Electric rooflights in a two-storey extension

The physics of a rooflight do not change between one storey and two. The glass still lets in the same light and the same heat for a given orientation. What changes is everything around the glass: the height it sits at, the distance from the floor, the volume of air underneath it, and the way that air moves.

On a single-storey rear extension the underside of the glass is usually somewhere between 2.4 and 3 metres up. A person of average height with a telescopic pole can reach a manual opener without much trouble. Put the extension up to two storeys and the roof is now sitting above a first-floor ceiling. If the rooflight is in a top-floor room, the underside might be 2.4 metres above that upper floor, but the moment you introduce a double-height void, a galleried landing, or a rooflight over a stairwell, the glass can be four, five or six metres above the nearest place you can stand. No pole reaches that. No ladder belongs there. The only sensible way to open it is a motor.

The second thing a second storey changes is the air. A tall space behaves like a chimney, and a rooflight at the top of it is the flue. That is a genuine advantage if you plan for it, and a source of overheating complaints if you do not. More on that further down.

There are three ways to open a rooflight, and the geometry of a two-storey extension usually decides between them for you.

A manual handle is fine when you can touch the frame. In a two-storey extension that is rare. It only applies where the rooflight is set low in the slope of a top-floor room and you can stand right under it.

A telescopic pole extends your reach to roughly three metres of opener height, which covers a lot of single-storey work and some top-floor rooms. It has a hard ceiling, though. Beyond about three metres the pole becomes long, heavy and awkward to locate onto the opening bracket, and nobody does it twice a day. A pole that is a nuisance to use is a rooflight that stays shut, which defeats the point of having an opening one at all.

An electric opener removes the height limit completely. A motor does not care whether the glass is three metres up or six. It opens on a wall switch, a handheld remote or a phone, and it closes itself when the rain sensor gets wet. For anything over a stairwell, in a double-height space, or high in a top-floor pitch, it is not really a luxury choice. It is the only version that gets used. Our electric skylight installation work is mostly exactly this: units placed where a human arm was never going to reach.

An electric opening rooflight is a standard rooflight with a motorised actuator fitted to the opening sash. The actuator is a small linear drive: either a chain that pushes out and folds back into a housing, or a spindle rod that winds in and out. It pushes the sash open against its hinges and holds it at any point you stop it.

A chain actuator is the common choice for flat-roof units and roof lanterns. It is compact, quiet, and gives an opening of around 200 to 250mm at the sash edge, which is plenty for ventilation. A spindle actuator can give a wider opening and more push, which suits larger or heavier sashes. Either way the motor runs on low voltage, typically 24 volts, stepped down from the mains by a small transformer or supplied by a battery.

Three things usually come with the motor and are worth having:

• A rain sensor. A small pad on the outside of the frame detects moisture and drives the rooflight shut within seconds. This matters far more on a two-storey extension than a single-storey one, because when the glass is six metres up nobody is going to notice a shower starting and run to close it. The sensor is the whole reason you can leave an unreachable rooflight open on a changeable day.
• Controls. A hard-wired wall switch, a radio remote, or a home-automation link. On a double-height space a wall switch at the bottom of the stairs is the sensible default, so the rooflight can be opened from where people actually stand.
• Position control. Good actuators open in increments rather than all-or-nothing, so you can crack a rooflight a few degrees for a background trickle or throw it wide to purge heat quickly.

There are two ways to feed an electric rooflight, and the choice often comes down to when in the build you are making the decision.

A mains-powered unit takes a low-voltage supply from a transformer wired back to a fused spur on the house circuit. It is the standard route on a new two-storey extension because the electrician is on site anyway and the cable can run inside the structure before it is closed up. It is the neatest solution and there is nothing to maintain.

A solar-powered unit carries a small photovoltaic panel on the outer frame, a rechargeable battery and the motor, and needs no mains cable at all. That is its advantage. If the extension is already built, or the rooflight is going into a roof where running a cable across a finished ceiling would mean cutting into plasterboard, a solar opener sidesteps the whole problem. The panel keeps the battery topped up, and on a south-facing Southend roof the panel is sitting in about as much sun as it could ask for, so charging is rarely the limiting factor here. The trade is that the battery is a component with a service life, where a mains unit has none.

The table below sets out the typical shape of the electric options, with figures that are industry-general rather than a fixed price list. Exact numbers vary by manufacturer, size and specification.

This is the single most common thing to get wrong on a two-storey extension, and it is entirely avoidable. A mains electric rooflight needs its cable run to the frame position during first fix, while the ceiling and the roof structure are still open. Once the plasterboard is up and skimmed, adding a concealed cable to a rooflight three metres over a stairwell means cutting the ceiling apart to do it.

So the decision about whether a rooflight opens electrically has to be made at the design stage, not when the room is nearly finished. If there is any chance a rooflight in a two-storey extension will be out of reach, and there usually is, the cable should be run to it as a matter of course. Running a cable you might not use costs very little. Retrofitting one you did not run costs a ceiling.

