Wiring an electric rooflight at first fix

What first fix means on this job

First fix is everything that happens before the surfaces close: structure, carcassing, pipework and cable runs. Second fix is the visible work afterwards, when accessories, faceplates and the units themselves are fitted and connected.

For a rooflight this splits into three distinct pieces of work. There is the aperture and the unit, which is roofing. There is the low-voltage cable from the power supply to the rooflight, which is a route that has to exist. And there is the mains supply feeding the power supply unit, which is electrical work for a qualified electrician. The three are done by different people at different times, and the coordination between them is where jobs go wrong far more often than any individual step.

The decision that must be made before plasterboard

Whether a rooflight will ever be powered is a first-fix decision, not a second-fix one, and it is not only about the unit you are buying now.

The useful question at survey is not “do you want electric” but “is there any chance you will want electric, or an electric blind, within the next ten years”. If the answer is anything other than a flat no, a cable is pulled to the aperture and left coiled. A few metres of low-voltage flex costs very little against the cost of opening a finished ceiling. This is the cheapest piece of future-proofing available on the whole job, and it is the one most frequently skipped because nobody asked the question.

Where the power supply unit lives

The transformer or power supply that converts 230V mains to the actuator’s 24V DC is a box roughly the size of a paperback. Where it goes is a real decision with real consequences.

It does not go in the roof void above the insulation, because that is a cold, sometimes damp space with poor access, and a supply unit there will eventually need reaching. It does not go where it cannot be got at without a scaffold. Good positions are inside the room in a cupboard, in an accessible eaves void with a proper hatch, in a services riser, or in a plant or utility space. On some integrated roof windows the supply is a small plug-in unit intended to sit near a socket, which changes the geometry entirely and is one reason the actual product should be chosen before the wiring is planned rather than after.

Cable for the low-voltage leg

The manufacturer specifies the cable, and their figure is the one to follow. In broad terms the low-voltage run is a two-core or three-core flexible cable, commonly 0.75mm² or 1.0mm² for short runs and 1.5mm² where the distance grows.

Two things separate it from lighting cable. It carries extra-low voltage, so it must be identifiable as such and kept clearly separate from mains cabling to avoid anybody later assuming it is a lighting circuit. And it is often a flexible type rather than flat twin and earth, because the last stretch has to bend at the unit. Where a flexible cable is run within a structure it needs mechanical protection: conduit, a capping, or a route that keeps it away from fixings.

Volt drop over a long run

At 24V a small voltage loss is a large proportion of the supply. This is the technical point that catches out people who assume low voltage means low consequence.

An actuator drawing 1A over 15 metres of 0.75mm² cable loses a meaningful fraction of its supply in the cable itself, and the symptom is a unit that runs slowly, struggles on the last few millimetres of closing, or trips its own current sensing early and stops short. The fixes are ordinary: a larger conductor, a shorter route, or a supply unit relocated closer to the rooflight. All three are easy at first fix and awkward afterwards. Where a run will exceed about ten metres, say so when the unit is being specified, because the manufacturer will usually state a maximum cable length for their actuator.

Routing through joists, rafters and insulation

The cable has to get from the supply to the aperture, and the route runs through structure that has rules attached to it.

Holes through joists sit within the permitted zone of the span and on the neutral axis, and notches are avoided where a hole will do. Where a cable crosses a rafter it goes through rather than over, so that the insulation and the boarding sit flat. Fixings are placed so that a plasterboard screw or a batten nail can never find the cable, which in practice means keeping the run in the middle depth of the timber and recording where it goes. Where a cable passes within 50mm of a finished surface, protection or a recognised safe zone applies, and that is the electrician’s territory rather than ours.

Insulation matters too. A cable buried in thermal insulation runs warmer and carries less current, and at 24V that combines badly with volt drop. Clip it to the structure, not inside the quilt.

Crossing the airtightness layer

In a modern warm roof or an insulated loft conversion there is a vapour control and airtightness layer on the warm side of the insulation, and every cable that crosses it makes a hole.

Each hole is a route for warm moist air to move into the cold part of the build-up, where it will find a cold surface and condense. One cable makes a small hole. A rooflight cable, a sensor cable, a blind cable and four downlights make a lot of small holes, and their combined effect is not small. The correct detail is a proprietary grommet or a taped sleeve at the membrane, formed as the membrane is installed rather than retrofitted through a torn hole afterwards. It costs minutes at first fix. Nobody can do it once the boarding is on.

Leaving a tail at the aperture

The cable arrives at the rooflight opening and has to be left in a state the next trade can use.

