Wiring an electric rooflight at first fix
The cable for an electric rooflight has to be in the ceiling before the plasterboard goes on. That single sentence is most of what there is to say about first fix, and it is the point at which a great many electric rooflight jobs go wrong, because the decision to have an opening unit is often made after the extension is already lined out. Getting it right means knowing where the transformer will live, what cable is needed, what route it takes through the structure, how far it can run, and what has to be agreed with the electrician and the builder weeks before the rooflight itself arrives on site. This page covers the wiring stage on a new build, an extension or a loft conversion, from the consumer unit to a coil of cable taped to a joist waiting for the unit. The rooflight side of the work is on the electric rooflight installation page.
What first fix means on a rooflight
First fix is everything that gets buried. Cables run, back boxes fixed, containment installed, structure notched or drilled, all before boarding and plaster. Second fix is what happens after decoration: the switch plate goes on, the control unit is connected, the rooflight is commissioned.
The rooflight is unusual among electrical items because the point of use is in the roof, which is the part of the building that gets closed up first and is hardest to get back into. A socket in a wall can be chased in afterwards with a morning’s mess. A cable to a rooflight in a finished vaulted ceiling with insulation, a vapour control layer and skim over it cannot, not without taking a section of ceiling down.
The two circuits, and which one matters
An electric rooflight involves two entirely different electrical runs and they get confused constantly.
The mains side runs from a fused spur or a lighting circuit to a transformer or control unit, at 230V. This is the notifiable part, carried out by a qualified electrician. The low voltage side runs from that transformer to the rooflight itself, usually at 24V DC, and it is the run that has to reach the unit in the roof. It is the low voltage cable that must be in place at first fix, and it is the one most often forgotten, because the electrician has wired the mains to a convenient point and assumed somebody else deals with the rest.
Where the transformer or control unit lives
The transformer is a box, typically 150 by 100 by 50mm, and it has to be somewhere accessible. It cannot be buried in a ceiling void with no access, because it is an electrical accessory that has to be reachable for isolation and for any later work.
Sensible positions, in rough order of preference: inside a boxed bulkhead with a removable panel, in an airing cupboard or store adjacent to the room, on the wall above a door head where a boxed detail already exists, or in the accessible part of a loft void where a proper walkway and a light exist. On a Cheltenham townhouse stairwell installation the transformer often ends up in the top floor cupboard, and on a vale extension it goes in the bulkhead above the bifold head.
Cable type for the low voltage run
The low voltage lead supplied with most rooflights is short, often a metre or two, and is there to reach a junction close to the unit. The run back to the transformer is site cable.
Two core flexible cable is used, and the size depends on the length of the run. On a short run of three or four metres, 0.75mm squared is adequate. Beyond about eight metres, 1.5mm squared is the sensible minimum, and on long runs 2.5mm squared is not excessive. The reason is voltage drop, which matters far more at 24V than it does at 230V, and which the next section deals with properly. The cable wants to be a proper flexible type rather than solid core twin and earth, because it terminates at a moving assembly and gets manipulated during commissioning.
Voltage drop, and why 24V punishes a long run
Voltage drop is proportional to current and to cable resistance. A rooflight actuator drawing one amp at 24V is drawing the same current as a 240 watt appliance at mains voltage, but it only has 24 volts to lose.
A two volt drop on a 230V circuit is under one per cent and nobody notices. The same two volt drop at 24V is over eight per cent, and the actuator slows noticeably, the current rises to compensate, and the controller’s current sensing can misread the load and stop the sash early thinking it has met an obstruction. That last symptom is the classic sign of an undersized cable, and it usually shows up months later on a cold morning when the seals are stiff. Size the cable generously at first fix; it costs almost nothing then and it is unreachable afterwards.
| Run length, transformer to unit | Minimum sensible conductor | Notes |
|---|---|---|
| Up to 5m | 0.75mm squared | Typical bulkhead to rooflight on an extension |
| 5m to 10m | 1.5mm squared | Loft conversion, cupboard to ridge |
| 10m to 20m | 2.5mm squared | Stairwell runs in a tall townhouse |
| Over 20m | Relocate the transformer | Move the mains closer rather than upsizing further |
The route through the structure
The cable has to get from the transformer to the head of the rooflight opening without crossing anything it should not. On a flat roof extension that usually means running along the underside of the joists inside the ceiling void, then up through the deck into the kerb.