If that moment has already passed and the extension is built, this is exactly the situation a solar opener is made for. It is worth designing in from the start all the same, because a planned mains unit is tidier and needs nothing replacing down the line. When we survey a two-storey extension before the build, the opener type and the cable route are settled on the same visit, alongside the glass.

Here is where a two-storey extension gives you something a single-storey one cannot. Warm air rises. In a tall space with a low opening near the floor and a high opening near the top, that rising air sets up a steady flow all by itself: cool air is drawn in low, warm air escapes high, and the taller the space the stronger the effect. It is called the stack effect, and it is the same principle that makes a chimney draw.

A rooflight at the top of a two-storey void is the perfect high outlet for this. Open a door or a window on the ground floor and open the rooflight at the top, and the space ventilates itself without a fan, quietly, on the temperature difference alone. On a hot day this clears the stratified layer of heat that would otherwise sit at the top of a tall room and radiate back down all evening. An electric opener is what makes it practical, because the high outlet is the one you could never reach by hand.

The reason this matters more upstairs than down is that heat collects at height. In a single-storey room the hot layer sits just above your head. In a two-storey space it banks up several metres above you, out of the way but still radiating, and the only way to shift it is an opening at the very top. A rooflight that opens on a switch turns that banked heat from a problem into a purge you run for ten minutes on a summer evening.

None of the mechanics matter if the glass is wrong for the roof it is sitting in, and on a two-storey extension the roof is usually the highest and least shaded plane on the whole house. There is no eaves overhang above it, no neighbouring roof casting across it, and often an open view of the sky in every direction. Whatever the sun is doing, this rooflight sees all of it.

That is fine on a north-facing plane, where every bit of daylight is welcome and there is no direct beam to worry about. It is a real consideration on a south or west-facing one. Southend faces south across the Thames Estuary, which is unusual for an English seaside settlement, and a great deal of the housing stock across south Essex, particularly the 1930s semi belt, has been extended to the rear with the garden and therefore the glass facing south. Lift that glass up to the roof of a two-storey extension and you have a solar collector at the highest point of the house, feeding heat into exactly the tall space that was already going to bank it up.

The answer is not to make the rooflight smaller or to skip the opening motor. It is to specify the glass for the aspect. A south or west-facing electric rooflight on a two-storey extension wants solar-control glazing with a low G-value, around 0.3 or below, so that most of the summer heat is turned away at the glass before the stack effect ever has to deal with it. A north-facing one can take clear glass and a higher G-value to make the most of the light. The G-value is the number that decides how the room feels on a July afternoon, and it belongs on the quote next to the U-value, per roof plane, not averaged across the roof.

Glare is the other half of it. A rooflight overhead in a tall space with a stairwell or a galleried landing throws bright light down through the volume, which is part of the appeal, but low-quality glass on a south plane turns that into a harsh, hot shaft in the afternoon. Selective solar-control glass keeps the brightness and takes out the heat, so the space stays light without becoming a greenhouse.

Electric opening rooflights come in the same size families as manual ones. Common individual sash sizes run from around 550 by 780mm up to 940 by 1600mm and beyond, with roof lanterns and large flat units made to order well past that. On a two-storey extension the useful figure is not just the glass area but the free ventilation area the opening gives, since that is what drives the stack effect. A larger sash and a wider opening move more air, which is why a generously sized electric unit at the top of a tall space does more than the same glass area split into smaller fixed lights.

A new or enlarged rooflight in a dwelling is notifiable under Building Regulations, and a two-storey extension brings a few parts into play at once:

• Part L, thermal performance, sets a maximum U-value for the rooflight. Any unit worth buying beats it. Typical rooflight U-values run from about 1.6 W/m²K for a basic double unit down to around 0.8 for a good triple.
• Part K, safety glazing, requires laminated glass on the inner pane of anything overhead, so a broken pane cannot fall into the room below. This is not optional on a rooflight at height over a space people stand in.
• Part F, ventilation, is where an opening rooflight can help the whole extension satisfy the airflow the room needs, particularly as a high-level outlet.
• Part O, summer overheating, applies to new dwellings rather than to an extension on a house you already own, so for most extension work nothing in the regulations forces anyone to think about solar gain. That is precisely why so many specifications ignore it, and why a compliant roof full of clear glass can still cook a top-floor room.

New rooflights on a house are generally permitted development where they project no more than 150mm beyond the roof plane and sit below the ridge, though flats, listed buildings, conservation areas and Article 4 zones are exceptions and a two-storey extension may need full planning permission in its own right. Treat that as general guidance and check with your local authority. Where a Building Control notification is needed, we make it to Southend-on-Sea City Council, or the relevant authority for your address, as part of the installation.

If you are planning a two-storey extension and there is a rooflight anywhere in the design, decide early whether it opens, and if it does, whether the reach makes it electric. On most two-storey work it will. We have been installing skylights and rooflights across south Essex for over fifteen years, every installation backed by a ten-year workmanship guarantee, and the opener, the cable route and the glass are all settled at survey. If you want the numbers for your own roof, including the G-value for each plane and the ventilation area of the opener, ask us for a specification before the first fix goes in.