Water Coming In Around A Velux Or Roof Window
Water around a roof window is usually the flashing kit or a blocked drainage channel rather than the window itself. Roof windows are designed to be watertight and generally are. What fails is the interface between the window and the roof covering, and that is a repair rather than a replacement.
Key takeaways
- The flashing kit around the window is a separate product to the window, and it is what normally fails.
- Roof windows have built in drainage channels that carry water away internally. When those block, water backs up and appears inside.
- Condensation on the inside of the glass is extremely common and is not a leak, though it looks like one.
- Water at the top of the window points at the head flashing. Water at the sides points at the side flashings or the covering beside them.
- A misted sealed unit is a failed window, but it does not cause water inside the room.
The window and the flashing are two different things
A roof window is a sealed frame and glass unit that arrives watertight from the factory. Getting it into a tiled or slated roof requires a flashing kit, a set of preformed metal sections that lap the covering above, beside and below the frame and channel water around the opening. The kit is specific to the covering type and profile, and it has to be installed with the right laps and the right upstand. Almost every complaint about a leaking roof window turns out to involve this kit rather than the window, which is why replacing the window without addressing the flashing frequently produces a new window that leaks in exactly the same place.
Where water gets in, in order of likelihood
The head, meaning the top of the window, comes first, because that is where water arriving down the slope hits an obstruction and has to be diverted. If the head flashing has lifted, been bent during a repair, or was never dressed properly under the course above, water goes behind it. Second is the sides, where the flashing has to lap the covering and where a slipped or cracked tile beside the frame breaks the seal. Third is the sill or apron at the bottom, which is the least likely because water there is already heading away. Fourth is the covering generally in the area around the window, since anything landing above it will find the window before it finds the gutter.
The drainage channels nobody knows about
Modern roof windows include drainage channels around the sash designed to collect the small amount of water and condensation that gets past the outer seal and take it back outside. Those channels can block with dust, insects, moss and general debris, particularly on a window that is rarely opened. Once blocked, the water that the design expects to drain away instead builds up and overflows inside, which produces a drip from the frame that looks exactly like a leaking roof. Opening the window fully and cleaning around the frame and the visible channels is a simple maintenance task that resolves a meaningful share of reported roof window leaks and costs nothing.
Condensation, which is the most common false alarm
A roof window is a large area of glass in a sloping surface, often in a bathroom or a loft conversion, and often in a room with limited ventilation. Warm moist air rises, hits cold glass and condenses, then runs down the pane and collects at the bottom of the frame. In cold weather this can produce a genuine puddle, and it appears without any rain at all. The tell is timing and location: condensation is worst on cold mornings, affects the glass surface rather than the frame surround, and improves when the room is ventilated. If water appears on a freezing dry night, the roof is not the problem. Using the window's vent flap, or improving extraction in the room, usually resolves it.
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How the age of the installation matters
Flashing kits and seals have a service life. The rubber gaskets around the sash harden over fifteen to twenty years and stop compressing properly, which lets water past the first line of defence and puts more load on the drainage channels. On older installations the flashing may have been formed in lead by hand rather than supplied as a kit, which can be excellent work or can be a fudge, depending on who did it. If the window is more than about twenty years old and the frame timber is soft or the seals are visibly hard and cracked, the calculation shifts from repairing the flashing to replacing the unit, because a new window comes with a new kit and a new set of seals.
What to check yourself, safely
All of the useful checks are from inside the room. Open the window fully in dry weather and look at the drainage channels around the frame for debris, and clean them out. Look at the seals for hardening, cracking or gaps where the sash no longer compresses them. Look at the timber of the frame for softness or dark staining, particularly at the bottom corners. Note where the water appears: at the top of the reveal suggests the head flashing, at the sides suggests side flashings, and on the glass itself suggests condensation. Do not go onto the roof to look at the flashing, and be particularly careful about leaning out of a roof window to inspect it, which is how people fall.
How soon a roof window leak needs attention
Most roof window leaks are slow and can wait for a planned dry day, which is when a flashing repair should be done anyway. Treat it as more pressing if water is running rather than seeping, if it is reaching a loft conversion's electrics or lighting, or if the plasterboard around the reveal has started to soften and bulge, because loft conversion reveals are usually plasterboard on a timber frame and they degrade quickly once wet. A misted or blown sealed unit, by contrast, has no urgency at all in terms of water: it is a failed seal between panes and it affects appearance and heat loss, not watertightness.
How a roof window leak is properly fixed
For a flashing problem, the covering around the window is lifted, the existing kit is inspected, and the flashing is refitted or renewed with correct laps under the course above and over the course below. Sealant smeared over the outside of a flashing is not a repair; it is a temporary measure that hides the fault and usually traps water behind it. For failed seals, the gaskets can often be replaced on their own, which is far cheaper than a new window. For a rotten frame or a blown unit on an old window, replacement with a new window and a matched flashing kit is generally the sensible answer, and it is a job measured in hours rather than days for a competent installer. One thing worth checking before any of that is whether the window was ever the right size for the opening. Roof windows fitted into a rafter spacing they do not suit end up with a trimmed opening and a flashing kit stretched to cover the difference, and that installation will keep leaking regardless of how many times the seals are renewed.
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About the author
David Boardman founded Boardman Roofing in 2018. Dave has 25 years on the tools. Read 150+ Google reviews or call 07879 736629.
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