On a pitched roof it means running between rafters, and here the trimmer around the opening is the obstacle. The cable is not run through the structural trimmer. It is taken alongside it, clipped to the side of a rafter, and brought to the head of the opening where the unit’s own lead will meet it. On a trussed roof at Quedgeley or Bishops Cleeve, the webs cannot be drilled at will, and the cable route is planned with the same care as the opening itself.
Notching and drilling, and the rules that apply
Where a cable has to pass through a joist, it goes through a drilled hole rather than a notch cut in the top edge. Holes are drilled on the neutral axis, at the centre of the depth, in the zone between 0.25 and 0.4 of the span from the support, and the diameter is limited to a quarter of the joist depth.
Those limits exist because a joist is strongest at its top and bottom faces and largely idle in the middle. A hole at mid depth removes material that was doing little. A notch in the top edge removes material in compression and is far more damaging. In a roof with a designed trimmer around a new opening, the load paths have already been altered once, and adding an unplanned hole to a trimmer is not on. The engineer’s detail governs, every time.
Safe zones and the mains run
The 230V side follows the standard rules. Cables buried in walls less than 50mm from the surface run in permitted zones, which are horizontal and vertical bands aligned with accessories, and within 150mm of the top of a wall or of an internal corner. Anything outside those zones needs mechanical protection or an earthed metallic covering, and everything is protected by a 30mm RCD in the usual way.
On a rooflight job the mains run is normally short, because the transformer is placed near an existing circuit rather than the other way round. That is a deliberate choice: it is far easier to extend a low voltage run than to bring a new mains circuit across a finished building.
Part P and Building Control
Electrical work in a dwelling is notifiable under Part P of the Building Regulations where it involves a new circuit or work in a special location. The mains connection for a rooflight transformer is carried out by a qualified electrician, and where the work is notifiable it is notified.
On an extension or a loft conversion this normally rides on the back of the main Building Control application for the works, which is already in place for the structural opening. That is the tidiest arrangement, because it puts the rooflight, the trimmer and the electrics on one notification. The Building Control side of the job is handled as part of the work rather than left with the householder.
Wiring for a rooflight that has not been chosen yet
It happens constantly. The extension is going up, the rooflight is a decision for later, and nobody wants to commit. The answer is to run a generous cable anyway.
A 1.5mm squared two core flexible run from a sensible transformer position to the head of each proposed opening, coiled with a metre of slack, labelled and photographed before boarding, costs very little and preserves every option. If the unit ends up being fixed, the cable is capped off and forgotten. If it ends up being electric, or gaining a blind later, the run is there. On a job where several rooflights go into a vale extension, running the cable to all of them is the sensible default.
Allow for the blind as well as the sash
An electric blind inside the rooflight is a second motor with its own supply, and it is a separate cable. On most ranges the sash actuator and the blind can share a control unit but not a conductor.
So a rooflight that may one day have both wants a four core run or two twin cables to the same point. Retrofitting a blind to a unit that only has a sash cable is the moment people discover this, and the options at that stage are a visible surface cable or a battery blind. Planning for it is a two minute decision at first fix and covered further on the electric blinds page.
The vapour control layer, and the hole the cable makes
A warm roof or an insulated pitched ceiling has a continuous vapour control layer on its warm side, and every penetration through it is a route for moist room air to reach cold structure. A cable is a penetration.
