What is a chain actuator?
A chain actuator is a motor and a folding steel chain in a slim aluminium casing, mounted along the head of a rooflight frame, which pushes the opening pane away from the frame and pulls it back. The trick that makes it work is that the chain is hinged in one direction only. Coming out of the casing the links lock straight against each other and behave as a rigid rod. Going back in they fold and coil into a housing a few centimetres deep. That single property is why a device the size of a curtain rail can lift a heavy glazed sash.
It is the most common mechanism on domestic opening rooflights, and if a unit is described as electric without further detail, this is usually what is on it. The alternatives and where each belongs are set out on the electric and opening rooflight page.
What is inside the casing
Four components in a run of extruded aluminium. A small direct current motor, almost always 24V, sits at one end. It drives a reduction gearbox, because the motor spins fast and produces very little torque on its own and the job needs the opposite. The gearbox turns a sprocket, and the sprocket engages the chain.
The chain itself is the interesting part. It is made of interleaved steel plates pinned together, shaped so that each link stops hard against the next when the chain straightens. Coiled inside the casing it occupies almost no space. Extended out of the aperture it forms a straight beam capable of carrying compression as well as tension, and the last link carries a bracket that pins to the sash.
Why a chain that folds one way can push
An ordinary bicycle chain is useless in compression: push on one end and it buckles. A push chain is machined so that the link plates have a flat shoulder on one face. Fold it one way and the shoulders open. Fold it the other and they meet and stop, and the chain becomes a column.
The consequence for the design is that the housing does not need to be as long as the stroke. A 400mm travel needs a casing barely longer than the width of the motor and the coil, which is what allows the whole assembly to be hidden inside a frame profile of a few centimetres. A linear ram achieving the same travel needs a body nearly as long as the stroke, which is why rams end up mounted at the side of a kerb rather than concealed.
Thrust and pull, and the numbers that matter
Actuators are rated in newtons, and domestic rooflight chain drives sit broadly in the 250 to 300 newton region, with heavier duty units going further. Push and pull ratings are usually similar but not always identical, and both are relevant: push lifts the sash against gravity and wind, and pull is what compresses the gasket at the end of the closing travel.
Sizing works from the weight of the glazed sash and the geometry of the hinge. A sash hinged at the top and pushed near the bottom rail gives the actuator good leverage. Pushing closer to the hinge multiplies the force needed. This is why the same 300 newton unit is ample on one window and marginal on another of similar weight, and why the honest specification names the unit model rather than just saying an actuator is included.
Stroke, and how the travel is set
Stroke is the length of chain the unit will push out, and it is what decides how far the pane opens. Common domestic strokes are 200mm, 300mm and 400mm, and many units are adjustable within their range by a switch or a control setting rather than being fixed at manufacture.
The relationship between stroke and opening angle depends on the depth of the sash. On a one metre deep pane, 200mm of chain gives roughly eleven degrees and 400mm gives about twenty three. Both are described as opening in a brochure, and they behave nothing alike: the first is background ventilation, the second is a genuine purge that clears a kitchen in minutes. Longer stroke also lifts the trailing edge higher into any wind crossing the roof, so an exposed site often wants the shorter setting.
Where the casing sits, and what you see
On a factory-built electric rooflight the actuator is inside the frame head, hidden behind a cover, and the only visible parts are the chain when the unit is open and the small bracket on the sash. On a retrofit installation the casing is surface mounted to the frame, which is entirely serviceable but visible from inside the room.
Orientation is decided by the unit, not by preference. On a top hung flat roof rooflight the actuator runs along the fixed frame at the hinge-opposite edge and pushes the sash up and out. On a centre pivot roof window in a pitched slope it usually sits at the head and drives the sash to rotate. Either way, the low voltage cable arrives at the head of the unit, which is why the cable route to that point is the decision that has to be made before a ceiling closes.
Two actuators working as one
Beyond about a metre and a quarter of sash width, one chain pushing at the middle starts to twist the pane, because the frame is not infinitely stiff and the corners lag behind the centre. The answer is two actuators mounted at the third points, driven by a synchronisation module that keeps them within a few millimetres of each other.
Synchronisation is not optional on a wide sash. Two independent drives on the same pane will drift apart over hundreds of cycles, and a pane held open by one unit further than the other is racking its own frame and its hinges every time it closes. The module is a small item in the control cupboard and it is the difference between a wide opener that stays square and one that does not.
Holding the sash shut, and holding it open
A chain actuator does its own locking in both positions. Extended, the straightened chain resists the sash being pushed further open by wind, and the gearbox resists it being pulled closed. Retracted, the same rigidity holds the pane down onto its gasket, which is why an electric rooflight generally needs no separate espagnolette or catch.
That holding force is finite and it is stated by the manufacturer. On an exposed roof, and much of the Essex coastal fringe qualifies, the wind load on an open square metre of glass at twenty three degrees is a serious number. Where that is the site, the sensible specification is a shorter stroke, a higher rated drive, or a mechanism that does not lift into the airflow at all.
Power, current and the cable that feeds it
Almost every domestic chain actuator runs at 24V direct current, drawing something like half an amp to one and a half amps while travelling and nothing at rest. The mains side stops at a transformer or a control unit, usually sited in a cupboard or a roof void, and everything past that point is extra-low voltage.
Two practical consequences follow. Volt drop over a long low voltage run is real, so the cable is sized for the distance rather than assumed, and a unit fed by a cable that is too thin over twenty metres will run slowly and close weakly. And a control unit has a stated total current output, which is what caps how many actuators can share it. The mains work and its certification are covered on the electrician page.
Where a chain is the wrong tool
Chain drives run out of road in two places. On a very heavy sash, typically a large triple glazed unit on a flat roof, the required thrust exceeds what a slim chain casing can deliver and a spindle drive is the correct component. And where the geometry gives poor leverage, a chain that would be ample elsewhere ends up labouring every cycle.
The diagnostic is straightforward and it happens at survey rather than on the day. Establish the glazed sash weight from the final glazing specification, take the hinge geometry from the unit, and check the required force against the rating with margin. That single calculation is what stands between a mechanism that runs for twenty years and one that groans at the end of every closing cycle from its first winter.
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