How do electric rooflights work? The parts, in the order they move

Everything starts with a command, and the command is only a small electrical signal. The switch on the wall does not carry the power that moves the glass. It tells a controller what you want.

The wall switch, handset or keypad is a low-effort device. It closes a circuit, or sends a coded radio message, that says open, close or stop. Inside the room this feels like turning on a light. Behind the plaster it is closer to a conversation between two small electronic parts.

A rocker or push-button wall switch usually has three positions in effect: open, stop and close. Hold it and the sash travels. Release it and the sash stops where it is, so you can leave a rooflight cracked open by a hand's width on a warm evening and fully open at midday. A wireless handset does the same job by radio, and a keypad or app adds a few refinements on top. The choices between them are laid out in our page on switches, remotes and keypads for electric rooflights, so here we only need the principle: the signal is small, and the controller decides what to do with it.

That small signal is a safety feature in its own right. Because the switch side of the system runs at a low voltage, a wall plate near a bath or in a child's bedroom carries far less risk than a mains-fed control would. The heavy lifting happens elsewhere.

Between the switch and the motor sits a box. It is the brain of the installation, and it does three jobs: it converts power, it obeys commands, and it keeps the motor inside safe limits.

The box takes mains electricity from a fused supply and steps it down. Electric rooflights often run their motor on a low-voltage direct current, commonly 24 volts, because a low-voltage motor is compact, controllable and gentle to have near a ceiling. The control unit does the conversion. Some products instead carry a mains-voltage motor with the control built in. Both arrangements end in the same place: a motor that turns when told to and stops when told to.

The controller also listens for other inputs. A rain sensor, a wind or temperature trigger or a smoke ventilation signal can all be wired in, and the controller decides which command wins. Usually the answer is that a safety input beats a comfort input. Rain outranks the switch when the glass is open, and a fire alarm signal outranks everything. We go through the usual rain input in rain sensors on electric rooflights.

Where the box lives matters more than people expect. It needs to be reachable for the electrician and for any later adjustment, and it needs a proper cable route to both the supply and the rooflight. That is a question of wiring rather than of movement, and it has its own page on wiring and powering an electric rooflight.

The motor is the part people picture as noisy, and it is also the part with the least to do. It spins. That is all. Everything else in the actuator exists to turn fast, weak spinning into slow, strong pushing.

A small electric motor produces a lot of rotation and very little force. A rooflight sash, with its double or triple glazing, weighs far more than a person can comfortably lift with one hand, so the motor needs help. The help is a reduction gear, usually a worm gear or a short train of gears, sitting directly on the motor shaft. Each turn of the output shaft takes a long run of motor turns. The speed falls, the force rises, and the sash can be lifted slowly and steadily.

A worm gear has a useful side effect. It is very hard to drive backwards, because the threads lock against one another when the load pushes on them. That means a sash held open by a worm-driven chain does not slide shut under its own weight or when the wind leans on it. The mechanism holds position without the motor drawing power, which is why a rooflight can sit half open all afternoon without a hum.

The gearbox and motor are sealed in a slim housing so that dust and damp from a loft or a kitchen stay out. In some systems the housing is hidden inside the frame of the rooflight. In others it is a visible but slender body fixed to the upstand or the kerb on one side, with the chain emerging from its end.

The component people associate with electric rooflights is the chain. It is worth understanding, because it looks like a bicycle chain and does not behave like one.

A chain actuator uses a stiff, interlocking chain that can be pushed as well as pulled. Its links are shaped so that, as they leave the housing, they lock into a rigid column. The output gear feeds the chain out of one end, the links straighten and lock together, and the free end pushes against the sash and lifts it. When the motor reverses, the chain is drawn back into the housing and the links fold away, pulling the sash down with it. Where the rooflight hinges, the chain end is fixed to the moving part through a small bracket with a pivot, so that the push follows the arc of the sash.

A spindle or rack actuator does the same job with a threaded rod or a toothed bar. The pattern is identical: rotation in, straight-line force out. Chain versions are common on hinged rooflights because they are compact and because the chain can be tucked away when closed. Spindle and rack versions appear more often on larger or sliding units, where a longer, steadier stroke matters.

The stroke is the distance the chain moves. It decides how far the sash opens, and it is fixed when the product is specified. A stroke that suits a bathroom may be too short for a stack-effect opening over a stairwell. We set it at the survey, by looking at what the opening is meant to do.

