Underlay collars, and the leak that only shows in driving rain
There is a leak on a roof window that appears only in wind driven rain from one direction, stops as soon as the wind drops, and produces nothing at all under a hose. It is the signature of a missing or badly made underlay collar, and it is one of the most commonly misdiagnosed defects in this trade.
The two layers on a pitched roof
A pitched roof has two lines of defence and most people only know about one of them.
The covering, whether slate, tile or stone, is the primary barrier and it sheds almost all of the water. Underneath it, over the rafters and under the battens, is the underlay: a sheet running the full slope from ridge to eaves.
The underlay is not a backup that sits idle. On every pitched roof in ordinary weather, a small amount of water gets past the covering, and the underlay carries it down to the gutter. That is its job and it does it continuously.
How water gets past a covering
Coverings work by lap and gravity, and both have limits.
- Wind driven rain. Water pushed sideways or upward under a lap by wind pressure, against gravity.
- Capillary action. Water drawn between two close surfaces, which is why cambered tiles exist.
- A lifted or slipped unit. A slate that has slid, a tile lifted by wind.
- Snow. Drifting into laps and melting there rather than running off.
- Debris. Moss or leaf litter damming water so it stands against a lap.
None of these is a fault. They are the ordinary conditions the underlay exists for.
What a roof window does to that layer
Fitting a roof window means cutting a hole in the underlay, and a hole in a drainage layer is a hole in a drainage layer.
Water running down the underlay arrives at the top edge of that hole. Without something to direct it, it goes through: into the opening, down the reveal, and onto the ceiling or the floor inside.
The underlay collar is the component that stops that. It is a preformed or site made piece that takes the cut edges of the underlay and connects them to the window frame, so that water arriving from above is carried round the window and back onto the underlay below it, on the outside of the frame.
Why it is missed
It is missed because it is invisible and because the roof works without it in ordinary weather.
The flashing kit is on the outside, it looks like the waterproofing, and it is what an installer is thinking about. The collar is underneath the covering, it takes a few more minutes, and a window fitted without one is perfectly dry in vertical rain.
The failure needs a specific combination of wind and rain to appear, which may be weeks or months after the work. By then nobody connects it to the installation.
The signature
| Observation | Missing underlay collar | Failed flashing |
|---|---|---|
| Weather | Driving rain with wind, usually one direction | Any heavy rain |
| Stops when | The wind drops, even if rain continues | The rain stops |
| Hose test | Nothing. Water runs off as designed | Reproduces it |
| Where it appears | Head of the window, running down the reveal | Varies with which part has failed |
| Quantity | Modest, but persistent through the storm | Can be substantial |
| Season | Autumn and winter, when the gales come | Any time of year |
The hose test row is the useful one. A hose runs water down the roof under gravity, which is exactly the condition the covering handles perfectly. It cannot reproduce wind pressure, so it cannot reproduce this failure.
Why a negative hose test is evidence
A negative hose test on a window that definitely leaks is not an inconclusive result. It is a positive indication.
It tells you the covering and the flashing are shedding water as designed under gravity, and that whatever is going wrong needs a condition the hose cannot create. On a pitched roof window that is almost always wind pressure, and wind pressure driving water past a lap is what the underlay is for.
So the sequence is: hose test, get nothing, stop looking at the flashing, and look at what is underneath it.
The prevailing wind here
On this coast the dominant wind is a south westerly straight off the Channel, and it carries the rain with it.
Which means the failure shows on slopes facing south west far more than on others, and it shows in the autumn and winter gales rather than in a summer downpour. A window on a north east facing slope with the same defect may go years without anybody noticing.
If your leak arrives with the gales and comes from one particular direction, that is consistent with this and worth saying when you call.
How the collar is made
There are two ways and both are legitimate.
A proprietary collar
Supplied with or alongside the window, preformed to suit the frame, with a skirt that is dressed onto the underlay above and below. It is the quicker option and it is what most current installations use.
Site made
Formed from underlay material, cut and lapped so that the water is directed round the frame and back onto the underlay below, sealed to the frame with a suitable tape. It takes longer and it is the answer on an older roof where the underlay is a material a modern collar does not bond to.
What both have to achieve
Water arriving on the underlay above the window must be carried round the outside of the frame and returned to the underlay below it, without passing over any cut edge that faces upward.
What the underlay is made of, and why it matters here
Underlay has changed twice in the last century and which generation is on your roof changes the detail.
