Guide · Electric and Opening Rooflights
Wiring an electric rooflight at first fix
First fix is the short window when the structure is up, the roof is on and the ceiling has not yet closed. On a rooflight job that window decides almost everything electrical, because a cable run through an open joist void takes twenty minutes and the same cable through a finished and decorated ceiling is a day’s work with a plasterer behind it.
This page is the detail of that stage. What gets run, in what size, to what point, and how it is left so that whoever comes back to connect the unit finds what they need. It applies to new extensions, loft conversions and any job where the ceiling is coming down anyway. The broader picture of electric and opening rooflights sits on the hub page.
What first fix covers on a rooflight
At this stage the opening is trimmed, the kerb or the trimmers are in, and the unit may not even have been delivered. Nothing is connected. What happens is that cables are run from their source to the opening, left long, protected and identified, and their positions recorded.
Second fix is the other half: the transformer goes in, the tails are cut to length, the actuator is terminated, the switch is fitted to its back box and the system is set up and tested. The two visits can be months apart on a slow build, which is exactly why the first one has to be done as though nobody will remember anything about it.
Extra low voltage at the unit, mains at the transformer
Almost every domestic motorised rooflight runs at 24 volts direct current. The actuator, the sensor and the control board all sit on that side. A transformer or switched mode power supply converts the mains down, and the boundary between the two systems is that box.
This split is useful. Everything above the ceiling, in the roof void and at the unit itself, is extra low voltage. The mains work stops at a fixed point in an accessible position lower down. It keeps the connections at the opening simple, and it keeps mains cable out of a build up where insulation, vapour layers and a moving mechanism all share the same space.
Where the power supply is allowed to live
The transformer needs three things: mains in, a route out to the unit, and permanent access without demolition. It runs slightly warm, so it wants air around it rather than being buried in loft insulation or sealed inside a plasterboard void with no hatch.
Sensible positions are the eaves cupboard of a loft conversion, the top of an airing cupboard, above a ceiling hatch, or inside a services void behind a bulkhead with a removable panel. On a single storey rear extension with a warm deck and no void at all, it commonly ends up in the adjoining house, in a cupboard on the party wall side, with the extra low voltage cable running from there out into the extension roof.
If the power supply cannot be reached without a hole saw, it is in the wrong place.
Cable size, run length and volt drop
At 24 volts, volt drop is a real constraint rather than an academic one. A drop that would be trivial on a 230 volt circuit is a large proportion of the supply here, and an actuator fed at 21 volts runs slower, draws more current and stalls earlier when the gasket compresses on a cold morning.
The working target is to keep the drop under about five per cent, which at 24 volts means roughly 1.2 volts across the whole run and back. That translates into these rough limits, based on a typical single actuator drawing about one amp and a heavier or doubled installation drawing two.
| Conductor size | Longest sensible run at 1A | Longest sensible run at 2A | Typical use |
|---|---|---|---|
| 0.75mm² | Around 25m | Around 12m | Short run, supply in the same room |
| 1.0mm² | Around 34m | Around 17m | The usual default on domestic work |
| 1.5mm² | Around 50m | Around 25m | Long runs, supply at the other end of the house |
| 2.5mm² | Around 85m | Around 42m | Several units on one supply |
Where the figure is marginal, go up a size. The cable is a small part of the job and it is the one component that cannot be changed later.
The cables a motorised unit may want
There are up to four, and it is worth deciding on all of them now rather than discovering a fifth later.
- Motor supply, two core from the controller to the actuator, sized as above.
- Sensor, two or four core from the rain or wind sensor position back to the controller.
- Wall switch, usually four core low voltage from the switch position to the controller, unless the switch is a radio one.
- Blind, a separate two core to the head of the unit where an integrated electric blind is going in, whether now or later.
Some control systems run a two wire bus instead, carrying power and data on the same pair to every device. Where that is the intent, the topology of the bus is set by the manufacturer and needs establishing before anything is pulled through.
Where the cable enters the frame
A factory motorised unit arrives with a defined cable entry, made through the frame in the works and landing in a channel designed for it. The installer’s job is to present the cable at that point, on the room side of the gasket line, with enough slack to reach the terminal.
On a lantern with a hinged vent the entry is generally through a hip bar or the ridge, and on a flat rooflight it is through the kerb below the unit. In every case the cable crosses the plane of the roof beneath the weathering, not through it. A cable that has to pass through a weathered surface has been routed wrongly, and the survey should have caught it.
The service loop, and how much to leave
Leave far more than looks necessary. A metre and a half of spare coiled neatly at the opening covers the unit sitting further into the reveal than the drawing suggested, a kerb that grew fifty millimetres, an actuator mounted on the other side of the sash, and the possibility of a different unit being ordered.
Coil it, tape it, and fix it clear of anything that will be cut, screwed or plastered. The commonest loss on a first fix is not a missing cable but a correct cable that has been cut off flush by whoever fitted the plasterboard, because it was hanging in the way and nobody had labelled it.
Crossing the vapour control layer
In a warm roof the vapour control layer sits on the warm side of the insulation, and its integrity is what keeps moist internal air out of the build up. Every cable through it is a hole, and holes in a vapour layer above a kitchen or a bathroom are how interstitial condensation starts.