A motor with no idea when to stop would drive the sash past its hinge. Two devices prevent that, and between them they also protect your fingers and the glass.

The first is the limit switch. Inside the actuator, a small switch is tripped at the fully open and the fully closed position. When the chain reaches either end of its travel the switch cuts the motor, so the sash finishes its journey without ramming into a stop. On modern units the limit is set by counting the turns of the motor shaft rather than by a physical switch. The effect is the same, and the benefit is that the position is repeatable from one day to the next.

The second is the load sensor. The controller watches how much current the motor draws. A sash moving freely draws a steady, modest amount. If something blocks the movement, such as a branch, a cord or a hand, the current rises sharply, and the controller stops the motor or reverses it. This is sometimes called overload protection, and it is why an electric rooflight will not crush what sits in its way. It also means the closing force is set with care, because a gasket needs firm pressure to seal and a hand needs very little.

The path between open and closed is not one straight lift. A hinged sash swings on an arc, and the chain adapts to it. A sliding rooflight glides along a track. In each case the motor does the same work. It turns, the gearbox reduces, the chain or spindle pushes, and the limits catch the ends of the travel. The difference between hinged and sliding lies in the frame, which we compare in hinged or sliding: choosing an opening rooflight.

A rooflight is watertight only because of one thin strip of rubber. It is worth slowing right down to watch the last few millimetres of the close, because that is where the motor earns its keep.

As the sash nears the frame, the chain is still pulling. The sash meets the gasket and begins to compress it. The motor keeps drawing until the compression is correct, and the current rises a little as it does so. The controller reads that rise as the signal that the glass is home and stops the motor. On some systems a set of locking points, small hooks or bolts that grip the frame in several places, engages at the same moment, so the sash is pulled tight along its whole edge rather than just where the chain meets it.

That evenness is the point. A gasket that is squeezed hard in one corner and barely touched in another lets water and air through the loose spot. Multi-point locking, a true and square frame and a chain with the right stroke all work toward a single result, a gasket compressed evenly all the way round. The second seal, a drip or a weather bar on the outer edge, keeps rain running off the glass rather than creeping in behind it.

For the room below, the seal has a second job. A gasket that closes well is also the barrier against wind whistle and against the fine buzz of rain on the outer frame. A tired gasket is the quickest way for a perfectly good rooflight to start sounding cheap. That is why the seal is one of the three things set out in every quiet spec, next to the glazing build and the upstand.

The controller is the only part of the system that can respond to rain while you are out. That is why a rain sensor is a sensible companion to a rooflight that is left open.

A rain sensor is a small plate, usually mounted on the roof or at the edge of the glass, that changes its electrical behaviour when a drop lands on it. A heater in the plate dries it again once the rain stops. When the plate reports wet, the controller overrides whatever the switch last asked for and sends the close command. The sash runs to its closed limit, the gasket seals, and the room stays dry. Once the plate is dry the system either waits for your next command or reopens, depending on how it is set up. Our answer page, do electric rooflights close by themselves when it rains?, takes that question in more detail.

Wind and temperature inputs work on the same principle. A wind trigger can close the sash before a gust loads it, and a temperature input can open it on a hot afternoon. Each is a sensor, a signal and a rule the controller follows. None changes how the motor and chain move. They only change who gives the order.

A motor needs electricity, and mains supplies fail. Good electric rooflights are designed around that fact rather than hoping it never comes up.

Because the worm gear locks, a rooflight that loses power simply stays where it is. If it was closed, it remains closed and sealed. If it was open, it remains open, which is where a rain sensor linked to a backup battery becomes more than a convenience. Some products offer a manual override, a crank socket or a release that lets you wind the chain in by hand, and some systems can be fitted with a small battery pack that finishes a close command after the mains drops. We set out the practical options in what happens to an electric rooflight in a power cut.

The safest habit is also the simplest. Before a long absence, or before a storm, close the sash. The motor holds position, and closed is the state in which no one has to be home for the rooflight to behave.

The brand we work under is built around quiet, so this is the part of the mechanism we pay closest attention to. An electric rooflight makes a small amount of sound when it moves, and almost none when it sits.

