Do electric rooflights need a socket in the roof?

Where the voltage actually changes

The system has two halves that meet at a box. On the house side there is a mains supply, an isolator and a cable running to the control unit. That control unit contains the transformer and the electronics that drive the mechanism. On the rooflight side there is a low-voltage lead from that box up to the actuator in the frame head, together with the leads for the rain sensor and any radio receiver.

Everything you can touch at the window is on the low-voltage side. That is why the rooflight lead can be dressed neatly along a rafter and taken through the frame without any of the containment a mains cable in that position would demand.

Why a socket in the void is a poor idea

Four reasons, and they compound.

• Accessibility. A socket buried above plasterboard cannot be reached, inspected or isolated without opening the ceiling. Accessories are meant to remain accessible.
• Environment. A cold roof void moves through large temperature swings and can see condensation. It is not where you want a plug and a general purpose accessory living.
• Load discipline. A socket invites something else to be plugged into it later, by somebody who does not know what circuit they are on.
• Traceability. Nobody selling the house in ten years will know that socket exists, and nobody surveying the house will find it.

A fused connection unit in a sensible position does the same job, cannot have anything else added to it, and can be found again.

Choosing where the control unit sits

It wants to be somewhere you could get to it in fifteen years with a step ladder and no tools beyond a screwdriver. In a loft conversion that is often in the eaves storage, behind a small access panel. In a vaulted kitchen ceiling it is more often in the void above an adjacent flat ceiling, with a neat hatch, or inside a cupboard on the floor above.

It should not be immediately above a shower, buried in insulation, or in a position that only exists while the scaffolding is up. And it needs to be within the cable length the manufacturer specifies for the run up to the mechanism, which is not unlimited.

What the cable run has to deal with

The lead from the control unit to the frame head travels through the roof build-up, and how it does that is worth agreeing at first fix rather than improvising. It should cross rafters through drilled holes at the neutral axis rather than through notches cut in the top, be kept clear of insulation compression, and arrive at the frame on the side the manufacturer’s connection point is on.

Two details cause most of the later trouble. The first is the vapour control layer, which the cable has to pass through, and which needs sealing properly at that penetration or you have made a small permanent route for warm moist air into a cold void. The second is slack. A run pulled tight leaves nothing for the day someone needs to disconnect the actuator, and a few hundred millimetres of coiled spare at the frame head costs nothing now.

The order of work that keeps this cheap

Cabling belongs to first fix, before insulation and plasterboard, at the same moment the lighting circuits are run. Our installation follows once the structure is closed and weathertight. Final connection and commissioning come last, with the electrician and ourselves on site together.

Get that order wrong and the cable becomes a chase through finished plaster, a second electrician’s visit and a redecoration. It is the single most common avoidable cost on an electric rooflight, and it is avoided with one conversation at the right moment. The sequencing detail is on wiring an electric rooflight at first fix.

Switches, sensors and what else needs a cable

A hard-wired wall switch needs its own low-voltage run back to the control unit, so its position has to be chosen at first fix along with everything else. Decide it by walking the room: by the door you actually use, at the same height as the light switches, not behind where the sofa will go.

A battery keypad needs no cable at all and can be moved later, which is why it is the usual answer where the decision has been left late or where the wall is solid masonry. A handheld remote needs nothing. The rain sensor is part of the rooflight and connects to the same low-voltage side as the mechanism.

Several rooflights on one arrangement

You do not need a control unit each. One unit will drive a group, within the limits the manufacturer publishes for the number of actuators and the total cable length. That means one mains connection, one isolator and one position to remember, which is better for you as well as cheaper.

What it requires is thinking about the whole group at once rather than adding units one at a time. A run planned for two rooflights and later asked to serve four is where the length limits get quietly exceeded. Grouping limits are covered on how many electric rooflights one switch can control.

The version with no cable at all

A solar-powered rooflight has no control unit, no isolator, no cable run and no connection to the house. The panel in the frame head charges a battery, and the battery drives the mechanism and the sensor. Where a ceiling is already finished, that removes the entire subject of this page from the job.

It is not free. The unit costs more to buy, the mechanism is slower, and the panel wants daylight, so a shaded slope is a weaker case. Whether it suits your roof is set out on whether solar powered rooflights are any good.

We install and replace rooflights across Oxford and up to twenty five miles by road, with fifteen years in the trade and a ten-year workmanship guarantee, fitting VELUX, Fakro, Keylite and Roto units. All electrical work is carried out by a qualified electrician. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.