Diagnosing a Leak Around a VELUX Window

Water appearing around a roof window is usually one of five things: the wrong flashing kit fitted from day one, a failed lap or termination in the right kit, roof covering displaced above the window, a perished sash gasket, or condensation that was never a leak at all. You can narrow it down a long way from inside the room. Where the stain sits, when it appears, and whether it tracks the weather or tracks your cooking and showering will tell you most of what you need before anyone puts a ladder up. What follows is the method we use at survey, in the order we use it.

One rule before anything else. Do not go on the roof. A wet pitched roof with slipped covering around a window opening is where people fall, and there is nothing you can learn up there that a survey from a tower or a scaffold will not tell you more safely. Look from inside, then look from the ground with binoculars.

Step one: where exactly is the water arriving?

Get in the room in daylight and look at the four sides of the reveal separately. Head, sill, and the two vertical returns. Then look at the ceiling for a metre out from the window on all sides. Most people describe a leak as being “around the window” when in fact it is arriving at one specific point, and that point is the diagnosis.

Take a photograph of each face. Then take one more from a couple of metres back so the whole opening and the surrounding ceiling are in shot. If the stain is old and dry, mark the edge of it with a pencil line and date it. When the next wet spell comes through you will know instantly whether it is still moving.

Step two: is it water or is it condensation?

This is the split that matters most, because it decides whether you are looking at the roof or at the room. Condensation around a roof window is extremely common and it is routinely reported to us as a leak.

The test is timing. Water ingress follows the weather. It appears during or shortly after rain, particularly wind-driven rain, and it dries back once the roof dries. Condensation follows occupation. It appears after a shower, after cooking, after a load of washing is dried indoors, and it is far worse from November through March than it is in summer. If the damp is at its worst on a cold, dry, still January morning with no rain for three days, you do not have a leak.

The second test is location. Condensation collects at the head of the reveal and in the top corners first, because that is the coldest part of the detail and warm moist air rises to it. It presents as a soft grey or black mould bloom rather than a hard brown tide mark. Water ingress leaves a defined edge with a darker rim where dissolved dust and timber tannin have concentrated as it dried.

We cover the condensation case properly on why VELUX windows mist up, so we will not repeat it here beyond saying that the usual cause is a missing insulation collar leaving a cold bridge right round the opening, and the usual cure has nothing to do with flashing.

Step three: the view from the ground

Stand back in the garden or across the street with a pair of binoculars and look at the window and the covering for a metre or so all around it. You are checking four things.

• Is anything displaced? Slipped, cracked or missing slates and tiles directly above the window are the single most common cause of a stain at the head. Water shed at a break in the covering runs down the underlay and meets the top of the window frame, and if the underfelt collar is not doing its job it goes into the timber rather than over it.
• Does the flashing sit tight to the covering? On pantiles and profiled concrete tiles you should see the flashing skirt pressed down into the valleys of the profile. If it bridges across the tops of the rolls and you can see shadow gaps beneath it, the wrong flashing kit was fitted.
• Is there mortar cracking or missing at a verge or abutment near the window? On a gable verge bedded in sand and cement, the mortar shrinks and cracks over twenty years. Water gets behind it, tracks along the batten line and emerges at the nearest interruption in the roof, which is frequently the roof window.
• Is there rust staining running down from fixing heads? Orange streaks below the flashing edge mean the fixings are corroding. Near the coast that happens fast with anything less than 316L stainless, and a corroding fixing is a hole in the making.

The five failures we find most often

1. The wrong flashing kit, fitted on day one

Flashing kits are made in families matched to the roof covering. A flat or low-profile kit suits natural slate, fibre cement slate and plain clay tiles up to around 8mm thick. A profiled kit, with a deeper pleated skirt, is required for pantiles and interlocking concrete tiles with a contour depth up to about 120mm. Suffolk has a great deal of pantile, and a flat kit on pantile leaves an open channel at every roll.

This failure has a signature. It leaks at the bottom corners, it leaks only in driven rain, and it has usually done so intermittently since the window went in. Owners often describe it as having “always done it a bit in bad weather”. That is not a quirk of roof windows. That is a specification error with a date on it.

2. The stapled underfelt bodge

The underlay beneath the covering has to be connected to the top of the window frame so that anything getting past the tiles is carried over the head of the frame and discharged back onto the covering below. There is a pre-formed underfelt collar made to do exactly that.

