Guide · Electric and Opening Rooflights
Retrofitting an electric opener to a manual unit
A manual rooflight can usually be motorised, and the deciding factor is almost never the motor. It is whether the sash, the hinges and the frame were built to carry one. On a unit under about fifteen years old from a mainstream range, the answer is normally yes. On the domed polycarbonate lights that went into Essex flat roofs in the seventies and eighties, there is nothing on the frame for a drive to hold on to.
This page is about the mechanics of the retrofit itself. What comes off, what goes on, where the bracket lands, and the handful of measurements that decide whether a drive off the shelf will suit the unit already in your roof. The wider subject of electric and opening rooflights sets the context; this is the detail of adding a motor to something that was never ordered with one.
What is actually being added to the unit
A retrofit is four components, not one. A drive, which does the pushing. A bracket set, which transfers the push into the sash without twisting it. A power supply, usually a low voltage transformer or a control box. And a control, which can be a wall switch, a handset or a link into a wider system.
Only the first two touch the rooflight. The other two live indoors and can be moved later without going anywhere near the roof. That split matters, because it means the fiddly part of the job is a couple of hours at the head of the unit and the rest is ordinary electrical work in a room.
Hinge type, and the movement the opener has to follow
A drive does not move the sash. It follows it. The hinge decides the arc, and the drive has to be capable of tracing that arc from a fixed point on the frame without binding.
A top hung sash swings on a hinge at its head and describes a simple arc, which is the easiest case. A centre pivot sash rotates about its middle, so the top edge travels inward as the bottom travels out, and a chain fixed to the top rail would slacken halfway through the stroke. Centre pivot units therefore take a spindle drive at the head rather than a chain. Getting this wrong is the most common reason a retrofit binds at half open.
Friction stays and the hardware that comes off first
Most manual opening rooflights hold their position through friction. Either a friction hinge with a controlled resistance, or a stay bar with notches, or a scissor stay with a locking arm. All of them are designed to resist movement, which is exactly what a motor does not want.
Before the drive goes on, the friction is reduced or the stay is taken out entirely. A friction hinge is usually adjustable at a grub screw and is backed off to the point where the sash still holds but no longer fights. A notched stay comes off and its screw holes are made good. Leaving the friction set for hand use is a reliable way to overload a drive within a season.
Measuring the sash before anything is ordered
Six figures settle the order, and they are all taken on site rather than from a brochure. Sash width and height on the outer edge of the frame. Sash weight, which for glass is calculated from the glazed area and the pane build-up rather than guessed. The rebate depth between the sash face and the frame face. The clear distance across the head where a chain has to run. And the opening angle you actually want.
The rebate depth is the one that catches people. A slim cast conservation frame can leave under twenty millimetres of usable depth at the head, which rules out most concealed drives and points at a slim surface-mounted chain unit instead.
Working out the force the pane will ask for
Force is not the same as weight. A drive pushing near the hinge line has a poor lever and needs more force than one pushing further out, and a sash on a shallow pitch has more of its weight resting on the drive than the same sash on a steep slope.
The working method is to take the glazed weight, apply the geometry of where the bracket lands, then add a margin for wind pressure trying to hold the pane shut. A drive that is comfortably inside its rating on paper runs quietly and lasts. One specified at the edge of its rating stalls on a cold February morning when the gasket has stiffened. How the drive itself develops that force is covered on actuators and how they work.
A drive chosen with headroom in its force rating is the cheapest reliability you can buy on this job.
Bracket geometry, and the fixing points that already exist
The bracket is where retrofits succeed or fail. On a factory motorised unit the sash bracket lands on a reinforced section that the manufacturer put there. On a retrofit, you are fixing into whatever the sash happens to be made of at that point.
In a timber sash there is usually solid material and the fixing is straightforward. In an aluminium extrusion the bracket wants to land on a screw port or a bolted through-fixing rather than a self tapper into a thin wall. In uPVC, the fixing goes into the steel reinforcement inside the profile if there is one, and if there is not, a spreader plate distributes the load. Screwing a drive to two millimetres of plastic will hold for a summer and pull out in a gale.
Surface-mounted drives against concealed ones
A concealed drive sits inside the frame profile and is invisible when the unit is shut. It is the neater answer and it is only available where the profile has the room and the manufacturer made a kit for that model.
A surface-mounted drive sits on the face of the frame at the head, visible as a slim housing perhaps forty millimetres deep. On a retrofit it is by far the more common outcome, and on a rooflight three metres up in a ceiling it is a great deal less noticeable than it sounds when described. On a roof window at eye level in a loft room, the choice matters more.
Where the transformer sits, and how far it can be from the motor
Most domestic rooflight drives run on 24 volts, which means a transformer or control box somewhere between the mains and the motor. It needs to be accessible, ventilated and dry.