The detail is straightforward: pass the cable through a grommet or a proprietary sleeve, then tape the membrane back to it with a compatible airtightness tape. It takes two minutes at first fix. Doing it properly matters most on the flat roof extensions common across Harlow, Basildon and the newer estates, where a warm deck sits directly over a kitchen with no ventilated void to forgive a mistake.
Warm decks, cold decks and rafter voids
The roof build up decides where a cable can physically go. In a warm deck, the insulation sits above the structural deck and the cable belongs below the deck, in the ceiling void or clipped to the underside of the joists. Running it above the deck puts it into the insulation layer and under the weathering, which is both awkward and unnecessary.
In a cold deck there is a ventilated void beneath the insulation and the cable runs in it, clipped to the sides of joists rather than lying loose. In a pitched slope, the cable runs down between rafters and out at the eaves or through the ceiling. On the trussed roofs across Great Notley, South Woodham Ferrers and Beaulieu Park, cable runs along the trusses and through the factory drilled service holes where they exist. Trusses are not notched or drilled to suit a cable.
Insulation, heat and derating
A cable surrounded by insulation cannot lose heat, so its safe current falls. On a 24 volt run carrying an amp this is rarely the binding constraint, but on the mains side feeding the transformer it matters, and a cable buried in three hundred millimetres of mineral wool is derated considerably against the same cable clipped to a surface.
The practical answer is to clip cables to joist sides above the insulation line where possible, or to size them for the condition they will actually sit in. It is another reason the mains work stops at an accessible transformer rather than being pushed further into the roof.
Terminating where the connection stays reachable
Any joint that will be permanently inaccessible has to be made in a maintenance free connection, or better, not made at all. Continuous runs from the controller to the unit avoid the question entirely, and at these lengths there is usually no reason to joint.
Where a joint genuinely helps, at the boundary between an existing house and a new extension for instance, put it in a box behind a hatch, in the eaves cupboard, or in a cupboard ceiling. Write on the box what it feeds. The person opening it in eight years may well be you.
Sequencing around the roofer, the plasterer and the delivery
The order that works is consistent. The roof structure goes up and the opening is formed. Cables are run, sensor and switch positions marked, service loops coiled and photographed. The roofer weathers the roof and, on a flat roof, the kerb is built. The unit is set and weathered. Only then does the plasterboard go up, with the cables emerging where they were left.
What goes wrong is a boarder arriving before the electrical first fix, usually because the unit was on a long lead time and the programme moved. Once the ceiling is on, every one of those runs becomes a chase, a trap or a compromise. The fix is to run the cables before the unit is even ordered, since the position of the opening is fixed long before the model is.
Testing before the ceiling closes
Every run is proved before anything is boarded over. Continuity end to end, insulation resistance on the mains side, and a physical check that the cable at the opening is the one labelled at the controller position. On a job with two or three units it is easy to swap a pair.
Where the actuator is already on site, connecting it temporarily and running a full cycle is worth the ten minutes. It confirms polarity, confirms the cable is the right size for the run, and gives an early sight of anything fouling the pane. Finding a problem at this stage costs nothing. Finding it after decoration is a different conversation.
Labelling, and the photograph that outlives the build
Every cable gets a written label at both ends: what it is, where it goes, and what it feeds. A marker pen and a strip of tape is adequate and a printed label is better.
The single most useful record, though, is a photograph of the open ceiling with a tape measure laid across it, taken from a fixed reference such as the corner of the room. It shows where every cable, joist and trimmer sits behind the plaster. It costs nothing at the time and it answers every question that arises later about drilling a ceiling, adding a light, or hanging anything from the structure.
Notification for the mains side of the work
The extra low voltage work at the unit is not a controlled electrical service. The mains connection to the transformer is, and in England it falls under Part P where a new circuit is created or where work happens in certain locations. It is carried out by a qualified electrician and certificated, and that certificate joins the paperwork for the job.
The Building Regulations application for the rooflight itself is separate again, covering thermal performance, structure and, where relevant, escape. Sunspire handles that notification as part of the job, so the electrical certificate and the completion certificate arrive together rather than one of them surfacing as a gap during a sale.
First fix on a house that is not being built
Most jobs are not new extensions. They are a rooflight going into a roof over a room that is already decorated, and the first fix window has to be manufactured. Sometimes that means lifting a floor in the room above. Sometimes it means a surface run in a slim trunking down a reveal, which is honest and neat if it is planned rather than improvised.
The Essex housing stock makes this specific. A Victorian terrace in Chelmsford or Colchester has lath and plaster ceilings and no service voids, so the route usually comes down from the floor above. A weatherboard cottage near Stock has a frame with clear cavities between studs. Interwar semis with hipped concrete tile roofs almost always have a generous loft to work from. Where no route exists at all, a solar powered unit removes the question, and where a manual unit is already fitted the options are set out under adding an opener to an existing unit.
Keep reading
More on electric and opening rooflights
Rain sensors and what they actually do
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 →
Get a quote
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements