to find its way through during a storm, which is exactly why so many leak calls come in the morning after bad weather rather than during a quiet drizzle.
Flashing that’s simply come loose at one corner, rather than corroded through, can sometimes be re-bedded rather than fully replaced, which is worth asking about since it’s a cheaper fix if it’s caught early.
Roof covering matters more here than people expect. Flashing is designed for a specific type of surrounding tile, and a kit made for plain tiles doesn’t seal properly against interlocking tiles or slate. If your roof has been re-covered since the window was originally fitted, mention that mismatch to whoever comes to look, since it’s a surprisingly common reason a leak keeps returning after a previous repair.
Perished Seals
Seals sit in two places on a Velux window: around the sash where it meets the frame, and within the glazing unit itself. A perished sash seal lets air and water in around the edge of the opening part of the window, and you’ll often notice it alongside a draught rather than as a dramatic leak. A failed seal within the double glazed unit is different again, it usually shows up as permanent misting between the panes rather than actual water ingress, and no amount of drying the frame out will fix that, since the moisture is sealed inside the glass unit itself.
Blocked Drainage Gutter
Every Velux window has a small drainage channel along the top of the flashing designed to carry rainwater away rather than let it pool. Moss, roof grit washed down from higher up the roof, and leaves in autumn collect there over years, and once it’s blocked, water has nowhere to go except backwards, under the tiles. This is genuinely one of the simplest causes on this list to both diagnose and, in mild cases, fix, and it gets mistaken for a much more serious fault more often than it should.
Ice Damming
Less common across most of the UK than in colder climates, but it does happen during a proper cold snap, particularly on north-facing roofs or in more exposed parts of the North East. Snow melts slightly from heat escaping through the roof, refreezes at the colder edge near the window, and builds up a small ridge of ice that then forces meltwater backwards under the flashing instead of down and off the roof. If a leak only shows up after snow rather than rain, consider this, as it often points to wider heat loss or a loft insulation issue rather than a fault with the window itself.
What You Can Safely Check and Fix Yourself
A few things on this list are genuinely safe for most homeowners to handle without any specialist tools:
- Clearing visible debris from the drainage gutter, provided you can do this safely without getting onto the roof itself, for example through a loft hatch or with a long-handled brush from a stable ladder against the wall rather than the roof.
- Checking and clearing any obvious blockage in guttering elsewhere on the roof that might be causing water to back up towards the window.
- Wiping condensation from the inside of the glass to rule out simple surface moisture before assuming it’s a structural leak.
- Improving ventilation in the room, which won’t fix an actual leak but rules out the most common false alarm.
What Needs a Professional, and Why
Anything involving the flashing, the glazing unit, or work at height on a roof pitch is not a sensible DIY job, and this is worth taking seriously for reasons beyond simple safety. Velux’s own product guarantee can be affected by unauthorised work on the flashing or seal, since it’s considered a modification outside the original certified installation. Getting a repair quote before attempting a fix yourself is nearly always the cheaper route in the long run, even before you factor in the risk of working on a roof.
A certified engineer will also be working from proper roof access equipment rather than balancing on tiles, and will have the correct flashing kit matched to your specific roof covering, since fitting the wrong type for your tiles or slate is a common reason a DIY leak repair fails again within a season. Callout costs for a leak diagnosis and flashing repair typically start from around £150, which usually works out far cheaper than the damp and timber repairs that follow a leak left unresolved over a winter. For a broader look at when a repair makes sense versus when it’s time to think about replacement, our guide to Velux window repairs and maintenance covers that decision in more detail.
If your leak turns out to be connected to a planning or building regulations question, for example if you’re considering enlarging the window while you’re having work done anyway, our roof windows UK FAQs page is worth a read before you go any further.
Frequently Asked Questions
How do I know if my Velux window leak is serious?
Water that’s tracking onto plasterboard, timber, or insulation needs attention quickly, since damp left untreated can cause rot and mould well beyond the window itself. A small amount of surface condensation is not urgent in the same way.
Can I reseal a Velux window myself?
Cleaning the drainage gutter and wiping condensation are safe DIY tasks. Resealing the flashing or glazing unit is not recommended without proper roof access and training, partly for safety and partly because it can affect the product guarantee.
Will fixing a leak myself void my Velux guarantee?
It can. Velux windows carry a 10 year product guarantee, and unauthorised alterations to the flashing or seal, even well-intentioned DIY repairs, can be treated as work outside the original certified installation.
How quickly does a leak need fixing once it’s spotted?
As soon as reasonably possible. A leak left for a full winter season tends to cause secondary damage, such as damp plasterboard or timber, that costs far more to fix than the original fault would have.
If you’re dealing with a leak and would rather have someone take a proper look than guess from a ladder, get in touch with our team for a straightforward assessment and quote, with no call-out charge just to have a look.