What happens to an electric rooflight in a power cut?
Nothing moves. The sash stays exactly where the power left it: closed if it was closed, open if it was open, and part way if the supply went during a cycle. An electric rooflight has no spring in it and no stored energy in the drive, so a cut does not cause it to fall shut or drift open. That is the correct behaviour for a hole in a roof, and it is designed in rather than incidental. What people actually want to know is the follow-on question, which is what happens if it was open and the weather turns while the power is off. That depends on how the unit is powered and what was specified. The deeper treatment of supplies and standby arrangements is on the electric rooflight installation page.
The sash holds its position, and holds it hard
A chain actuator is a geared motor driving a rigid link chain, and the gearbox between the motor and the chain is what keeps the sash still. The reduction ratio is high enough that the drive cannot be back-driven: force applied to the chain from outside, whether that is wind under an open sash or somebody leaning on it, does not turn the motor backwards. The mechanism is self locking under load by virtue of its own geometry.
A spindle actuator on a kerb mounted unit behaves the same way for the same reason. The threaded rod turning inside its tube has a lead angle low enough that axial load on the rod will not spin the thread. So a flat roof rooflight lifted 350mm at its leading edge on a Severn Vale extension stays lifted 350mm through a cut, a gale and the following morning, and the glass is carried by the actuator rather than balanced on it.
A closed unit stays weathertight without any power at all
The important half of the answer is that a closed electric rooflight is not held shut by electricity. Once the closing cycle has finished, the motorised locking bar has driven home mechanically and the sash is compressed against its gaskets by a physical latch. Cutting the power does not release it.
This is worth being clear about because it is the scenario that worries people most: a February storm across the wolds, the lights go out, and there is a large piece of glass in the roof above the bed. That unit is as sealed during the cut as it was before it, and it will be as sealed at the end of the week if the supply is still off. Weathertightness on a rooflight comes from the gaskets, the drainage channels in the frame and the perimeter detailing around it, and none of those four things has a wire attached.
Solar units carry their own supply and keep working
A solar rooflight is unaffected by a house power cut, because the house was never powering it. The photovoltaic panel on the outside of the top rail charges a battery inside the sash, and the actuator, the rain sensor and the radio receiver all run from that battery. During a cut the unit opens, closes and responds to rain exactly as it does on any other day.
That is a genuine argument for solar in some positions, and it applies most in the parts of the county where cuts are least rare. Overhead supplies through the Forest of Dean and the wolds villages take tree damage in a south westerly, and the rural runs out towards Newent and the Leadon valley are exposed in the same way. A unit over a stairwell in a house on one of those supplies is one that carries on regardless if it is solar. Battery behaviour and charging in winter are set out on the energy efficient rooflights page and the hub.
The rain sensor, and the unit that was open when the lights went out
On a mains unit the sensor is the thing the cut actually takes away. The pad on the outside of the sash works by detecting water bridging two conductive tracks, and it needs the control electronics live to act on that. With no supply, an open unit stays open through rain.
How much that matters depends on the position. An open roof window over a landing in a Hucclecote semi, with a shower arriving, means water on the reveal and the floor below and nothing worse. An open kerb mounted rooflight lifted 350mm over a kitchen island in Cheltenham is a much larger aperture pointing straight up, and heavy rain through it is a genuine mess. The practical answer is habit rather than hardware: units are closed before leaving the house or going to bed if the forecast is unsettled, which is the same discipline a manual unit needs and which most people already have.
Standby arrangements, and what they actually buy
Some manufacturers offer a battery backup accessory for their mains ranges. It is a small rechargeable pack wired into the control system that holds enough charge to run a limited number of closing cycles after the supply goes.
Be clear about what that is for. It is not a way of continuing to use the window through an outage. It is one job, done once: closing an open sash when the power fails, so that a rooflight left open at eleven in the morning is shut by the time the storm that caused the outage arrives over it. Where a unit is large, high and over a finished room, that is worth having. Where it is a modest roof window in a bedroom that gets closed by hand each night anyway, it is an accessory that will sit there for a decade. The choice is made at survey against the position, and the deeper detail on standby options is covered separately in the guides.
Escape windows never depend on a motor
Where a roof window forms the means of escape from a loft room, the regulations do not permit the escape function to rely on power, and manufacturers build those units accordingly. An escape rated window opens by hand, from inside, in the dark, with no supply, using a handle and a stay. A motor can be added to one, and units combining both are made, but the manual function is the primary duty and the actuator is an addition to it.
So a cut has no bearing on getting out. The dimensional requirements are the ones that matter instead: a clear openable area of at least 0.33 square metres with a minimum of 450mm in both height and width, and the bottom of that opening between 800 and 1100mm above the finished floor. Those are checked at survey before anything is ordered, which is where the common and expensive mistake gets caught.
When the supply comes back
Nothing dramatic happens. The control board powers up, re-establishes its radio link with the paired remotes and wall pad, and waits. The sash does not move on its own, which is the behaviour you want: a window that closed itself the instant the power returned would be an alarming thing to have above a stairwell.
Two things are worth knowing. If the unit was interrupted mid-cycle, the first command afterwards will usually complete the movement it was making rather than starting a fresh one, so press the button you actually want rather than assuming. And where a gateway is fitted for phone control, that box has to reconnect to the router before app control returns, which typically takes a few minutes longer than the window itself. Neither affects the remote in your hand, which talks to the unit directly.
Specifying against it before the order goes in
All of this is a survey conversation rather than a problem to solve afterwards. Three questions settle it.
- How exposed is the supply. Overhead lines through the Forest, the wolds and the rural vale go down more often than an underground urban feed in Gloucester or Quedgeley, and that shifts the case for solar or for a standby pack.
- What is under the unit. A landing with a hard floor tolerates an open sash in rain. A kitchen island, a stairwell over a carpeted hall, or a bedroom does not.
- Is the position reachable at all. Where nobody can get to the sash by hand, the automatic closing behaviour is the only closing behaviour there is, and that is the case for either solar power or a backup arrangement.
Those three answers, taken with the room and the roof, decide the specification. Email info@severnskylightsgloucester.co.uk with photographs of the roof and the room and the discussion starts from the actual position rather than a general one.
Next on this subject.
- Question Can rooflights be opened electrically? Yes. Almost every category of glazed unit that goes into a roof is available with a motor in it, and…
- Question How much does an electric rooflight cost? No figure is published on this site for an electrically operated rooflight, and the reason is that the motor is…
- Question Do electric rooflights need an electrician? A mains powered rooflight does. A solar one does not, because nothing in it is connected to the house supply…
- Question Can you convert a manual rooflight to electric? Often yes, and the answer is decided by the model rather than by how much effort anybody is willing to…