Wiring an electric rooflight at first fix

The difference

At first fix, before plasterboard, a cable to the rooflight position is a few metres of twin and earth clipped to a joist. Twenty minutes and the cost of the cable.

Afterwards, it means lifting flooring above or taking down ceiling below, chasing a wall for a switch drop, and making good. A day and a half, plus decoration.

People routinely fit a manual rooflight to save a few hundred pounds, want a motor two years later, and find the wiring costs more than the upgrade would have.

Run it even if you fit manual

This is the important part. A capped cable in a ceiling void costs almost nothing and is worth a great deal later.

Terminate it properly in an accessible enclosure, note where it is, and photograph it before the ceiling goes on. That photograph is worth having.

What to run

A supply to the rooflight position and a switch drop to a sensible wall position. Fit the wall switch even if you also have a remote: the remote is always in another room.

If a rain sensor is likely, it usually sits on the roof near the unit and its cable runs back to the control point.

Your electrician will size and protect the circuit. Most rooflight actuators run on low voltage from a transformer, so what reaches the window may be extra low voltage rather than mains, which affects cable specification.

Blinds

If electric blinds are a possibility, they need their own supply. Run it at the same time.

Sequence

Structure, then kerb, then first fix wiring, then ceiling, then the rooflight. Getting the wiring in before the ceiling is the whole point, and it is easy to miss because the rooflight is not there yet to remind anybody.

Twenty minutes now, a day and a half later

That is the whole of this guide, and everything else is detail.

During a build or a loft conversion, at first fix, running a cable to a rooflight position is a few metres clipped to a joist before the plasterboard goes on. It is trivial work and the material cost is negligible.

Into a finished room the same cable means lifting flooring above or ceiling below, chasing a wall for a switch drop, making good and redecorating, plus an electrician’s time and certification. A day and a half plus decoration, and frequently several times the cost of the motor it was meant to serve.

Same cable, same window, same actuator. Completely different job.

The advice, stated plainly

Run a capped cable to every rooflight position at first fix, even where you are fitting manual units and have no intention of motorising anything.

Terminate it in an accessible enclosure. Photograph where it is before the plasterboard goes on. Note it on a drawing if there is one.

It costs almost nothing and it covers a future motor, a future blind, a future rain sensor and a future control unit. If none of those ever happens, you have lost twenty minutes and a few metres of cable.

The single most common regret in this trade is saving the electric premium during a build and wanting a motor two years later.

What a mains system actually consists of

Three parts, and only one of them is difficult to add later.

A spur from a convenient circuit to a control unit. Ordinary electrical work.

A run from the control unit to the actuator on the window. Usually extra low voltage, which is why the cable is modest and why the arrangement is safe in a position that sees condensation.

A cable to a wall switch, if one is being fitted.

Nothing sits outside. There is no socket on the roof and no mains voltage anywhere near the weather.

Where the control unit goes

Somewhere you can reach: a loft, an airing cupboard, a service void. Not buried behind plasterboard.

It is the part most likely to need attention over twenty five years, and burying it turns a ten minute job into a disruptive one.

Agree the position before the wiring is run rather than putting it wherever is convenient on the day, and record it with the rest of the specification. In fifteen years nobody will remember.

The wall switch, and why its position is a first fix decision

A handheld remote comes with most systems and is permanently in another room.

A wall switch is always in the same place, anyone can use it without being shown, and it works when a remote battery is flat. On a high level rooflight it is the difference between a window the household uses and one only the person who knows where the remote lives ever operates.

It belongs beside the light switches at the room entrance rather than under the rooflight, which is where people instinctively reach.

That position needs a cable drop in a wall, which is straightforward at first fix and disruptive afterwards. It is the reason the switch position has to be decided with the electrician rather than with the window supplier.

Grouping, and what it means for cabling

Where several rooflights are being installed, grouping them so one switch operates a set is configured at the control unit rather than at the window.

