What happens to an electric rooflight in a power cut?

Why the sash holds rather than drops

The mechanism is not holding the sash up against gravity in the way a prop does. A chain or spindle actuator drives a rigid element out of a housing and that element is self-locking: it resists being pushed back down the way it came. Remove the power and the drive simply stops turning, with the chain still extended and still taking the load.

That is deliberate design, and it is also why you cannot pull a powered sash closed by hand while it is extended. Forcing it is the one thing that genuinely damages the mechanism, because you are loading a gearbox backwards through a train that was never meant to be driven from that end.

The three positions the cut can catch you in

Closed is the easy case, and it is where the unit spends most of its life. Nothing happens. The gasket stays compressed, the unit stays weathertight, and when the supply returns you carry on.

Open is the case people worry about. The rooflight stays open, the room ventilates, and if it rains, rain comes in. How much depends on the pitch, the opening angle and the wind direction. On a shallow flat-roof vent the exposed area is small. On a steep pitched window at full travel it is not.

Mid-travel is the least common and the most awkward. The sash sits part open, the seal is not made, and the system will resume from that point when the supply comes back rather than needing a reset.

What happens the moment the supply returns

Most systems simply become responsive again. They do not run a homing cycle, they do not close automatically, and they do not restore a previous position. The unit stays where the cut left it until somebody presses something.

That is worth knowing, because a house that loses power at three in the morning and regains it at five will have a rooflight still open at breakfast. There are two exceptions. Where a rain sensor is fitted and the pad is wet when power returns, the system will usually drive the unit shut on its own. And where the rooflight sits in a smoke ventilation system, the control panel has its own battery and behaves to a different set of rules entirely.

Rain sensors during and after an outage

A rain sensor draws its power from the same control unit as the mechanism. In a cut it is dead, so the vent that would ordinarily have closed itself will not. This is the single practical argument for thinking about outages at all: the protection people rely on is the protection that goes first.

Once power returns the sensor is live again and will act on water sitting on the pad. It will also refuse to reopen the unit while the pad is still wet, which reads as a fault and is not one. Sensor behaviour in general is covered on whether electric rooflights close automatically in rain.

Manual override, and its honest limits

Some systems offer a way to disengage the mechanism at the frame and move the sash by hand. Where it exists it usually means releasing a clip or a pin on the actuator body at the head of the window, which restores free movement until you re-engage it.

The catch is obvious once you say it out loud. The rooflights that most justify a motor are the ones nobody can reach: a vaulted kitchen ceiling, a stairwell, a galleried landing. A manual override three and a half metres above a stair tread is not a plan. If reaching the unit in an outage genuinely matters to you, say so before the unit is specified, because it changes what should be specified. Whether a given unit can be worked by hand at all is set out on whether an electric rooflight can be opened manually.

Escape windows are a separate duty entirely

If a rooflight in a loft conversion is serving as the means of escape from that room, it has to be operable immediately, by hand, by whoever is in the room, with no power and no remote. A motor does not satisfy that requirement and no amount of backup arrangement changes it.

In practice that means an escape window keeps a proper manual handle at reachable height and a clear opening of the required size. You can have powered rooflights elsewhere in the same room. What you cannot do is let the motor become the way out.

Designing the outage out at specification stage

There are three routes, and the right one depends on the unit’s position rather than on how often your street loses power.

• Battery backup on the control unit. Available on several systems, it holds enough charge to drive the unit shut and is worth specifying over anything unreachable.
• A solar unit. Independent of the house supply by construction, so a household cut has no effect at all. Covered on whether solar powered rooflights are any good.
• Reachability. The cheapest answer of the three. A rooflight you can get a pole to is a rooflight an outage cannot strand.

The wider comparison of backup options sits on backup power and what happens in a cut.

What to do while the power is off

Very little, and that is the correct response. Leave the sash alone. If the room is getting wet, deal with the water below rather than with the window above: something absorbent on the floor and anything valuable moved clear. If the unit is over a stair or a landing, keep the area below it clear of foot traffic while it is unattended.

When power returns, cycle the unit once through a full close and a full open. That confirms the travel limits have not shifted and that the sash is seating evenly on its gasket, which is the thing that actually keeps water out. If it does not respond, check the remote battery and the isolator before assuming anything worse.

We install new rooflights and full replacements across Oxford and up to twenty five miles by road, with fifteen years in the trade and a ten-year workmanship guarantee, fitting VELUX, Fakro, Keylite and Roto units. Electrical connection is carried out by a qualified electrician. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.