Electric rooflight wiring: planning the cable before the ceiling closes

There are only four parts to the system. A supply from the house, a control unit that steps it down or switches it, a cable to the motor, and a way for you or a sensor to tell it what to do.

Most motorised roof windows and flat rooflights run their motors at a low voltage, usually 24 volts direct current. The house supplies 230 volts alternating current, so somewhere between the consumer unit and the glass there has to be a transformer, which is normally housed in a small control box. Some ranges build the control electronics into the frame or into a separate plastic box that is screwed to a rafter or a joist. The exact arrangement changes from product to product, so the installation manual for the unit you have chosen is the authority for the details. What stays the same is the order of things: mains in, conversion, low voltage out, motor.

A few products break that pattern. A solar powered unit carries a small panel and a battery, and needs no cable from the house at all. We cover that route separately in solar powered opening rooflights. Others use a mains motor directly. The survey tells us which you are dealing with before a single cable is ordered.

It helps to think of the job as two separate cable runs with different rules:

• The mains run, from a fused connection near the control box back to the house supply. This is fixed wiring and sits under electrical safety law.
• The low voltage run, from the control box to the motor, the rain sensor and the wall switch. This is lighter cable, but it still has to be routed, supported and protected from the weather and from insulation.

The mains feed is almost always a short extension of a circuit that already exists, taken from a spot close to where the rooflight will go. Which circuit is the first thing to decide, and the decision is made by the electrician, not guessed from the ceiling.

In a loft conversion, the natural candidate is the lighting circuit for the floor below, or the upstairs ring or radial if the loft room has sockets. In a flat roofed kitchen extension, the feed usually comes from the lighting circuit serving the new ceiling, or from a spur off the ring main, run in through the ceiling void. Either way, three questions are asked before the feed is chosen:

1. Does the circuit have spare capacity for a small extra load? A rooflight motor draws very little, but the circuit has to be tested and found sound first.
2. Is there a convenient, accessible point to connect? A junction or a fused connection unit that can be reached later, without lifting a floor.
3. Is the protective device suitable? Cables concealed in walls or partitions at a shallow depth need residual current protection under the wiring regulations, and the test of the existing circuit shows whether that is already in place.

The connection is normally made through a fused connection unit, a small switched and fused box, so that the rooflight can be isolated on its own. The fuse rating follows the manufacturer's instructions. It is never chosen by habit.

Part P of the Building Regulations covers electrical safety in homes. The electrical side of a rooflight installation is carried out to Part P and to the current edition of the wiring regulations, BS 7671, by a qualified electrician.

That is worth saying plainly, because the fitting of a rooflight and the wiring of one are different trades. The carpenter and roofer who set the unit in the roof do not wire it. The electrician who connects the supply tests the circuit, completes the connection and records what was done. Fitting and wiring are planned together so that they do not get in each other's way.

Whether a particular piece of work has to be notified to Building Control depends on what the electrical work is. Adding a new circuit to a home is notifiable. Extending an existing circuit is generally not, unless it is in a special location such as a room with a bath or shower, and even then the work must still meet the regulations. The government guidance in Approved Document P sets out the detail, and it is worth a read if you want to see the wording for yourself. Where the work we carry out is notifiable, we handle the Building Control notification, and the rooflight itself is installed to current Building Regulations as a matter of course.

If you already employ an electrician you trust, say so when you ask for a quote. Our quote then states clearly who does which part, what has to be in place before we arrive and what stays as it is. For the plain-question version of this topic, see does an electric rooflight need an electrician.

Cable is cheap and plasterboard is not, so the route is drawn while the roof structure is still open. Once the ceiling is boarded and skimmed, any change means cutting into a finished surface.

The route starts at the rooflight and works backwards. The motor is on the frame, or in the light well beside it, and the cable leaves by the shortest concealed path to the control box. From the box, the mains cable runs to the connection point. On a new build or an extension with exposed joists, this is straightforward: the cables lie in the joist void, clipped at regular intervals and kept clear of insulation edges and plumbing.

