What happens to an electric rooflight in a power cut?
Nothing moves. A mains-fed actuator with no supply holds the pane exactly where it was when the power went, and it stays there until either the supply returns or somebody releases the drive by hand. If the unit was shut, that is precisely the right outcome and the building is weathertight. If it was open, the rooflight is open until something changes.
That is the whole answer for a wired unit. Solar powered units are unaffected, because the cell in the frame is the supply. The wider subject sits on electric rooflight installation.
Why the pane holds position rather than falling shut
An actuator is not a spring under tension. Chain drives are rigid in compression and hold the sash out against gravity through the chain itself and the gearing behind it. Spindle drives are self-locking, because a threaded rod through a nut will not back-drive under load.
The practical consequence is that a power cut does not produce a sudden movement. There is no bang, the glass does not drop, and the unit does not creep shut over the next hour. Whatever position it was in when the supply stopped is where it will be found. That is a deliberate design property, because an uncontrolled closing pane overhead would be a far worse failure mode than an open one.
The case worth planning for is an open unit in bad weather
Power cuts and weather correlate. The storms that bring down overhead lines across the rural Essex network, around the Blackwater villages, the Rodings and out towards Thaxted, are the same storms you would want a roof opening shut for.
So the scenario to think about is not a quiet loss of supply on a still afternoon. It is a unit sitting at twenty degrees on a flat roof with wind rising and rain arriving, and no motor available to bring it down. On an exposed site that is a reason to favour a shorter stroke, or a sliding panel that does not lift into the airflow at all, rather than to rely on being at home when it happens.
Battery backup, and what it is actually sized for
Several control systems offer a backup cell that sits in or beside the control unit and holds enough charge to drive the unit through a small number of cycles with the mains off.
Read what it is rated for. Backup packs are generally sized for closing a unit a handful of times, not for running it normally for a week. That is the sensible design: the purpose is to get the roof shut and keep the escape function available, not to preserve the convenience. On systems where the rain sensor is powered from the same unit, backup also keeps the automatic close alive through the cut, which is the case where it earns its cost.
Solar units carry their own supply
A solar opener has a photovoltaic panel on the outside of the frame and a rechargeable cell inside it. The mains network is irrelevant to it. When the street loses power the unit keeps working exactly as before, including its rain sensor if it has one.
That is one of the quieter arguments for solar, and it is stronger than the usual one about avoiding a cable. On a rural supply with a history of interruptions, or on any unit that serves as a means of escape, a self-powered opener removes an entire failure mode. What solar delivers day to day is set out on whether solar powered rooflights are any good.
Escape rooflights, and what Building Control expects
Where a rooflight is the means of escape from a habitable room, usually a loft conversion, Part B sets requirements on the clear opening dimensions and on the height of the opening above the floor. It also expects the opening to be operable without power and without a key.
An electrically operated escape rooflight therefore needs a way of being opened by hand, and how the sash releases is part of the conversation with Building Control before the unit is ordered rather than after it is fitted. Chain drives usually release at the sash bracket with a hand tool. Spindle drives generally do not, because the thread is what carries the sash. The manual side of that is covered on opening an electric rooflight manually.
The sensors and controls go down with the supply
It is easy to think of the rain sensor as a separate safety device. It is not. On a wired system the sensor is powered from the same control unit as the motor, so when the mains stops the sensor stops with it.
Radio handsets and wireless wall plates run on their own cells and keep transmitting cheerfully, which can be misleading. The plate lights up, the command goes out, and nothing at the rooflight receives it because the receiver sits in the unpowered control unit. Anything running through a network gateway is doubly affected, since the router and the broadband line will usually be down as well.
What happens when the supply comes back
Most control units restore to a known state rather than resuming what they were doing. The pane stays where it is, the controller re-initialises, and the next command is obeyed normally.
Two behaviours are worth checking on the system being specified. Whether any scheduled automation resumes on its own, because a timed opening that fires at seven the next morning after an overnight cut is a surprise if nobody expected it. And whether the limits have to be reset, which on most modern units they do not, since the positions are held in memory rather than in a mechanical stop. Running a full cycle after a long cut is a sensible habit either way.
Specifying so a power cut is uneventful
Three choices make the whole question dull. Fit a rain sensor and a backup cell together, so the roof closes itself whether or not the supply is present. Keep at least one hard-wired wall plate on the system, because it is the control least likely to be the thing that has failed when something else has.
And choose the drive with the escape function in mind where the room needs one, so the hand release is a designed feature rather than a discovery. On a rural supply, or on a unit that will regularly be left open while the house is empty, solar removes the question entirely. Sunspire settles this at survey, and a range by email becomes a fixed written figure once those answers are known.
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