The bodge is to slit the felt, lap it roughly over the frame and put a row of staples through it. It holds in vertical rain. In an easterly, water driven up under the covering hits an unsealed lap and finds the timber. Stains at the head, appearing only in bad weather, on a window that has otherwise behaved, very often come back to this. You cannot see it without lifting covering, which is one reason a proper survey involves getting up there safely rather than guessing from a photograph.

3. Displaced pantiles above the window

Pantiles are single-lapped and they rely on their own weight and interlock. Around a roof window, where they have been cut to fit the flashing, the cut pieces are smaller, lighter and easier to lift. Twenty winters of wind lift and thermal movement walk them out of position. A pantile does not have to fall off to leak. Five millimetres of slip at the side lap is enough.

4. Mortar-bedded verge failure

Older Suffolk roofs finish at the gable with a bed of sand and cement mortar over the verge. It is a rigid finish on a structure that moves, and it cracks. Once water gets behind the verge it travels along the battens, and it will emerge wherever the batten run is interrupted. If your roof window sits within a couple of metres of a gable end and the stain is at the top corner nearest that gable, look at the verge before you look at the window.

5. Perished sash gaskets

The sash closes onto a compressible gasket run around the frame. That gasket is a rubber or foam section, and after fifteen to twenty years of ultraviolet exposure, heat cycling and compression it goes hard, flattens and loses its recovery. At the same time the closure mechanism that pulls the sash down wears, and the friction hinges lose the tension that holds the sash square.

The result is a closure line that is no longer continuous. In still rain nothing happens. In driven rain, water crosses it and appears at the sill, because that is where gravity takes it. Sill stains with a dry head and dry corners are gasket and hardware, not flashing. Test it with a sheet of paper: close the sash on it and pull. If it slides out easily at some points around the perimeter and grips at others, the closure is uneven.

When replacing the whole unit and the flashing kit is the honest answer

There are cases where a roof covering repair by a roofer is the right and only intervention. A slipped tile above an otherwise sound window is a roofer’s morning, not a new window. We will tell you that.

The picture changes when two or more of the following are true at once, and on a fifteen to twenty year old installation they usually are.

• The flashing kit is the wrong family for the covering, so the leak is designed in and dressing it down will not create a lap that was never there.
• The underfelt collar is absent or stapled, which cannot be corrected without stripping back and lifting the frame.
• The frame timber at the sill is soft, which means water has been present long enough to change the material.
• The sealed unit has blown, which dates the installation and tells you the whole assembly is at end of life.
• The unit is old enough that the sash, gaskets, hinge set and glass are no longer obtainable in the profile you have.

At that point the sensible unit of work is the whole assembly: window out, opening and trimmers inspected and made good, insulation collar, underfelt collar, and a flashing kit matched to your covering with laps mechanically fixed rather than sealed. That is a one-day job on a like-for-like opening, the roof is watertight the same day, and it carries a 10-year workmanship guarantee. Prices start at £1,150 to £1,650 for a single unit up to MK06, with the full range on our roof window replacement page.

What we check at survey

A leak survey on a roof window is about forty minutes of work and it happens in a specific order, because each stage rules something out.

1. Inside first. Moisture meter readings at the head, sill and both returns, and into the plasterboard away from the opening. Wet plasterboard with dry timber behind it points one way, wet timber points another.
2. Sash and closure. Gasket condition all round, closure evenness, hinge play, whether the sash sits square in the frame.
3. Glass. Whether the haze is on the inner face, on the outer face or between the panes. Three different problems.
4. Loft void where there is one. Staining on the underside of the underlay above the window shows you where water is landing, which is often nowhere near where it appears in the room.
5. External, from safe access. Flashing family against covering profile, lap dimensions, termination method, fixing grade and condition, covering integrity for a metre around, verge and abutment condition.
6. Water test. Hose applied low first then progressively higher, one zone at a time, with someone watching inside. Start at the sill and work up, or you learn nothing.

Our installers have 25 years fitting VELUX, Keylite and Roto roof windows across Suffolk and north Essex. We are not part of any manufacturer’s approval scheme and we do not carry a manufacturer badge, so what you get instead is the reasoning written down and a photograph of the defect.

Common questions about roof window leaks

Get it looked at properly

Send us photographs of each face of the reveal, one of the whole window from a couple of metres back, one of the roof from the garden showing the covering type, and the data plate if you can reach it. That is usually enough for a first opinion on whether you are looking at the roof, the window or the room.

Then we survey, water test, and give you one written price that does not move. If the answer is that a roofer needs to reset three tiles and your window is fine, we will say that too.

Request a fixed quote, or read up first on VELUX window installation, rot in timber VELUX frames, what a flashing kit actually does and when a roof window is past replacing.