Getting Power to a Rooflight in a Finished Flat Roof

Electric rooflight wiring on a flat roof almost never runs through the roof itself. On a finished warm roof the cable comes from below, through the ceiling void, up beside the kerb and into the rooflight from inside the insulated envelope. Routing it over or through the deck means cutting the waterproofing after it has been laid, and that is the one thing worth avoiding. Where no internal route exists without wrecking a finished room, a solar unit removes the problem entirely.

That is the short version. The long version is the part that decides your budget, because on a flat roof the difference between an easy route and a hard one is a couple of hundred pounds against the best part of a thousand. Our electric skylight installation page covers the whole job. This page is only about the cable, and only about flat roofs.

Why the roof build-up decides everything

You cannot sensibly plan a cable route on a flat roof without knowing which of three build-ups you have, because each one gives the cable a different amount of room and a different set of rules.

Warm roof

The standard detail on any extension built in the last twenty years or so. Structural deck goes down on the joists, then a vapour control layer, then rigid insulation, then the waterproofing on top. Everything structural sits inside the warm, dry side. The joist void underneath is not insulated and not ventilated, and it is where services live.

This is good news for wiring. The joist void is a genuine service void, the cable stays in the warm, and nothing has to pass through the insulation or the membrane. On a warm roof, the cable comes along the joist void and turns up the inside face of the rooflight kerb.

Cold roof

Older construction. Insulation sits between the joists and the void above it is ventilated to the outside air to stop condensation. The void is cold, it is damp in winter, and it is exactly where you do not want a control unit or a connection. Cable can pass through it, but any joint or transformer needs to be inside the warm side or in a properly sealed enclosure. We site control gear inside the warm envelope on principle, and on a cold roof that principle stops being optional.

Inverted or ballasted roof

Insulation sits above the waterproofing, with ballast or paving over the top. Uncommon on domestic extensions and awkward when it turns up, because getting to the membrane means lifting ballast. Cable routing is still from below, but any penetration is close to unrecoverable once the ballast is back down.

The four routes, in the order we like them

At survey we walk these in sequence and stop at the first one that works. On a finished room, we walk them before anything is ordered.

1. Along the joist void from an existing circuit

The best outcome by a distance. If the ceiling below the flat roof is coming down anyway, or there is an access point, or the extension ceiling was formed with a service void under the joists, the cable runs along the void to a fused connection unit and up the kerb. No structural work, no making good, nothing visible.

The catch is joist direction. Cable running parallel to the joists is easy. Cable running across them means either notching or drilling every joist in the run, and both are governed: holes on the centre line within the middle half of the span, notches only in the top edge in the first quarter of the span, and neither anywhere near the bearing. A run of eight joists across a side-return extension is eight holes, each drilled from an access hatch you have to make. That is the point where the figure moves from £180 to £400 up towards the £350 to £900 band.

2. Down from a floor void above

Applies where the flat roof is single storey off the back of a two-storey house and the supply is easier to reach from the first floor. Boards come up in the bedroom, the cable runs in the floor void to the party wall or the rear wall, then drops down inside the cavity or a chase to the fused spur. Practical on a carpeted chipboard floor. Genuinely unwelcome on engineered oak laid last month, and we say so at survey rather than on the day.

3. Up an internal wall to the ceiling line

Where the nearest circuit is a socket or a switch drop on a wall in the same room, the cable can go up the wall in a chase, into the ceiling void at the wall head and across to the rooflight. This is often the shortest run on paper and the most disruptive in practice, because it adds plastering and decorating to the job. It is also the route that gets chosen by default when nobody surveyed properly, which is why we would rather find route 1.

4. Surface containment

Trunking or conduit on the surface, down a wall, to a spur. Fast, cheap, entirely visible. In a garage, a utility room or a workshop nobody cares. In the kitchen extension you have just spent forty thousand pounds on, it undoes the point of the extension, and we will tell you that rather than quietly pricing it.