The detail is to bring the cable through a grommet or a purpose made cable collar and tape the membrane to it with a proper airtightness tape, not gaffer tape. On a rooflight the membrane is already being dressed and taped to the frame around the whole perimeter, so the cable entry is done at the same time and by the same hand. Getting it wrong produces condensation in the roof build up above a warm room, which shows up as a damp area on the ceiling near the reveal a winter or two later and is genuinely awkward to diagnose. Insulation and airtightness at the reveal are covered under energy efficient rooflights.
Sequencing with the builder and the electrician
Three trades have to agree on one coil of cable, and the sequence is the same on every job.
- Rooflight positions and sizes fixed at survey, before the roof is framed.
- Transformer position agreed with the electrician, and a mains point provided for it.
- Low voltage runs pulled in while the ceiling void is open, with slack left at both ends.
- Everything labelled at both ends and photographed against a tape measure before boarding.
- Board, insulate, tape the membrane, plaster.
- Rooflight fitted, weathered, reveal formed.
- Second fix: transformer connected, unit connected, remotes paired, rain sensor tested.
The photograph in the middle of that list is worth more than it sounds. A dated picture of a cable coil with a tape measure against the nearest wall means the next person to need it can find it through a skim coat without exploratory holes.
First fix on a Gloucestershire loft conversion
A loft conversion is the hardest case, because the finished ceiling follows the rafter line and there is no void to work in afterwards. Everything happens in the window between the structural work and the boarding, which on a typical conversion in Longlevens or Hucclecote is a matter of days.
The transformer usually goes into the eaves storage or a purpose built cupboard on the new landing. The runs go down the rafter line to each roof window head. On the Regency and Victorian townhouses of central Cheltenham the same job is complicated by a stairwell unit four storeys above the hall, where the run is long enough to want a heavier conductor and the transformer position needs genuine thought, because getting back to it later means a tower inside the house.
New extensions across the vale
Single storey rear extensions in Gloucester, Tewkesbury, Quedgeley and the vale villages are the commonest first fix situation, and they are the easiest, because the flat roof deck is open and the joists are accessible from below until the day the boarders arrive.
The usual arrangement is a warm roof deck with the kerbs already built, cables brought up through the deck inside each kerb before the insulation and membrane go down, and the penetration sealed into the waterproofing as part of the roof rather than after it. On a made to size flat roof unit the manufacturer will state where the cable entry is expected, and building the kerb to suit that point is easier than making the cable find its way afterwards. That work runs alongside everything described on the flat roof rooflight installation page.
When the cable was never run
Where a ceiling is already finished and there is no cable, three routes remain. A surface run in trunking or conduit, which works and is visible. A route through an adjacent void, up a boxed corner or behind a bulkhead, which is often achievable with a little joinery and looks deliberate. Or a solar powered unit, which needs no run at all and is frequently the right answer in a finished Cotswold stone roof where opening up the covering to chase a cable is a serious undertaking.
That third option is worth taking seriously rather than treating as a compromise. On a finished building it is often the least disruptive path to an opening rooflight, and it is set out fully under solar powered rooflights.
Next on this subject.
- Guide Electric rooflight actuators and how they work An electric rooflight opens because a small low voltage motor drives a mechanical arm that pushes the sash off its…
- Guide Rain sensors and what they actually do A rain sensor on an electric rooflight is a small plastic block, usually 60 to 80mm across, fixed to the…
- Guide Solar powered rooflights without a mains run A solar powered rooflight carries its own photovoltaic panel, its own rechargeable battery and its own control electronics, all built…
- Guide Remote controls, wall switches and smart home links Every electric rooflight needs something to tell it what to do, and there are four ways of doing it: a…
- Guide Electric blinds fitted inside the rooflight A blind on a roof window is not the same object as a blind on a vertical window. It hangs…
- Guide Opening rooflights for the stack effect An opening rooflight moves far more air than its size suggests, and the reason is height rather than area. Warm…