The short answer
A mains or low-voltage motorised rooflight with no power usually stays where it was. If it was shut when the supply dropped, it is still shut and still weathertight. If it was open, it remains open until power returns or you move it by hand. The part to plan for is the second case, because a rain sensor is only a switch that tells the motor to close, and a motor with no supply cannot act on it.
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Why an open unit stays open
The actuator is the motor and arm that lifts the sash. It moves only while it is fed, and it holds its position when the feed stops. Nothing springs back, so a vent left cracked open on a warm afternoon is still cracked open at midnight if a storm takes the street's supply out.
The rain sensor is the reason people worry. It is a small detector on the outside of the glass, and it works by sending a close signal to the controller. That signal needs the same supply as everything else. A sensor on a dead circuit can get as wet as it likes and nothing will happen. Rooflights that run from a battery pack or a small solar panel behave differently, because the battery carries on after the mains goes, and our answer on solar powered skylights covers where that suits.
The practical habit is simple. Before you go out or go to bed in unsettled weather, set the glass to closed or to a small gap. A rooflight left on a wide opening when you leave for the day carries the most risk, and it is worth making the closed position the default for the house.
Opening or closing it by hand
How a unit can be moved without power depends on its make and its drive, so the answer for yours sits on the specification we write into your quote. Some roof windows and flat-roof lids can be worked manually once the motor is disengaged or the control is bypassed. Others rely on a chain or spindle actuator that resists being forced, and pushing against it can strip the drive. Never lever at a motorised sash until you know which type you have.
On a landing or stairwell the glass is often well over head height, which turns a manual release into a job for a pole or a stepladder. We think about this when we choose the unit and where the controls go, because the best answer to a power cut is a release you can reach from the floor. A unit over a bathroom or utility room, where the glass is nearer, is easier.
At fitting, ask us what the unit does with no supply. It helps to write the answer on a card kept with the remote: where the release is, and who to call.
| Unit was | Mains power fails | What you can do |
|---|---|---|
| Shut | Stays shut, still weathertight | Nothing; it opens again when power returns |
| Open a crack | Stays open, rain sensor cannot close it | Close by hand if the unit allows, or cover the room below |
| Open wide | Stays wide open | Same as above, and check the room for rain on return |
| Battery or solar drive | Keeps working while the battery lasts | Use the controls as normal |
Choosing a unit that copes with an outage
A few choices at the survey reduce the problem. A battery back-up on the control side, where the system offers one, lets the glass close for a period after the mains drops. A low-voltage motor behind a transformer is no better in a cut, since the transformer still needs the house supply, but it does keep the wiring in the roof void at a gentler voltage. Where a room really cannot be left open, such as a bedroom under a large unit, we will talk about a fixed rooflight with a smaller opening vent elsewhere.
Position matters as much as hardware. A unit on a north-facing slope above a hall gets less weather driven against it than one facing the prevailing wind over a conservatory, and a sash that lifts at the top edge sheds rain differently from one that hinges at the side. Those details sit in the specification, so we look at the roof before we recommend anything.
The Building Control notification and the electrical work run in parallel for an opening like this. We handle the notification where the work is notifiable, and the person doing the fixed wiring works to the electrical rules for homes; our guide on whether an electric rooflight needs an electrician explains who does what.
Coming back after the cut
When power returns, a motorised rooflight does not usually move by itself. It sits at its last position until you or a sensor tell it otherwise. Check the controller first: some keypads and timer systems lose their programmed settings in an outage and need setting again, and a handheld remote may need its battery changed if the cut was long. Look up at the glass, and then at the ceiling and floor beneath it.
If rain came in, dry the area, and mention it when you are in touch. A short spell of water inside the room is a nuisance, but staining that reappears after the next heavy shower points at the roof junction, and that is a question for a survey. For wiring and control questions, see the main electric opening rooflights section, or start with a free survey and we will check how your unit behaves.
