Do electric rooflights close automatically in rain?
Yes, where a rain sensor is fitted, and on manufactured electric roof windows it is fitted as standard rather than as an option. The sensor is a small pad on the outside of the sash, the unit reads it continuously, and the closing cycle starts within a few seconds of the first drops landing on it. Made-to-size opening rooflights on flat roofs take a sensor too, though on those it is more often specified rather than assumed, so it is worth confirming on the order. What the sensor is not is a weather forecast: it reads water on a pad, and it will act on any water at all. The full account of how the electronics behave is on the electric rooflight installation page, and this is what it means in a real house.
The pad, the two tracks and the few seconds it takes
Physically the sensor is unremarkable. Two conductive tracks run across a small plate, separated by a millimetre or two of insulation, and the electronics measure the resistance between them. Dry, that resistance is very high. Water bridging the gap drops it sharply, the control board sees the change, and it commands the sash shut.
Response time from the first drops to the motor starting is a couple of seconds on most systems, and the closing cycle itself takes somewhere between fifteen and thirty seconds depending on the stroke. So the whole event, from rain arriving to the unit being latched, is under a minute. On a normal shower that is fast enough that the water which gets in lands on the reveal and the frame rather than the floor, and it dries without leaving a mark.
Where the water goes in the seconds before it latches
Being honest about the gap matters, because the phrase “closes automatically” implies an instant that does not exist.
On a pitched roof window the sash is closing through an arc, so the aperture is shrinking the whole time and the covering above the frame is already shedding most of what falls. Water entering during those seconds runs down the inside of the sash and into the frame’s own drainage channel, which is designed for exactly this and carries it back out onto the tiles. On a kerb mounted flat roof unit the geometry is less forgiving, because the aperture points straight up and 350mm of lift is a large gap. Rain that gets through lands inside the reveal or on whatever is beneath. Over a kitchen island in a Cheltenham extension that is a wipe. Under a Stroud valley squall that arrives with real force behind it, it is a bit more than a wipe.
Water is water, whatever brought it
The sensor cannot tell why the pad is wet, and there is no filtering in it. Several things set it off on a dry day.
- Heavy dew on a clear autumn morning, which on a north facing slope can sit on the pad until mid morning.
- River fog in the vale, which condenses on everything and does not lift quickly along the Severn or around Tewkesbury.
- A garden sprinkler, a hose on a car, or somebody cleaning windows on the elevation.
- Melting snow and frost running off the covering above.
The behaviour to expect is a unit that shuts itself at seven in the morning in October. That is the sensor working correctly, not a fault, and understanding it in advance saves a call. What it does mean is that in a room where continuous background air matters, the vent flap in the head of the frame is doing that work, not the sash.
The override, and the deliberate limit on it
Every system has some way of reopening a unit with the sensor wet, because otherwise a light drizzle would make the window unusable for an afternoon. Typically it is a long press on the open button, or a specific sequence on the remote, and it holds the unit open for a set period before normal sensor control resumes.
Note what the override does not do, and note that this is deliberate. It is time limited, so it lapses. There is generally no way to disable the sensor permanently from the handset, and where a system allows it at all the setting is buried in an installer menu. The design view behind that is straightforward: an override is for a person standing in the room who can see the shower has passed, not for someone leaving the house. A rooflight is a hole in a roof, and the failure mode of an override you can set and forget is a bad one.
Kerb mounted units, and where the sensor sits on a flat roof
On a flat roof rooflight the sensor position needs thought that a pitched unit does not. The pad has to be somewhere water reaches quickly, which means high on the sash or on the kerb capping, and it has to be somewhere water leaves quickly too. Sited in a hollow, or at the low corner of a pane that drains across it, the pad stays wet long after the shower has gone and the unit refuses to open.
Falls are part of that. A flat roof is designed to fall at around 1:40 so the finished fall after deflection is no shallower than 1:80, and a deck that ponds will hold water against a kerb for months at a time. Where an existing roof already ponds at the position somebody wants a unit, the deck needs re-falling before a rooflight goes on it, and that gets said at survey rather than discovered later. Kerb heights and falls are on the flat roof rooflight installation page.
Wind, which the sensor does not read
A rain sensor detects water. It has nothing to say about wind, and on an exposed Gloucestershire roof that distinction matters. A top hung sash open to 20 degrees presents a real area to a south westerly, and the Cotswold escarpment above Cheltenham, the ridge from Birdlip to Painswick and the whole of the Forest take that wind with nothing in front of them.
The mechanism copes: the actuator has a rated holding force, it is self locking under load, and the sash and hinges are designed for it. What it will not do is decide for you that a front is coming. Dry gales exist, and a unit will sit open through one quite happily. On exposed positions the sensible specification leans towards a shorter opening stroke and a modestly sized top hung sash rather than a large unit opened wide, and the operating habit is to close it before a front rather than after.
Manual units, and the shower that arrives while you are out
This is the comparison that decides whether the sensor is worth having, and it has nothing to do with reach. A manual roof window within easy arm’s length of the floor still cannot close itself.
Anyone who has left a rear addition roof window open in Gloucester on a bright June morning and come home to a wet mark across the landing carpet knows the arithmetic. It is not a serious event, and it happens repeatedly. In a house where windows get opened in the morning and everybody leaves for the day, an automatic closing unit changes the behaviour: the window actually gets used, because using it carries no risk. A manual unit in the same position gets left shut on any day that looks uncertain, which in this county is a good many of them.
Keeping the sensor reading correctly
There is very little to do and it takes two minutes. Twice a year the pad gets wiped clear of debris, and that is essentially the whole task.
What it prevents is worth knowing. Under trees, and Gloucestershire has plenty of roofs under trees through the Forest, the wolds villages and the older Cheltenham suburbs, the pad collects leaf fragments and moss spores. A mat of green growth across the tracks holds moisture continuously, so the unit reads rain permanently and will not open. Left longer, a fully fouled pad can stop reading altogether, which is the worse failure because the unit then stays open in a shower. On a solar unit the photovoltaic panel gets cleaned at the same time, since a film of algae on a north slope costs real charging capacity. Both jobs are done from inside on a pitched unit with the sash rotated, and the maintenance detail sits in the guides.
Next on this subject.
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