Bitumen felt
The traditional material, used from the mid twentieth century onward. It is impermeable, it is reasonably robust when new, and it becomes brittle with age. On a fifty year old roof it frequently tears when disturbed, which means the area that has to be opened up around a window is larger than expected.
Vapour permeable membranes
The modern material, which sheds liquid water while letting vapour through, so the roof space does not have to be ventilated in the same way. Light, tough, and easy to form a collar in.
Nothing at all
Common on genuinely old roofs, and covered below. The absence is not a defect on a roof designed without it.
Mixing generations
A collar formed in permeable membrane, lapped into brittle old bitumen felt, is a junction between two materials that behave differently. It works, and it needs the lap made generously and taped rather than assumed.
Sarking boards, and the Scottish exception
Occasionally a roof has boards laid over the rafters under the covering rather than an underlay membrane. It is standard practice in Scotland and unusual here, and it turns up on a small number of older and grander buildings.
Where it exists, the boarding is the drainage plane and the detail around a window is different again: the water runs on top of the boards, and the connection to the frame has to be made at that level rather than in a membrane.
It is rare enough in Sussex that the main thing is to recognise it rather than to assume a standard collar will do, and it is one of the reasons we look inside the roof space before quoting.
Roofs with no underlay at all
A great many older roofs in the villages here have no underlay. They were built before it was normal and they have performed for a century on the covering alone.
That works because the roof is generously lapped and because the whole assembly breathes. It also means a window fitted into such a roof has nothing to connect a collar to.
The answer is to introduce underlay locally around the opening, taken far enough up the slope and out to either side that water is intercepted well above the frame. It should be vapour permeable, so that a roof which has been drying through its covering for a hundred years continues to.
Where the water actually appears
Because the water enters at the head of the window and runs, it does not necessarily appear at the head inside.
It can run down the outside of the frame and appear at a lower corner. It can run along a rafter and drop somewhere else entirely. On a lined reveal it can travel behind the plasterboard and appear at the bottom of the reveal or on the ceiling beyond it.
Which is why chasing the visible stain is unproductive and why the weather conditions are better evidence than the position of the mark.
What it does if it is left
The quantity of water is usually modest, which is why people live with it.
What that water is doing is wetting the timber around the opening every time there is a gale, and drying it in between. Over years that produces decay in the trimmers and the rafter ends, staining and mould on the reveal, and repeated redecoration.
It also wets the insulation, which stops it working, which makes the reveal colder, which adds condensation to the original problem. The two then get confused with each other.
Fixing it on an existing window
The collar is under the covering, so putting one in means lifting the covering around the window.
- Confirm the diagnosis: driving rain only, negative hose test, one wind direction.
- Lift the covering above and to the sides of the window, far enough to reach sound underlay.
- Inspect the underlay for cuts, tears and previous attempts at a repair.
- Form or fit the collar, connecting the underlay properly to the frame.
- Check the flashing kit while everything is open, since it costs nothing extra at that point.
- Reinstate the covering with the original slates or tiles.
It is a half day on most roofs, and it is a great deal cheaper than the replacement window that frequently gets fitted instead.
Why replacing the window does not fix it
This is the expensive mistake, and it is common.
The window is leaking, so the window gets replaced. The new unit goes into the same opening, with the same underlay still cut and unconnected, and frequently with the same new flashing kit fitted equally carefully on the outside.
The next gale produces the same leak. The customer has bought a new window and solved nothing, and confidence in the trade goes with it.
What to say when you call
Four pieces of information make the diagnosis almost immediate.
Whether it happens in all rain or only in windy rain. Which direction the wind is coming from when it does. Whether it stops when the wind drops even though rain continues. And whether anybody has already run a hose on it and got nothing.
Those four answers separate this failure from every other cause of water at a roof window, and they can all be given from indoors.
On new work
None of this arises where the collar goes in as part of the installation, and it takes minutes.
The underlay is cut, the collar is fitted and connected to the frame, the flashing kit goes on over it, and the covering is reinstated. Three components doing three jobs: the collar for water that gets past the covering, the flashing for the covering junction, and the insulation collar for thermal continuity.
Fitting one of the three is how a window ends up leaking, draughty or cold, and it is why we fit all three as standard rather than as extras.
What we fit on a pitched roof
We fit an underlay collar on every pitched roof installation, and where the roof has no underlay we introduce a vapour permeable one locally around the opening. On a diagnosis visit we ask about wind direction before we ask about anything else, and we run a hose to rule the flashing in or out rather than to confirm a hunch.
A leak that only comes with the wind is not a leak in the covering. It is water that got past the covering and found nothing underneath to catch it.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.