What happens if the power fails?

An electric rooflight is opened by a small motor driving either a chain or a rigid spindle out of the frame. That mechanism is self-locking. The motor turns a worm gear, and a worm gear will drive the chain but the chain cannot drive it back the other way. So the glass is held wherever the motor last left it, and it takes powered rotation of that gear to shift it. Cut the supply and the gear simply stops turning. The weight of the sash does not force it open or pull it shut.

This matters because it means an outage is not a weather event on its own. People imagine an electric skylight going limp when the power dies, the way a powered gate might swing free. It does not. A closed rooflight seals against its gasket under the same pressure it always did, the locking points that the motor drove home stay home, and rain runs off it exactly as before. You could lose power for a week in the depth of winter and the closed units in the roof would not know the difference.

The one state that is not self-resolving is open. If the glass was standing open at the moment the supply failed, it stays standing open, because holding position is all the mechanism does. That is the situation worth planning for, and it is the subject of the next two chapters.

Most electric opening rooflights are fitted with a rain sensor, a small contact on the outside of the frame that tells the control unit to close the glass the moment it gets wet. It is one of the best reasons to have the electric version rather than a manual pole-operated one. But the sensor and the motor both run on the same supply. When the power is off, the sensor has nothing to send a signal to and nothing to drive the glass with, so an open unit will sit open through a shower it would normally have closed against.

In practice a short cut mid-afternoon on a dry day is a non-event. The risk is the overnight outage, or the one that coincides with a storm, where an open rooflight lets weather into the room below for hours before anyone notices. If a cut happens and you can see or reach the roof, the sensible first move is to close any open units by hand. If you cannot, moving anything precious out from directly under the opening buys you time.

There are two ways to take the mains out of the equation, and both are decided at specification, not afterwards.

The first is a solar-powered rooflight. Instead of a cable back to the consumer unit, it carries a small photovoltaic panel on the frame that trickle-charges an onboard battery, and the motor runs off that battery. A mains cut has no effect on it at all, because it was never drawing from the mains. It keeps opening, closing and responding to its rain sensor through a power failure and a dark winter afternoon alike, running the battery down and topping it back up when the light returns. For a rooflight in a difficult position, or one where you would rather not run a spur across the loft, it is often the tidier answer as well as the more resilient one.

The second is a backup battery on a mains unit. Some control systems accept a battery pack that carries the motor through an outage, and a number of them are wired to do one specific thing when they detect the mains has dropped: drive every open unit closed and then sit tight. That turns the worst case, glass left open in a storm, into a non-issue, because the system shuts the roof for you the instant the supply goes. It is worth asking for on any unit that will spend real time standing open.

Every mains-wired opening rooflight can be operated manually, and knowing how yours works before you need to is worth five minutes. The method depends on the make. Some use a hook on the end of a telescopic pole that engages a manual ring or eye on the sash. Some have a release that disengages the chain so the glass can be pushed shut and then re-latched. A few have a small crank or winder. The manufacturer’s manual for your exact unit shows the specific release, and it is worth keeping the pole or handle somewhere you can find it in the dark rather than at the back of the garage.

If a unit sits high over a stairwell or a double-height space and there is genuinely no way to reach it by hand, that is a reason to specify battery backup or a solar unit from the start, so the roof can look after itself. Retrofitting resilience to a rooflight you cannot reach is far harder than building it in.

Before an outage ever happens, find out which of your units open, where the manual override is and whether any of them tend to be left open unattended. If the answer is a rooflight that is regularly open and hard to reach, that is the one to put battery backup or a solar-powered motor on.

This question comes up most on south-facing rooms, and for a good local reason. Southend faces south across the estuary, and a great many rear extensions here run an electric opening rooflight specifically to vent the heat that builds up under south-facing glass on a summer afternoon. If that heat purge is the whole point of the unit, a power cut on the hottest day is exactly when you least want it stuck shut, which is a strong argument for a solar-powered opener that carries on regardless. We size and specify these as part of our electric skylight installation work, weighing the glass specification and the opening strategy together rather than in isolation. Solar gain is the problem we are built around, and how a room stays cool when the power drops is part of the same conversation, which we also cover in our energy-efficient skylight installation and flat roof skylight installation pages.

If you are planning new units and want the electric side to survive a power cut, tell us the position and how the room is used. Ring 01702 898232 or request a quote, and we will specify the opening, the backup and the glass together.