Guide · Electric and Opening Rooflights
Electric rooflight actuators and how they work
An electric rooflight is an ordinary rooflight with a small ram bolted to it. That ram is the actuator: a motor, a gearbox and a moving member that pushes the opening pane away from the frame and pulls it back down onto the seal. The switch, the sensor and the cable all exist to tell that one component when to run.
This page is about the component itself. Knowing what is inside the housing explains why one unit opens 200mm and another 600mm, why a wide pane takes two drives instead of one, and why a motorised unit is still watertight after ten years of daily cycling. The wider picture of electric and opening rooflights sits on the hub page; this is the mechanism in detail.
What sits inside the housing
Four things. A small direct current motor, in domestic units almost always 24 volt. A reduction gearbox, which is where nearly all of the engineering lives. A drive member, either a chain or a threaded spindle, that converts rotation into a straight push. And a control board carrying a current sensor, the wiring terminals and, on better units, a position encoder.
The motor on its own would be no use at all. It spins at a few thousand revolutions per minute and produces almost no usable torque. The gearbox trades that speed away in exchange for force, at a ratio in the order of several hundred to one, and the result is a member that creeps out at a walking pace far slower than walking and shifts a pane weighing thirty or forty kilogrammes without complaint.
Gearing, and why a small motor lifts a heavy pane
The reduction is usually done with a worm and wheel, sometimes with a worm followed by a spur train. A worm gear has an unusual property: it drives easily in one direction and resists being driven backwards. Turn the worm and the wheel moves. Push on the wheel and the worm does not turn, because the thread angle is too shallow for the load to overcome friction.
That property is the reason an open rooflight does not slam shut under its own weight the moment power is removed, and the reason a gust cannot force a closed pane open. The mechanism holds position with no power, no brake and no latch. It is a quiet piece of design that most owners never think about once.
Chain drives, and how a chain manages to push
A chain actuator is the common choice on roof windows and on smaller hinged flat units. The chain lives coiled inside a slim housing on the frame, and a sprocket feeds it out through a slot.
A bicycle chain would fold. This one cannot, because the links are shaped so that they hinge freely in one direction and butt hard against each other in the other. Once the chain leaves the housing it behaves as a rigid strut. The advantage is that the whole drive folds away into a housing barely thicker than a finger, so the actuator disappears against the frame when the unit is shut.
Spindle drives, and the units that need one
Where the pane is heavy, wide or triple glazed, a chain runs out of stiffness and a spindle takes over. A threaded rod turns inside a nut carried on a telescopic tube, and the tube extends. It is the same principle as a car jack, geared for speed rather than brute force.
Spindle drives are stiffer laterally, which matters on a large flat rooflight where the pane can twist as it lifts. They are also longer when retracted, because the thread has to exist somewhere, so they need a bay of clear frame to sit in. On a lantern with a hinged vent pane, a spindle drive is usually what is on the drawing.
Force ratings, and what a pane actually asks for
Actuators are rated in newtons, and the figure quoted is the force the drive can deliver at the end of its travel. Domestic chain actuators commonly sit in the 200 to 300 newton band. Spindle drives for heavy glazing run considerably higher.
Two separate demands set the number. Opening a pane means lifting part of its weight through a hinge, and the geometry is worst in the first few millimetres when the pane is still nearly flat. Closing it means compressing a gasket around the full perimeter until the pane sits home. On a large flat rooflight the compression demand is usually the larger of the two, which surprises people who assume weight is the whole story.
| Drive type | Typical stroke | Suits | Retracted footprint |
|---|---|---|---|
| Chain, single | Around 200 to 400mm | Roof windows, small hinged flat units | Slim housing on the frame |
| Chain, twinned | Around 200 to 400mm | Wide vents on a lantern ridge | Two housings, synchronised |
| Spindle | Around 150 to 600mm | Heavy or triple glazed flat panes | Long body, needs a clear bay |
| Concealed frame drive | Set by the unit | Factory motorised roof windows | Invisible, inside the sash head |
Stroke against opening angle
Stroke is the distance the drive member travels. Opening angle is how far the pane swings. They are related by the geometry of the bracket, not by a fixed conversion, and confusing them is behind a good many disappointed customers.
A 300mm stroke acting close to the hinge line produces a wide swing and a low mechanical advantage. The same stroke acting further out along the sash produces a smaller angle and far more force. Manufacturers publish a bracket position for each unit size for exactly this reason. Moving the bracket to suit a cable route quietly changes both the angle and the force available at the seal.
Speed, and the seconds a cycle takes
Slower than most people expect. A typical chain actuator extends at somewhere between ten and twenty millimetres per second, so a full opening cycle takes between fifteen and forty seconds depending on stroke.
That is deliberate, and it is not a limitation worth engineering away. Slow travel keeps the gear train quiet, keeps the current draw modest so the cable can stay small, and means the pane arrives on the gasket gently rather than slamming. It also gives a rain sensor time to be useful, because a shower that starts now is met by a pane that begins closing within a second or two and is home well before the roof is wet.
How the actuator knows where it has got to
Simple actuators know nothing. They run until a mechanical end stop is reached, the motor stalls, the current rises and the board cuts the supply. It works, and it is why some older units make a short buzz at the end of travel.