That means a generous tail, at least 500mm and preferably a metre, coiled and secured to the structure rather than dangling. It means the end protected, taped and kept dry, because an open conductor sitting in a roof void through a wet fortnight is not a good start. It means the tail positioned at the corner or head where the unit’s own gland will be, which requires knowing the make and model. And it means labelled: a strip of tape reading “rooflight 24V” prevents a great deal of guesswork three weeks later when four unidentified cables emerge from the same ceiling.

Switch drops and where the wall control goes

If the control is a hard-wired wall switch rather than a radio keypad, it needs its own cable drop, and that drop is also first-fix work.

Position it by the door of the room, at the height of the light switches, on the handle side. That is where people’s hands already go. Putting a rooflight control on the far wall because the cable route was easier produces a switch nobody uses. Where the control is a battery radio keypad the drop is unnecessary, but the fixing position still deserves the same thought, and there is more on that in remote controls, wall switches and smart home links.

Several units on one supply

Where two or more rooflights are to open together, the wiring topology has to be decided at first fix because it changes the number of cables in the ceiling.

The usual arrangement is a single power supply of adequate rating with a home run to each unit, so that every actuator has its own pair back to a common control point. That is more cable than daisy-chaining but it keeps each unit independently testable and lets a controller drive them in step. The supply has to be sized for all the actuators starting at once, because inrush current when three motors begin moving together is considerably higher than three times the running current. Our page on how many rooflights one switch can control deals with the control-side limits.

Sequencing the trades

The order of visits is the part that most affects whether the job runs smoothly, and it is worth writing down at survey.

• Aperture formed, unit fitted, roof weathered and closed the same day.
• Electrician’s first fix: mains cable to the intended supply position, low-voltage route pulled and tailed at the aperture.
• Insulation and airtightness layer installed, with the cable penetrations sealed as they are made.
• Continuity and insulation testing on the runs while everything is still visible.
• Plasterboard, then plaster.
• Electrician’s second fix: supply unit fitted and terminated, controls fitted, mains connection made and certified.
• Actuator connected, cycled and commissioned, sensor proved with water.

Testing before the ceiling closes

Every cable in that ceiling should be proved before it disappears. Continuity end to end, insulation resistance, and where possible a live functional test with the actuator temporarily connected.

A functional test is worth the ten minutes it takes. A cable can be electrically perfect and still be the wrong one, in the wrong place, or a metre too short. Running the motor once while the ceiling is open converts every one of those problems from an expensive discovery into a small adjustment.

Recording the route before it disappears

Photograph the ceiling before it is boarded. Every ceiling, every time.

A handful of photographs on a phone, taken from a consistent position with a tape measure laid across the joists for scale, is the most useful document produced on any first fix. It tells whoever comes next exactly where the cable runs, which means a future light fitting, loft hatch or partition does not go through it. It takes two minutes and repays itself the first time anybody drills that ceiling.

When the ceiling is already up

Sometimes the decision arrives late. It is not fatal, but the options narrow and each one has a visible cost.

A cable can sometimes be fished from an adjacent void or eaves cupboard, which works where there is a clear path and fails where noggins or firewalls intervene. Surface trunking is honest and cheap and looks exactly like what it is. A section of ceiling can be opened and repainted, which is often less disruptive than people fear when the run is short. Or the specification changes: a solar powered rooflight without a mains run avoids the question entirely, which is precisely the situation those products exist for. We will tell you plainly which of those applies to your ceiling rather than promising a hidden route we cannot deliver.

What we do and what the electrician does

We form the aperture, build the upstand where one is needed, fit the unit, form the flashings and weather the roof. We set out the cable route with the ceiling still open, mark where the tail must land, and mount the actuator and any factory sensor. That work carries our ten-year workmanship guarantee, and the roof is never left open overnight.

The mains side is different. The supply to the power supply unit, the connection to the fixed wiring, the testing and the certification are all done by a qualified electrician, and Heritage Skylights does not carry out or certify that work. What we do is sequence it, so that the electrician arrives when the ceiling is still open and not a fortnight after the plasterer. Our page on whether electric rooflights need an electrician sets out the split in more detail.

Getting the sequence agreed at survey

Bring this up at the survey rather than after it. We will walk the route with you, identify where the supply unit can sensibly live, measure the run, and tell you whether the length pushes you towards a larger cable. If the ceiling is going up in a fortnight, that conversation is urgent rather than optional.

We install and replace rooflights in Oxford and within 25 miles by road, which takes us into Buckinghamshire, Berkshire, Northamptonshire and Warwickshire. We have been doing it for fifteen years. Call 01865 704245 or email info@heritageskylightsoxford.co.uk, and you will get a range by email first, then a fixed price once we have seen the roof.