That means grouping can be changed later without rewiring. What cannot be changed later is that each window needs its own cable back to the control unit, so the cabling is dictated by how many units there are rather than how they are grouped.

Run a cable to every position, including any you are undecided about. Adding a fifth window to a group of four later means a cable run to the fifth, which is the disruptive job all over again.

What else to run while you are there

A spare to any position that might take a unit later. On an extension where the client is considering two rooflights but might want three, run three.

A cable for a blind, or at least check whether the control unit will have a spare output. Adding an electric blind to an existing motorised window is frequently a matter of connecting and configuring rather than running new cable, and that only works if the capacity was provided.

A cable to a sun tunnel diffuser position, where a light kit inside the diffuser might be wanted. Same principle: trivial now, means opening a ceiling later.

Certification and notification

The connection to the household supply is electrical work. It needs a competent electrician and it needs certifying, and depending on the situation and location it may be notifiable under Part P.

Where a bathroom is involved, the electrical zones apply and a fitting near a bath or shower has particular requirements. That is settled at design stage rather than on the day, so tell whoever is coordinating the work where the rooflight sits relative to the shower.

The certification comes to you with the rest of the paperwork and belongs with the house records.

Coordinating with the rooflight installer

The two trades meet at the actuator, and it is worth them talking before the cable is run rather than after.

The rooflight supplier knows where the actuator sits on the frame, which end of the window the cable should arrive at, and what the control unit needs. Getting the cable to the wrong side of the opening is an avoidable irritation.

Where a builder is running first fix and a rooflight installer is coming later, we are happy to supply the requirements in advance so the cable arrives in the right place.

The alternative, which needs none of this

A solar powered opener carries its own photovoltaic panel and rechargeable battery. No spur, no control unit, no cable, no electrician and no making good.

It works better than people expect: the panel needs daylight rather than direct sun, so it functions on a north facing slope through an English winter, and the energy to open a window a few times a day is small. It opens more slowly, thirty to sixty seconds against fifteen to thirty, which nobody notices. And it keeps working in a mains power cut.

For a retrofit into a finished room it is almost always the sensible answer.

During a build it is still worth running the cable, because mains is faster, allows a hard wired switch, and costs almost nothing at that stage. Fit solar if you prefer; leave the cable capped in case a future owner wants mains.

Why this is the cheapest decision on the whole job

While the roof is open and the ceiling is off, running a cable to a rooflight position costs almost nothing. It is a length of cable and twenty minutes.

Once the reveal is plastered and the ceiling is decorated, the same cable means either surface trunking, which is visible, or opening up and making good, which means replastering and redecorating.

That difference is why the decision belongs at first fix even if you have not decided whether you want a powered unit.

Run the cable even if you fit a manual window

This is the advice worth taking away.

A capped and labelled cable terminating near the rooflight position costs very little now and preserves the conversion option for the next thirty years. Somebody may want a powered opener, or a powered blind, long after the room is finished.

Label it clearly and record its position with the house paperwork, because an unlabelled capped cable in a ceiling void is a mystery rather than a provision.

What to plan while the ceiling is open

The control positions. Wall switches go where somebody standing in the room would reach for them, not where the cable run was convenient. Decide before the plasterboard.

The grouping. Which units operate together. On a hardwired system, changing this afterwards means going back into the ceiling.

A supply for a powered blind, which can share the arrangement and be grouped with the window on one control.

Where a gateway would live, if smart home control is a possibility, because it needs power and a position within range.

The regulatory part

It is notifiable electrical work and it needs certifying by a competent person. Ask which route is being used and make sure the certificate reaches you rather than staying in a file.

It surfaces at sale, and obtaining it retrospectively is far harder than getting it at the time.

The cable entry

On a properly designed unit the cable enters through a designed penetration in the frame, not a hole somebody made. That penetration is detailed to stay weathertight.

Where an installer is drilling the frame to get a cable in, something has gone wrong with either the product choice or the sequence.