Why does my rooflight drip in winter?
Because the room side of the glass has fallen below the dew point of the air touching it, water has formed on it, and once enough has collected it runs. On a horizontal or shallow unit there is no vertical face for it to sheet down, so it gathers into beads along the frame and lets go. Almost every winter drip reported on a rooflight in Essex is the household’s own moisture arriving on the coldest surface in the ceiling.
That is not the whole answer, because a small number of drips genuinely are water from outside, and the two look identical from a sofa. Telling them apart takes about a week of attention and three simple observations. This page covers those observations, what can be changed quickly, and the perimeter details that decide whether a unit runs wet every January. The specification side is set out across energy efficient rooflights.
The temperature the inner pane reaches on a January night
A room at twenty degrees and fifty per cent relative humidity has a dew point near nine degrees. A modern double glazed rooflight with a soft coat and argon holds its inner surface comfortably above that on a cold night, which is why a well specified unit in a dry room shows nothing at all.
Change either input and the picture changes fast. Push humidity to seventy per cent, which a kitchen reaches in the time it takes to boil pasta with the extractor off, and the dew point climbs to around fourteen degrees. Now the inner pane is below it, and so is the frame, and so is the plaster at the edge of the opening. Nothing has failed. The air simply arrived carrying more water than the surface could tolerate.
Drips that start at the frame, not at the glass
Watch where the first bead forms. On a unit that is behaving normally the glass mists at its edges and clears by mid morning. On a unit with a cold frame, the water appears on the metal or the timber first, sits in the corner of the sightline, and grows until surface tension gives up.
An aluminium profile without a continuous thermal break runs several degrees colder than the glass it carries, so it collects water while the pane is still clear. That is a diagnostic worth knowing, because it points at the frame specification rather than at the room. The same applies to a sash that has an ageing gasket letting room air into the drainage channel.
The seven day test that separates the two causes
Three observations, taken over a normal week, settle it.
- Timing against the weather. Condensation appears on cold, still, clear nights and is worst at dawn. Water from outside appears during or shortly after rain, and particularly rain driven from one direction.
- Timing against the room. If it follows a shower, a wash load dried indoors or a long session at the hob, the moisture came from inside.
- Where it sits. Condensation forms across the whole cold surface as an even film. Water from outside arrives at a point, usually the same point every time, and often the highest corner of the opening.
A week of notes on a phone is more useful than an hour of speculation, and it is the first thing a survey asks for. The broader diagnosis of surface water sits on what causes condensation on a rooflight.
The moisture load a winter household generates
The numbers surprise people. Cooking a family meal releases well over a litre of water vapour. A shower puts out a couple of hundred millilitres in a few minutes. A load of washing dried on an airer indoors releases two litres or more as it dries, slowly, into the room it stands in.
All of that has to leave the house in air, and if it does not it circulates until it finds a cold surface. In a modern extension with good doors and a tight membrane, there is less accidental ventilation than there was in the draughty rooms people grew up in, so the moisture stays. The rooflight is not causing the problem. It is the first place the problem becomes visible.
Clear nights, still air and the sky the glass faces
A rooflight faces the open sky, and on a clear still night the sky behaves as a very cold body. Long wave radiation leaves the outer pane faster than the air around it can replace the heat, so the outer surface can drop below the outside air temperature.
That is why a rooflight frosts before the tiles beside it, and why the coldest interior surface is the one lying flat rather than the one standing up. A vertical window in the same room, with the same glass, will often stay clear on a night that has the rooflight running. Cloud cover changes it completely: an overcast night at the same air temperature produces far less.
The reveal below the unit, and the mark that appears there
Water running off a frame lands on the plasterboard lining of the opening. Plasterboard absorbs it, holds it, and shows a dark wet mark that dries out slowly and comes back the next cold snap. After two or three winters the paper face is stained and mould has started in the corners.
Where that happens, the cause is very often the kerb or the trimmed opening itself rather than the glass. An upstand insulated on three sides and left bare on the fourth, or a rafter trimmer with no insulation packed against it, gives a straight thermal path from the warm room to the cold outside. The inner face of that path sits below dew point all winter, and the reveal is where it shows.
What can be changed in a week
The quickest measures cost very little and they work. An extractor over the hob that ducts outside rather than recirculating through a charcoal cartridge. A bathroom fan with an overrun that keeps running after the room empties. Laundry dried outdoors, or in a machine that vents, rather than on an airer under the rooflight.
Beyond that, purge the room. Ten minutes of a wide open opening after cooking removes more moisture than an hour of background trickle, because the exchange rate is so much higher. An opening rooflight is unusually good at this, since it sits at the top of the room where the warmest and wettest air has collected. Options for powered openers are on electric rooflight installation.
What needs the covering lifted
If the room has been dried out, the ventilation is working, and the unit is still running with water every cold morning, the answer is in the build-up. That means looking at the insulation continuity around the kerb, the vapour control layer where it meets the frame, and whether an old single glazed or thermally unbroken unit is simply past what it can do.
None of that is visible from inside. It is established at survey, and the work is done with the covering open, the insulation made continuous around the full height of the upstand, and the vapour layer taped to the frame so warm air cannot get behind it. Where the unit itself is the limit, replacing it into the existing opening is often a single day.
Essex stock that runs wet, and the stock that does not
The pattern across the county is consistent enough to predict. Seventies and eighties domed units on flat roofs across Harlow, Basildon and the Canvey bungalows are single skin or thin twin wall acrylic, and they run with water from November to March as a matter of routine. Victorian terraces in Chelmsford and Colchester with a roof window fitted into a cold loft conversion show it at the frame and the reveal.
The estate housing at Great Notley, Beaulieu Park and South Woodham Ferrers is a different case. The fabric is tight, the air changes are low, and the moisture has nowhere to go, so a good rooflight in a well built house can still drip if the ventilation strategy was never thought through. Tighter building makes the ventilation more important, not less.
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