Moving, a quality actuator gives a soft, low hum from the motor, and a faint tick as the chain links engage and release. In a bedroom at night the sound is less than a blind being drawn. It is also brief, since a typical stroke takes under a minute. What you should not hear is grinding, a rhythmic clunk or a rattle from the frame as the sash passes through its arc. Those point to a misaligned chain bracket, a frame that is out of square or a fixing that has loosened. They are installation matters, and the place to address them is the fit. Our page on whether electric rooflights are noisy when they open and close goes through what is normal.

Sitting still, the concern changes. The rooflight is no longer a machine but a hole in the roof with a seal in it. The sounds that matter then are rain on the glass and the outside world passing through it. Here the glazing build counts for more than the motor does. A laminated acoustic inner pane, a sensible gap between the panes and a well-compressed gasket do the work, and we explain the choice in can an electric rooflight have acoustic laminated glass. The aircraft on the Gatwick routes, the M23 and the Brighton main line are all heard through the glass and the seal, never through the motor.

A motor and chain can only lift what the frame is built to be lifted. The parts around the mechanism decide how well it will live.

An opening sash is heavier than a fixed pane of the same size, because it carries its own frame, hinges and locking hardware as well as the glass. A laminated acoustic build adds weight again. The chain and gearbox are chosen with that total in mind, and the frame has to be stiff enough that the sash does not twist as it travels. A sash that flexes leaves one end of the gasket unsealed, however powerful the motor.

The upstand, the raised kerb the rooflight sits on, plays its part too. It has to be square, level and rigid, because the actuator pushes against it and the gasket closes onto it. An opening unit may need a little extra height so that the sash clears the roof finish as it swings, a point we set out in whether an opening rooflight needs a taller upstand. The upstand is also insulated and sealed to the roof covering, so that the mechanism above it stays dry.

The mechanism asks for very little. It does not ask for nothing, and a few habits keep it moving smoothly for years.

The chain and any visible track should be kept clear of leaves, grit and cobwebs. A dry cloth across the gasket now and then removes the grit that would otherwise scratch it. Hinges and locking points can take a thin smear of suitable lubricant once a year, applied as the manufacturer recommends, never poured on. The motor itself has a finite life, measured in cycles of open and close rather than in years, which is why daily use wears it differently from occasional use. How long an electric rooflight motor lasts explains the factors involved. When a motor reaches the end of its life, we renew the actuator and keep the glass.

If you already have a manual rooflight and like the sound of the above, the same principle can often be applied to it. Motorising a manual opening rooflight depends on the frame, the hinge and the position of the handle, and it is something we judge on site.

Once you know the parts, a quote or a data sheet stops being a wall of jargon. A few lines are worth picking out.

• The supply voltage tells you whether the motor runs on low-voltage direct current behind a control unit or on mains directly.
• The stroke tells you how far the sash opens, which decides the ventilation it gives.
• The locking points tell you how evenly the gasket will be compressed when closed.
• The override tells you what you can do by hand when the power is off.
• The inputs tell you whether rain, wind or alarm signals can be added later.

None of these is a reason to be intimidated. They are the same five ideas as the ones above, written in a trade shorthand. If a description leaves one out, ask. A written quote from us names each of them for the product proposed, together with the glazing and the seals in the quiet spec. To begin with a survey, use the quote form on this page.

These are the small queries that come up once the mechanism has been explained, each answered in a few lines.

Does the motor run all the time the rooflight is open?

No. The motor runs only while the sash is travelling. Once it reaches the requested position, the controller cuts the power, and the locking action of the gear holds the sash where it is.

Can the chain be seen when the rooflight is closed?

Usually not. On a chain actuator the links fold back into the housing as the sash closes, so the closed rooflight looks like a plain unit. On some frames a slim casing stays visible along one edge.

Will the rooflight stop if something is in the way?

Yes. The controller watches the current the motor draws. A blocked sash makes that current rise, and the controller stops or reverses the motor before the obstruction is crushed.

What decides how quickly it opens?

The reduction gear. A higher ratio gives a slower and stronger movement, a lower ratio gives a faster and lighter one. We choose the ratio for the weight of the sash, since a heavy acoustic build wants the slow, steady version.

Is an electric rooflight worth having on a fixed pane of glass?

The mechanism needs an opening sash, so a fixed pane has nothing to move. Whether to swap a fixed unit for one that opens is a question of the frame and the upstand, which we assess at the survey.