Coming up the kerb: the last 300mm

Every route above ends the same way. The cable arrives in the ceiling void beside the opening and has to get from there into the rooflight, which sits on top of an upstand built to a minimum of 150mm above the finished roof surface at its lowest point, and 200mm on exposed coastal sites at Felixstowe, Harwich or Aldeburgh.

That last stretch is where installations get spoiled, so it is worth being specific about how it is done properly.

• Inside the kerb, never outside it. The cable runs up the internal face of the upstand, inside the insulated and waterproofed envelope. A cable dressed up the outside face and tucked under the flashing is a leak waiting for a hard winter and it is the detail we most often find on units somebody else fitted.
• Through the kerb at the head, not the base. Where the cable has to pass through the timber upstand, the hole goes near the top, above any conceivable water line, and gets sleeved and sealed both sides.
• Sealed cable entry into the frame. Modern opening rooflights have a designed cable entry point into the frame profile. Use it. On coastal work we specify marine-grade glands where the design allows, because a cable entry is a route for damp salt air into the connector block, and corrosion there kills more actuators than the mechanism ever does.
• Terminate inside the warm side. The fused connection unit and any control gear sit inside the room or inside the warm roof void, not in a cold ventilated void where they will condense. This matters more than the cable does.
• Leave slack. A service loop at the frame means a future actuator swap does not mean a new cable run.

Sequencing: the order that costs nothing and the order that costs a lot

On a new extension, the cable is trivial and the only thing that matters is doing it in the right order. Get the order wrong and the cheapest part of the job becomes the most expensive.

1. Rooflight position fixed on drawings. Not “somewhere over the island”. A dimensioned position, because the electrician needs to know where the spur lands.
2. Cable first fix, before the vapour control layer. Cable pulled through the joist void to the opening and left coiled. Ten minutes.
3. Deck, vapour control layer, insulation, kerb. The kerb is built with the cable already inside it.
4. Membrane. Roof closed and watertight with no penetration required, because the cable never crosses it.
5. Rooflight set, actuator fitted, travel checked mechanically.
6. Electrical second fix, test, certificate, commissioning.

On a retrofit into a finished room the sequence inverts in one important way: the rooflight goes in and the roof is made watertight first, then the cabling happens. The room is never left open to weather waiting on an electrician. That single decision is why a retrofit runs two to three days rather than a week of tarpaulin.

When there is genuinely no route

It happens more often than the industry admits, and it is the honest reason solar exists as a product. A single storey flat roof extension with a plastered ceiling tight to the joists, a tiled or engineered floor above, no loft, no accessible void, and a consumer unit at the far end of the house. There is no cable route that does not damage something you have paid for.

At that point the answer is not to chase a wall and apologise. A solar opening rooflight carries its own panel, battery, actuator and rain sensor, needs no cable at all, and removes the electrician from the job entirely. It also keeps working in a power cut, which a mains unit does not. The honest caveat is Suffolk in midwinter: Ipswich sees roughly 45 to 55 hours of bright sunshine across the whole of December and about eight hours of daylight a day, so what you get is fewer operating cycles per day rather than a dead unit. The rain sensor draws so little that it keeps working regardless. A north-facing or heavily shaded unit that you want to cycle several times daily is the one marginal case, and we check orientation and shading in person before recommending it.

The detail sits on our solar powered rooflight installation page, and the straight comparison is on mains or solar powered rooflight. If your unit is an existing manual one, why solar rooflights suit retrofit projects is the place to start.

Common questions about flat roof rooflight wiring

Get the route surveyed before you order anything

The cable is the part of an electric rooflight job that people find out about last and pay for most. We survey it first: joist direction, void access, floor coverings above, where the consumer unit sits, whether the roof is warm, cold or inverted. Then you get one written price covering the rooflight, the actuator, the controls and the electrical connection as a real figure rather than a caveat, and that price is fixed. Every installation carries a 10-year workmanship guarantee.

Worth reading alongside this: electric opening rooflight installation for the unit itself, flat roof skylight installation for upstands and falls, and how do you get power to an electric rooflight for the short version. Costs across the whole hub are on what an electric rooflight costs to install.

Request a fixed quote and we will walk the cable route with you at survey, across Ipswich, Suffolk and north Essex.