In a loft conversion the route is harder, because the ceiling below is usually already finished. There are three ways through:

• Down the sloping plane, between rafters, into the eaves space and out to a wall.
• Along a stud wall that is being built anyway for the new room.
• Through a service duct or boxed-in section, where one exists beside a chimney breast or a stair.

Whichever is chosen is marked on the drawing we keep with the job, and photographed before the boarding goes on. That photo matters later, when somebody wants to fix a shelf to the wall and needs to know where the cable sleeps.

Cables cross timber, and every hole in a joist or rafter takes away a little strength. The general guidance on holes keeps them small, on the centre line of the member, and within a zone away from the ends and the middle of the span.

In practice this means three habits. Holes are kept to a modest diameter relative to the depth of the timber, and never cut close to another hole or to a notch. They are drilled, not chiselled. And where a cable has to cross an engineered joist, a trussed rafter or a beam, it goes through a gap or a service zone that the manufacturer allows, because those members are designed to tight tolerances and must not be altered. If a route cannot be found without cutting a structural member, the route changes.

The same thinking applies to the structural opening for the rooflight itself. Trimming the opening is a structural job, and the cable path is decided around it. Where an opening rooflight needs a taller upstand, the wiring has to be threaded through the kerb before it is closed, as the upstand height allows.

A roof is a layered thing, with a waterproof skin on the outside, insulation in the middle and an air and vapour layer on the warm side. Any cable that crosses those layers is a potential weak spot, in both weather and warmth.

The preferred answer is to avoid crossing the waterproof layer at all. The cable from the motor goes into the frame or the kerb from the inside, and it leaves the same way, so it never passes through the roof covering. Factory-fitted units come with the motor cable already routed through the frame, which is one reason to choose them. Where a site-fitted motor or a rain sensor on the outer face needs a cable, the manufacturer supplies a sealed entry, and we fit that, following the instructions.

Through the insulation and the vapour layer, the same care applies. A hole left open in a vapour control layer lets warm, moist house air reach the cold side of the roof, where it can condense. The hole is taped and sealed around the cable with the right tape for the membrane. In a warm flat roof, where the insulation sits above the deck, the cable sleeve is fixed and sealed before the insulation is laid. In a cold roof or a loft, the cable is kept clear of the ventilation path and clipped out of the way.

There is one more effect worth knowing. A cable surrounded by thick insulation cannot lose heat as easily, so it carries less current safely than the same cable in open air. The electrician takes that into account when sizing the cable, and keeps the run as short as the route allows.

The control box has to be close to the rooflight, reachable without a ladder and a torch, and not buried under a finished surface. Getting all three right is the main job of the wiring plan.

Because the low voltage cable loses strength over distance, the box sits near the motor, usually within a few metres of the unit. The manufacturer states the maximum cable length between box and motor, and we stay inside it. A long run also needs a thicker cable, so a modest move of the box can save a lot of copper.

Practical places that work are a dry loft void beside the opening, a joist bay close to the light well, or a small access panel set flush in the ceiling or the wall of the light well. A box that has to be reached for a reset or a change of fuse needs an access route. Nobody plasters over a box that will be wanted again, and nobody leaves one loose in an insulated void where the lagging can cover it.

In a room where noise matters, the position counts for a second reason. A control box with a transformer can hum faintly, and a motor can produce a low whirr as it runs. Placing the box in a void rather than against the plasterboard of a bedroom keeps that to a minimum, and fixing it on a resilient mount helps more. Motor noise gets its own treatment in are electric rooflights noisy.

The wall switch, the keypad or the receiver for a remote all need connecting, and the rain sensor needs to reach the control box. These are the small cables, and they are the ones most often forgotten.

A wired wall switch needs a low voltage cable back to the control box, run in a conduit or a chase where it is concealed. A wireless remote needs only a receiver, often inside the box, so that cable run disappears. A rain sensor sits on the outside and is linked to the box with its own short cable, which passes through the flashing or the frame at a point the manufacturer specifies. The choices between these are laid out in controls for electric rooflights, and the sensor side is covered in rain sensors on electric rooflights.

