The short answer
In a power cut, a mains-powered electric rooflight stops responding to its controls and stays wherever it was. If it was shut, it stays shut. If it was open, it stays open unless the model has a battery or a manual way to move it. What you can do depends on the product, so it is worth knowing before you choose one, not on the night the lights go out.
Why it stays where it is
An electric rooflight uses a motor or actuator that moves the sash and holds it in place. When the supply drops, the motor has nothing to drive it. The gearing inside it is built to hold the window at the angle it was left, so it does not creep. That is deliberate: a window that drifted shut or wandered open under its own weight would be a nuisance and a safety issue.
The same is true of the sensors. A rain sensor needs power to tell the window to close. If the window was open when the power failed and the weather turns, the sensor cannot act. This is the main practical risk, and it is why some households would rather close the window before they go out on a day when rain is forecast and the supply is uncertain.
Ways round it
Manufacturers deal with this in different ways, and the options vary between ranges. Check each one against the product you are being quoted.
- Battery backup
- Some control units carry a small battery that lets the window close, or move to a safe position, when the mains drops. Ask how long the battery lasts, and whether it only closes the window or can open it too. Batteries wear out, so ask how it is replaced.
- Manual release
- Some operators can be disengaged and the sash moved by hand, using a pole, a handle or a release key. Others cannot, and the sash only moves when the motor does. This feature is worth asking about by name.
- Solar-powered units
- A window that runs from a panel on the unit and a battery does not rely on the house supply, so a mains failure does not affect it. It can still run flat after a long run of dull days, so ask about the capacity. We describe the idea in our guide to solar-powered rooflights and windows.
- Low-voltage systems
- A transformer takes mains power down to a low voltage for the motor. A cut at the mains still stops the transformer, but a battery pack can sometimes be wired into the same circuit. It is a design question for the quote, not an afterthought.
What it means for ventilation and safety
If a rooflight is part of how a room is ventilated, a power cut leaves it fixed in position. Closed, it is safe against the weather but ventilates nothing. Open, the room gets air but is exposed to rain. Neither is a danger in itself, but a room that depends on the window for air, such as a kitchen or a bathroom, deserves another route for ventilation, which an extract fan or a trickle vent can provide.
Loft rooms add a further point. A roof window in a bedroom can be an escape route in a fire, when it meets the size and position rules in Approved Document B. An escape window needs to be openable by hand without relying on power. If an electric window is to serve in that role, check that it can be opened manually and that the opening is of the right size, and say so at the quote stage.
Choosing a unit with a power cut in mind
When you compare electric rooflights, put three questions to every supplier.
- Can the window be moved by hand, with the power off, and how?
- Does it have a battery, and what does it do when the mains fails?
- What happens to the rain sensor when the power drops?
Written answers are better than verbal ones. We note the product's behaviour in a power cut in the quote, so you are not discovering it on the day. Some people decide that an electric rooflight over a landing is worth the trade-off, since the alternative is a manual one they cannot reach. Others choose a manual window with a long pole for a simple, power-independent opening. The comparison is laid out in electric or manual opening rooflights.
Long outages and storms
Most power cuts are short, and a window that holds its position for an hour does no harm. Longer outages are more likely in winter storms, when wind and rain arrive together. A rooflight left open in that weather is the case to plan for, so on a day with a storm warning, close the window and leave it closed. A closed unit with good seals gives the same protection whether or not the power is on.
Reducing the risk day to day
A few habits take most of the risk out of a power cut. Close the window when you leave the house for the day if rain is forecast. Know where the manual release is, if there is one, and keep any pole or key where you can find it. If the window has a battery, replace it on the schedule the maker gives. And if a long outage is likely, as can happen in a storm, close the window before the weather arrives, not after.
It is also worth testing the window with the power off at the consumer unit, once, on a dry day, so you know how it behaves. That takes a few minutes and can be done at handover.
What we put in writing
Where a rooflight is electric, we record the control, the power source and what the window does without power in the finish schedule and the quote. We also confirm where the supply is taken from and where any cable runs, since that affects the ceiling we have to make good.
If you want to ask about a particular model, ask for a quote and tell us which rooms the windows are for. Our page on electric opening rooflights covers the full range. The wiring side is answered in do electric rooflights need an electrician.