Better units carry a position sensor, usually a magnet on the motor shaft passing a hall effect device. The board counts pulses, so it always knows where the pane is in millimetres. That is what allows an intermediate position to be held, a favourite ventilation gap to be recalled, and a soft slowdown in the last stretch of travel before the gasket is met.
Current sensing, and the moment a motor stops
The control board watches how much current the motor is drawing, because current is a direct proxy for the load on the drive. A free running motor draws little. A motor pushing against something draws a great deal, and it does so within a fraction of a second.
Set a threshold and you have obstacle detection. If a closing pane meets a propped ladder, a bird’s nest or a hand on the frame, the current spikes, the board sees it and the drive stops or reverses. The threshold has to be tuned above the load of normal gasket compression and below anything that would hurt, which is a narrower window than it sounds and is one reason a factory motorised unit behaves better than a drive matched to a pane by guesswork.
A pane that stops short and reverses has usually found something, not failed.
Self-locking gears, and holding a pane in wind
Once open, a rooflight pane is a sail. On an exposed Essex roof a moderate south westerly puts a real load into the hinge line, and the actuator is what resists it.
The worm gearing does that passively. There is no solenoid holding anything and no power being consumed. The force path runs from the pane, through the sash bracket, up the drive member, into the gear train, where the worm thread angle simply refuses to be back driven. The practical limit is not the motor at all: it is the fixings holding the actuator to the frame and the bracket holding it to the sash, which is why those screws go into the frame reinforcement rather than into whatever timber is nearest.
The sash bracket, and where the load goes
The bracket is the least glamorous part of the assembly and the one that decides whether the installation lasts. It carries the entire push and pull of the drive into an aluminium or timber sash that was extruded with that load in mind.
Three things matter. It must sit at the published distance from the hinge, so the geometry is what the manufacturer calculated. It must be square to the frame, so the drive is not asked to push at an angle and wear one side of its bearing. And it must be fixed into structure rather than into a cover trim. On a factory motorised unit all three are settled in the works. On a drive fitted to a pane on site, they are decisions, which is the subject of adding an opener to an existing manual unit.
Cold mornings, grease and winter behaviour
Gearboxes are packed with grease, and grease gets thicker as it gets colder. On a January morning at minus three, an actuator that normally takes twenty seconds may take twenty five and sound slightly more laboured for the first cycle of the day.
That is normal and it settles as the mechanism works. What is not normal is a unit that stalls in the cold and runs fine at noon, which usually means the drive is already close to its force limit at room temperature and the extra drag has pushed it over. The fix is a correctly rated actuator rather than a lighter grease, and this is why the force calculation is done from the actual glazing weight rather than from the frame size.
A wide pane, two drives and keeping them in step
Beyond roughly a metre and a half of sash width, one drive at the centre starts to bow the pane rather than lift it evenly, and the gasket compresses in the middle before it compresses at the corners.
The answer is a pair of drives set in from each end, running from a controller that keeps them synchronised. Synchronisation is not optional. Two actuators of the same model do not run at identical speeds, and a few millimetres of difference over a full stroke will twist the sash and load the hinges unevenly. A proper controller either compares position feedback from both or drives them from a shared output designed for the pairing.
Which side of the seal the actuator lives on
Almost always the inside. The housing sits on the room side of the gasket line, so the mechanism spends its life in the same conditions as the ceiling below it and never sees driven rain.
That is why an actuator on a rooflight is not a weathering component and why a motorised unit is no more exposed than a fixed one. The only penetration is the cable entry, and on a factory unit that is made through the frame in the works, into a channel designed for it. Where a drive is external, and a few lantern vents are, it will carry an ingress protection rating and a drip loop in the cable, and it wants a wipe when the glass is cleaned.
What wears over ten years
Very little, if the drive was correctly sized. The wear points, in order, are the plastic bearing at the chain exit, the grease in the gear train, and the microswitch or encoder if one is fitted. The motor itself will typically outlast the sealed unit it is fitted to.
Symptoms of a tired drive are consistent: a cycle that takes noticeably longer than it used to, a pane that no longer pulls fully home so the gasket whistles in wind, or an actuator that runs its full stroke without the pane moving because the bracket screws have loosened. Checking the bracket is a two minute job and it is covered under keeping the mechanism working.
Essex exposure, and the roofs that work an actuator hardest
Not every roof asks the same of a drive. A vent pane on a sheltered rear extension in central Chelmsford, surrounded by other roofs, opens into still air. The same unit on a bungalow at Canvey, on reclaimed land with nothing between it and the estuary, opens into wind that arrives with salt in it.
Three Essex situations raise the specification. Exposed coastal and estuary positions around Canvey, Maldon and the Blackwater villages, where wind load on an open pane is the governing case and a higher force rating earns its keep. Tall extensions on the newer estates at Beaulieu Park and Great Notley, where the unit sits high and is operated daily for ventilation rather than occasionally. And weatherboard and timber frame properties around Stock and Coggeshall, where the sash bracket needs a proper structural fixing rather than a screw into a lining. On any of these, the drive is chosen from the glazing weight and the exposure, and the honest starting point is a look at the roof.
Keep reading
More on electric and opening rooflights
Wiring an electric rooflight at first fix
Read the guide →Solar powered rooflights without a mains run
Read the guide →Remote controls, wall switches and smart home links
Read the guide →Electric blinds fitted inside the rooflight
Read the guide →Opening rooflights for the stack effect
Read the guide →
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