Switch position is a design decision. A switch by the door is easy to find, while one on a rafter above head height is not. Near a bath or shower, switches have to follow the rules for that zone. The switch goes on the plan along with the light fittings and sockets, so that the room reads as one scheme.

Some rooms change the wiring plan. A rooflight over a shower, a kitchen extension or a utility space raises its own questions about moisture, zones and heat.

In a bathroom, the wiring regulations divide the space into zones around the bath or shower, and any equipment inside those zones has to have a suitable level of protection against water. A motorised rooflight directly above a shower may fall within a zone, so the product, the control position and the cable route have to be chosen with that in mind. Often the answer is to keep the switch and the control box outside the zones and to use a product whose manufacturer approves it for a damp room.

In a kitchen, steam and cooking heat are the issue. The control box stays away from the hood and the hob, while the ventilation benefit of an opening unit is kept, as discussed in the piece on electric rooflights over a kitchen extension. On a stair, the challenge is reaching the rooflight to service it, so the control box tends to go on a landing wall or in the loft hatch zone, as described on the page about stairwells and landings.

Where a cable passes through a ceiling or wall that forms part of a fire separation, the hole has to be sealed so that it does not become a path for smoke or flame. In a loft conversion, the floor and the stair enclosure are the usual places this matters.

Approved Document B governs fire safety in dwellings, and a loft room adds a storey that is normally protected by a fire-resisting floor and an escape route. A cable that goes through that floor is passed through a neat hole and sealed around with a suitable fire-stopping product. Recessed lights, if any are fitted in the same ceiling, are chosen and boxed in to match. This goes on the plan so that each trade knows what is expected.

While on the subject of fire and escape, a motorised rooflight is not the same thing as an escape window. A motor can fail and a power supply can drop, so a unit used for escape has to open by hand and meet the dimensions in the regulations. The relevant discussion is in can an opening rooflight be used as a fire escape.

Before the ceiling is finished, the work is tested. The circuit is checked for continuity and insulation, the protective device is checked, and the motor is run through a full opening and closing cycle.

After that, the plasterboard goes up, and the decorator can follow. The finish around the light well is made good, with neat reveals and clean corners, so the wiring disappears into the building. A small, labelled access point stays wherever a box or a connection must remain reachable.

At handover, you get three things. The electrical certificate or installation record for the work carried out, which you should keep with the house papers, since a buyer's solicitor will ask for it. A note of where the cables run and where the box sits. And the manufacturer's instructions, including the reset procedure for the motor. The written quiet spec that comes with the quote is a separate document, and it records the glazing build, the seals and the upstand, not the wiring.

The 10-year workmanship guarantee covers our installation work. The guarantee page, our guarantee, states what it covers and how it works, and the wording on that page counts.

Can the wiring be added after the rooflight is fitted?

It can, but it takes more effort. If the ceiling is already finished, the cable has to be fished through voids or chased into the plaster. Planning the route first lets everything be hidden from the start. A fixed unit that is to be motorised later is worth wiring for now, even if the motor comes later, which is covered in can a manual opening rooflight be motorised.

Is there a plug, or is it fixed wiring?

The supply is fixed wiring. It is connected to the house circuit, normally through a fused connection unit, rather than a plug and socket. A plug on a rooflight motor would be easy to pull out and awkward to place in a roof void.

What if the power goes off?

The rooflight stays where it is. Some units have a hand crank or a battery back up, and some have neither, depending on the model. The page on what happens to an electric rooflight in a power cut explains the options.

Does a wireless control mean no cables at all?

No. The remote is wireless, but the motor and its control box still need power, and the receiver has to be fed. The wireless part removes the wall switch cable, not the supply.

Where should I start?

With the room, the ceiling and the cable route. Our parent guide to electric and opening rooflight installation sets out the wider picture, and you can get a quote once you know which room you have in mind. The survey covers the electrical plan alongside the glazing plan.