The short answer
They do when a rain sensor is part of the system. The sensor detects moisture on its surface and tells the control to drive the motor shut, usually within a minute of the first drops. A rooflight with a motor but no sensor stays exactly where you left it. So the useful question to ask at quote stage is whether the sensor is included and how it is wired.
A motor opens the rooflight, a sensor decides when to shut it
Two separate parts do two separate jobs, and people often assume one gives them both.
The motor is the part that moves the glazed sash. It is commanded by a switch, a remote, a keypad or an app. If nothing gives it a command, it does nothing. A rooflight that opens to air the kitchen at breakfast and is left ajar when you leave for work will still be ajar at noon, whatever the sky is doing.
The rain sensor is a small flat plate with a sensing surface, often with a fine grid of conductive tracks. Water bridges the tracks and changes the signal. The control reads that change as rain and sends a close command to the motor. In many models the plate also carries a tiny heating element, so that it dries quickly once the shower ends and so that morning dew or frost does not give a false alarm.
Some manufacturers build the sensor into the rooflight frame. Others supply it as a separate plate fixed to the roof close by, wired back to the control unit. Either way it needs to be fitted in a place that is truly exposed to the rain the glass receives, not tucked in the lee of a chimney. Our page on rain sensors on electric rooflights explains where it is best placed and how it is powered.
What happens between the first drop and a closed sash
The sequence is short, and it is worth knowing what you would see and hear.
- Drops land on the sensor plate and the moisture reading crosses its threshold.
- The control unit overrides any open command you gave earlier and starts the motor in the closing direction.
- The sash travels to the closed position, where the seals compress against the frame.
- The motor stops itself, either on a limit switch or when it senses the resistance of a fully closed unit.
The travel takes some seconds on a small unit and longer on a large sash, because the motor has a fixed stroke rate. There is a short gap between a light spit of rain and a closed rooflight. A sensor is set to react to a first few drops, but a sash that is fully open, in a sudden cloudburst, can let in a little water before it seals. So we look at what lies beneath an opening rooflight at the survey, and a timber floor directly under it is worth a thought.
The sound is modest. A well-fitted motor hums for the duration of the stroke, then the seals meet with a soft tap. The wider question of the sound an opening makes is covered in are electric rooflights noisy when they open and close.
Checking what you would actually be buying
Not every electric rooflight on the market is sold with a sensor, and the wording on a brochure can blur the difference. These are the options in plain terms.
| System | Closes in rain? | Note |
|---|---|---|
| Motor with switch only | No | Shuts when you tell it to |
| Motor with rain sensor | Yes | Needs a live supply to act |
| Solar powered with sensor | Yes | Runs from a battery charged by the sun |
| Manual opening rooflight | No | Shut by hand |
A sensor is usually an optional extra rather than part of the base price, so ask for it by name. On a rooflight over a bedroom, a kitchen or a stairwell it earns its place. A rooflight over a sheltered porch that you operate only when you are at home may not need one.
When you are out, and when the power is off
The sensor is most valuable when nobody is at home, and that is also when its limits matter.
If the house is empty and it begins to rain, a rooflight with a working sensor and a live supply closes itself. That is the main reason people choose one. It covers the quick afternoon shower that you did not see coming.
A sensor needs power to act, just as the motor does. If the supply is cut when the rooflight is open, it stays open until power returns, or until you use the manual override fitted to the unit. We describe how that works on the page about what happens to an electric rooflight in a power cut. Battery backup and solar-powered units reduce that gap, and solar powered opening rooflights are the simplest case, since they run from their own cell.
The habit that goes with a sensor is to close the rooflight yourself for a long absence, such as a holiday. The sensor is for the surprise shower, not for a fortnight away.
VELUX make a roof window with this built in: the VELUX INTEGRA electric roof windows work on the same principle, and the sensor details are covered in what the VELUX rain sensor does.
Short questions about rain and opening rooflights
Can the sensor be set to react only to heavy rain?
The sensitivity depends on the make. Some controls have an adjustable threshold and others react to the first drop. We choose the control for the room during the survey, and it is noted in the quote.
Will it close if I am using the rooflight to let steam out?
Yes. The rain command overrides your earlier open command, so the rooflight closes even if you opened it to clear the kitchen. Extractor ventilation is a sensible partner for a sensor.
Does the sensor work in snow?
Snow melts on a heated sensor and registers as moisture, so it closes. On an unheated plate it may not register until thaw.
Can a manual rooflight be given a sensor later?
Only if it is first motorised. Our page on adding a motor to a manual opening rooflight explains the steps.
If you want an opening rooflight that looks after itself in a shower, get a quote and we will include the sensor in the spec. The wider range is on our electric and opening rooflight installation page.