Distance is the constraint people miss. Voltage drop over a long low voltage run reduces the force at the motor, so cable size has to increase with length. A four metre run in the standard cable supplied with the kit is fine. A fourteen metre run to a cupboard on the other side of the house wants a heavier conductor, or the drive will open slowly and stall against a cold gasket. Running the cable while the ceiling is open is covered under wiring at first fix; on a retrofit the ceiling is already closed, which is the whole difficulty.
Cable entry through a frame that was sealed at the factory
A factory motorised unit has its cable exit formed and weathered in the works. A retrofit has to create one, and it has to be on the dry side of the seal line.
On a kerb-mounted flat rooflight the usual route is through the kerb itself, below the unit, from inside the room into the void under the frame, with the entry sleeved and sealed and dressed so water running down the inner face of the kerb cannot track along the cable. On a pitched roof window the cable comes down the inside of the reveal and enters the frame at the head, entirely within the warm side. Drilling through an outer frame face at the top of a rooflight creates a hole that faces the weather, and it is not a route worth taking.
Setting the end stops on a drive fitted years after the unit
A drive needs to know two positions: shut, and fully open. On a factory unit those are set in the works. On a retrofit they are taught on site.
Most modern drives learn by running a calibration cycle and sensing the current rise when the sash meets its seal. Older or simpler drives use adjustable limit switches set by hand. Either way the closed position is set so the gasket compresses properly without the motor straining, and the open position stops the sash short of the point where the hinge or the stay would take load. A drive left to run to its own mechanical end can be pushing against a hinge that stopped moving twenty millimetres earlier.
Gasket compression once a motor takes over from a handle
A person closing a rooflight by hand feels resistance and leans on it. A motor stops at a preset point. If that point is a millimetre generous, the gasket sits proud of full compression along one edge and the unit whistles in wind and lets water in under pressure.
The check is straightforward and worth insisting on. A strip of paper closed in the seal at each corner and at the mid point should offer even resistance when pulled. Uneven resistance means the sash is closing out of square, which is adjusted at the hinges before the drive is signed off rather than after the first storm.
Keeping a handle, a latch or an escape release in the picture
Many manual rooflights have a handle that operates a multi point latch. Once a drive is fitted, that latch either has to be permanently released or replaced with a version the drive can work with, because a motor pushing against a shot bolt will damage something.
Where the rooflight serves as a means of escape under Part B, the retrofit has to preserve a way of opening it that does not depend on power, and that route has to be operable by a person standing in the room. That is a design decision taken before the drive is ordered, not a detail resolved on the day.
Older units, obsolete profiles and universal drives
Manufacturers support motor kits for current and recent ranges. Go back far enough and the kit no longer exists, at which point the choice is a universal drive with a bracket set adapted to the profile.
Universal drives are honest engineering and they work, provided the sash is sound and the fixing points are real. What they cannot do is rescue a unit whose hinges have worn, whose sash has dropped, or whose sealed unit is already misting. Spending on a motor for a rooflight with five years left in it rarely reads well later. A replacement unit ordered motorised from the works arrives with the drive concealed, the cable exit weathered and the stops already set.
Testing the retrofit before the access comes down
The test is not one cycle. It is five or six, with the sash watched through the full stroke from outside as well as inside.
What is being looked for is a chain running square rather than sideways, a bracket that does not flex under load, an even closing line, and a motor that sounds the same at the end of the stroke as at the start. A rising note near the closed position means the drive is working harder than it should be, and that is adjusted then, while there is still a platform to stand on. Adding a rain sensor or pairing a handset or wall switch happens after the mechanical side is proven, not before.
What the retrofit changes about the guarantee
The rooflight keeps whatever product warranty it already had, subject to the manufacturer’s own position on approved kits. The workmanship on the retrofit itself carries the same ten year cover as any other Sunspire installation, which covers the bracket fixings, the cable entry and its weathering, and the closing set.
That distinction is worth understanding at the outset. The motor is a component with its own life. The detail that keeps water out where the cable passes through the build-up is workmanship, and it is the part that has to still be right in a decade.
Essex roofs where a retrofit is the sensible route
The strongest case is a roof window in a converted loft in one of the Chelmsford or Colchester Victorian terraces, fitted ten years ago with a pole in mind, now sitting above a bed where nobody wants to stand on a mattress in January. The unit is sound, the profile takes a kit, and the cable can run down the reveal into the room.
The second case is the interwar semi across the commuter towns where a bathroom roof window is high on a hipped slope and the pole has been leaning against the wall for three years. Where a mains run genuinely cannot be made without opening a finished ceiling, a solar powered opener is the route that avoids the chase. Where the ceiling is coming down anyway for other reasons, the cable goes in and the job is simpler and quieter for it.
Keep reading
More on electric and opening rooflights
Rain sensors and what they actually do
Read the guide →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 →
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