Electric rooflight actuators and how they work

The motor, the gearbox and the arm

Strip an actuator back and there are three parts in series. A small permanent magnet DC motor spins fast and produces very little torque. A reduction gearbox, usually a worm and wheel or a train of spur gears, trades that speed for a large increase in torque. The output of the gearbox then drives the arm, which converts rotation into a straight push.

The gear ratio is high, often in the region of 200:1 or more, which is why a motor the size of a marker pen can lift a glazed sash weighing forty kilogrammes. The worm gear also brings a second property that matters: it is self locking in the reverse direction, so pressure on the sash from wind cannot back drive the motor and force the unit open or shut. The mechanism holds position with no power applied at all.

Chain actuators, and the folding link that makes them work

The chain actuator is the commonest mechanism on manufactured roof windows, and it is a neat piece of engineering. The chain is not a bicycle chain. It is a stack of interlocking links shaped so that they can fold in one direction and lock rigid in the other.

Inside the housing the chain is coiled or stacked in a compact loop, folded flat. A sprocket driven by the gearbox feeds it out through a guide, and as each link leaves the guide it swings straight and locks against its neighbour. The result is a rigid strut that can push, made from something that stores in a box a fraction of its extended length. Retracting reverses it: the sprocket pulls the chain back, the links break at the guide and fold back into the housing.

The practical consequence is size. A chain unit that extends 300mm can live in a housing around 350mm long and 30mm deep, which is why it can be built into the head of a roof window sash without adding depth to the frame.

Spindle and screw actuators, and where the extra push is needed

A spindle actuator replaces the chain with a threaded rod running in a nut. Turn the rod and the nut travels along it, and the nut carries the pushing arm. It is the same principle as a machine vice.

Screw drives produce far more force than a chain for the same motor, because the thread pitch is itself a second reduction. They are also stiffer in bending, so they cope better with a heavy sash on a shallow pitch where a good deal of the load acts sideways on the arm rather than along it. Against that, they need the full extended length as physical space when closed, so the housing is longer, and they are generally slower.

On rooflights they turn up on larger flat roof units, on hinged access units, and on anything where the sash weight is beyond the comfortable range of a chain.

Linear rod and scissor arms on flat roof units

Made to size flat roof rooflights and hinged glass access units use a heavier class of mechanism again, usually a linear rod actuator mounted inside the kerb or along the frame, sometimes paired with a scissor arm that multiplies the travel.

A scissor arm is a pair of crossed links pinned at their intersection. A short push at the base opens the arm through a much longer arc at the tip, which is how a unit with only 250mm of actuator stroke can lift its sash 400mm clear. The mechanical trade is exact: the extra travel is bought with a proportional loss of force, so a scissor arrangement wants a stronger actuator behind it than the raw stroke suggests. On flat roof rooflights the arm is normally hidden inside the kerb upstand so nothing is visible from the room.

Stroke length, and what it means for the opening you actually get

Stroke is the distance the arm extends, and it is not the same as the gap the sash opens to. On a top hung roof window the actuator pushes near the bottom rail and the sash swings about hinges at the top, so the arm length and the hinge position set the arc. A 300mm stroke on a 1180mm tall sash gives roughly 30 degrees of opening.

That angle matters for two reasons. It sets the free area for ventilation, and it sets how far the sash projects above the roof line, which is worth thinking about on a Stroud valley conversion where the window is overlooked from the slope behind. Manufacturers publish the opening angle per size code, and it is the figure to check rather than the stroke.

Force, load and the mass of a triple glazed sash

A sealed unit is heavy. Double glazed low emissivity glass in a 1140 by 1180 roof window sash comes out around 35 to 45 kilogrammes with the frame; a triple glazed equivalent can be half as much again. The actuator does not lift all of that, because the hinges carry most of the weight, but on a shallow pitch it carries a much larger share than on a steep one.

The rule of thumb is that the flatter the roof, the harder the actuator works, because the sash is being lifted against gravity rather than swung. At 15 degrees the mechanism is doing something close to a straight lift. At 55 degrees it is mostly pushing the sash out sideways. That is why manufacturers publish different maximum sizes for electric units at different pitch bands, and why a size that is fine on a Forest of Dean roof at 45 degrees may not be offered for a shallow rear addition in a Gloucester terrace.

Limit switches, and how the unit knows it has arrived

Something has to stop the motor at each end of the travel. Older and simpler actuators use mechanical limit switches: a cam or a striker on the moving part trips a microswitch at full extension and again at full retraction, cutting the supply.

Better units use current sensing instead. The controller watches the current the motor draws, and when the sash reaches its stop the mechanical load spikes, the current rises sharply, and the controller cuts power. That method needs no adjustment, it adapts as seals compress and relax over the years, and it works at any intermediate position. It is also what makes controlled part opening possible, because the controller can count motor revolutions from a known closed position and stop at a percentage of the travel.

Obstruction detection and what the motor does when something is in the way

The same current sensing gives an obstruction response. If the closing sash meets something before the current profile says it should, the controller sees an unexpected load and stops, and on most ranges it reverses a short distance.

It is a genuine safety function rather than a marketing one, and it is the reason an out of reach unit over a stairwell can be closed by remote without anybody being able to see the aperture. It is not infinitely sensitive: a soft obstruction such as a curtain or a hanging plant may not generate enough load to trip it before it is pinched, so the aperture still wants keeping clear.

Low voltage, and why the sash carries no mains

Almost every rooflight actuator runs on extra low voltage direct current, usually 24V DC and occasionally 12V. The mains supply terminates at a transformer or a control box in a fixed accessible position, and only the low voltage pair travels up to the moving sash.

There are three good reasons for it. A flexible cable that crosses a hinge and flexes several thousand times over its life is safer at 24V than at 230V. The motor can be small and the electronics simple. And the shock risk in a location that will get wet, cleaned with a damp cloth and occasionally rained on is much reduced. The mains side of that arrangement is set out on the first fix wiring page.

Where the actuator sits, and what it does to the sightline

On a manufactured roof window the actuator is built into the top of the sash behind a cover, so from the room the unit looks like any other roof window with a slightly deeper head trim. Nothing crosses the glass.

On a retrofit or a made to size unit the arm may be surface mounted on the frame, which puts a visible housing and a moving chain in the corner of the aperture. It is not unsightly, but it is visible, and on a formal Cheltenham interior it is worth knowing before the unit arrives rather than after. Where the appearance genuinely matters, a concealed mechanism in the sash head is the specification to ask for, and it usually means choosing a factory built electric unit rather than adding an opener later.

Locking, and what holds the sash shut

An actuator that has retracted fully has pulled the sash onto its seals, and the self locking gearbox holds it there. On smaller units that is the whole locking arrangement, and it is more secure than it sounds, because there is no way to back drive a worm gear from the sash.

Larger units add mechanical locking points that engage as the sash closes: a hook or a claw on the frame that the sash catches, driven by the last part of the actuator travel. Multipoint locking of this kind is normal on big flat roof access units, where seal compression across a two metre sash cannot be achieved by one central pull. It also improves the weather seal, which is the part that matters most in a Gloucestershire winter.

Speed, noise, and what it sounds like from the room below

A rooflight actuator is slow on purpose. Fifteen to thirty seconds from shut to fully open is normal, and it is set by the gear ratio rather than by any control decision. Slow travel means low forces, quiet running and time for the obstruction sensing to react.

The noise is a low geared hum in the region of 45 to 55 decibels at the unit, which is comparable to a fridge compressor. Three metres below in a room it is quiet. In the stairwell of a Cheltenham townhouse, where the unit sits four storeys up in a hard plastered shaft with no soft furnishings, the same sound carries and reflects and can seem louder than the specification suggests. Nobody has ever asked for it to be changed, but it is worth expecting.

Brackets, guides and the fixings that carry the load

The actuator is only as good as what it is bolted to. On a factory built unit the mounting is engineered into the sash and the frame and there is nothing to decide. On a made to size flat roof rooflight the bracket positions are set out at manufacture, and the fixings land in the kerb.

That puts a load into the kerb that a fixed unit never sees. A timber kerb carrying an electric opener wants to be properly built and properly fixed down to the deck, because every cycle applies a push at the top of it. Where a kerb is being built new as part of the job, the specification accounts for it. Where an existing kerb is being reused, its construction gets checked before an opening unit is ordered for it.

Actuators on Gloucestershire roofs

The county throws up three recurring situations. High level units over stairwells in the Regency and Victorian townhouses of central Cheltenham, where the sash is far out of reach and the mechanism has to be right first time because getting back to it means a tower or a scaffold inside the house.

Steep slopes in the Stroud valleys and around Chalford, where the pitch is generous and the actuator has an easy mechanical job, but access for the fitter is the hard part. And shallow rear additions across the vale in Gloucester, Tewkesbury and Longlevens, where pitches down near 15 degrees put the mechanism under the most load and the size options narrow accordingly.