An electric opening rooflight installation is the fitting of a powered opening roof window or flat rooflight, where an actuator drives the sash instead of a pole or a handle. The install is decided by four measurements taken at survey: the actuator stroke, the resulting opening angle and free area, the clearance the sash needs to swing through, and the upstand or trimmer height it sits on. Get those right and the controls are the easy part.
Our electric skylight installation page covers the service as a whole, the prices and the electrical split. This page is the fitting detail underneath it: what the numbers mean, where they bite, and what we measure before anything is ordered.
Stroke, angle and free area are three different numbers
People use them interchangeably and they are not the same thing. Confusing them is how a rooflight ends up opening beautifully and still failing a Part F purge ventilation calculation.
Stroke is how far the actuator physically pushes, measured in millimetres along the chain or the arm. A chain actuator gives 200mm to 500mm depending on model. Opening angle is how far the sash rotates about its hinge as a result, and that depends on stroke and on where the actuator is mounted relative to the hinge line. Free area is the measured area of the gap the open sash creates, which is the number the Building Regulations care about.
A worked example
Take a 1500 x 1000mm flat rooflight, top hinged along the 1000mm edge, with a 300mm stroke chain actuator. At full stroke the leading edge of the sash lifts roughly 300mm. The opening is not 1500 x 300mm, because the two side gaps are triangular wedges rather than full rectangles, and the hinge edge contributes nothing at all.
The usable rectangular gap at the leading edge is about 1500 x 300mm, so 0.45m². Add the two triangular side gaps, each roughly half of 1000 x 300mm, so about 0.15m² each. That is a geometric total in the region of 0.75m², and manufacturers will publish a tested figure lower than the raw geometry because of the frame profile intruding into the gap.
Now check it against Part F. Purge ventilation for a habitable room works to an openable area of at least 5 per cent of the floor area. A 20m² kitchen extension needs 1m². That one rooflight on a 300mm stroke does not get there on its own. It contributes most of the way and the glazed doors finish the job, which is exactly the calculation a building control officer will want to see rather than a promise that the rooflight is big.
| Sash width | 200mm stroke | 300mm stroke | 400mm stroke | 500mm stroke |
|---|---|---|---|---|
| 800mm | 0.26m² | 0.39m² | 0.52m² | 0.65m² |
| 1000mm | 0.30m² | 0.45m² | 0.60m² | 0.75m² |
| 1200mm | 0.34m² | 0.51m² | 0.68m² | 0.85m² |
| 1500mm | 0.40m² | 0.60m² | 0.80m² | 1.00m² |
| 2000mm | 0.50m² | 0.75m² | 1.00m² | 1.25m² |
Treat that table as a sizing aid at survey stage, not as a submission figure. The manufacturer’s tested free area for the specific unit is what goes on a Building Regulations application, and it will be a little under the geometry because of the frame.
Choosing the actuator
The chain against hinge comparison sits on the hub page in table form. What that table does not tell you is how the choice actually gets made on a roof, so here is the sequence we work through.
Start with the free area you need
If the Part F number, or a Part O overheating assessment, demands a large opening from a modest sash, a chain actuator will run out of stroke before it gets there. Above roughly 0.9m² from a single domestic sash, you are into hinge or spindle territory with angular opening up to 60 degrees. Below that, a chain almost always wins on noise, on concealment and on cost.
Then check the exposure
A chain actuator holds the sash out on a chain that is stiff in compression but not infinitely rigid. On an east-facing roof at Felixstowe or Harwich with full open-sea exposure, a large sash at full stroke can flutter in a gust, and that flutter loads the chain and the hinges in a direction the design does not favour. Two answers: fit a restrictor so the sash never reaches full stroke on that elevation, or specify a hinge actuator with its 400N to 800N closing force and rigid hold at any position. On an estuarine site at Woodbridge or Pin Mill, where wind across open water is real but salt loading is moderate, a chain unit with a restrictor and a wind sensor is usually enough.
Then check whether it needs to be solar
Solar versions of chain actuators are widely available. Solar hinge actuators are rare, because the current draw is generally too high for a small panel and battery to sustain. If there is no cable route and the room needs a hinge actuator’s free area, that conflict has to be resolved at design stage rather than discovered on site. See solar powered rooflight installation for what the panel and battery will and will not do.
Then check the noise
Chain actuators run at roughly 40 to 45dB, which is quiet enough to sit under during a film. Hinge and spindle units run at roughly 50 to 55dB and are audible across a room. Over a bedroom this matters more than the specification sheet suggests, and it is worth hearing one run before you commit.
Restrictors, and why we set them at handover
A restrictor limits how far the actuator travels, either mechanically or as a programmed stroke limit in the control unit. Most installers leave the factory setting and go home. We set it deliberately, for four reasons.
- Clearance. If the sash swing arc reaches a parapet upstand, a soil vent pipe, an adjacent rooflight or an overhanging branch, the restrictor is what stops the glass finding it. This is the commonest reason we limit stroke on a terraced Ipswich rear return where the rooflight sits close to a party parapet.
- Wind loading. On exposed elevations, capping stroke at 60 or 70 per cent removes most of the flutter and costs you very little free area, because the last third of stroke adds less than you would think once the side triangles are accounted for.
- Rain ingress. A sash open 120mm takes far less driven rain than one open 400mm, and for background ventilation on a wet Suffolk week that is often all you want. Several systems allow two stored positions, a vent position and a full purge position, which is the arrangement we specify wherever the controller supports it.
- Part K guarding. Where an opening rooflight is within reach of a floor, a low sill or a raised platform, an opening that a person could fall through needs consideration under Part K. On a domestic roof this is rare, but on a rooflight served by a roof terrace it is not, and a restrictor is part of the answer alongside guarding.
Do not fit a restrictor to an AOV. A smoke ventilation rooflight has to reach its specified geometric free area every time it operates. Restricting its travel defeats the purpose of the installation and it is a life-safety failure, not an inconvenience. See AOV smoke ventilation rooflight installation.
Upstands, kerbs and the clearance envelope
An opening unit needs more room around it than a fixed one, and the requirement is different on a flat roof from a pitched roof.
Flat roofs
The insulated upstand goes to a minimum of 150mm above the finished roof surface at its lowest point, and 200mm on exposed coastal sites. Measure at the lowest point, because a flat roof has a fall in it and the upstand is level. A 1:60 fall across a 2 metre rooflight puts 33mm of difference between the two ends, and an upstand that measures 150mm at the high end is only 117mm at the low end, which is where water actually sits.
On top of that, an opening unit adds three clearances that a fixed unit does not need:
- Swing clearance. The arc the leading edge sweeps through. On a 1000mm deep sash at 400mm stroke, the leading edge travels outward as well as upward, and it needs roughly 150mm of clear space beyond the frame line on the opening side.
- Actuator housing clearance. The chain housing sits in the frame profile, but the mounting bracket and the cable entry need access for future service. Butting a rooflight tight against a parapet on the actuator side makes a battery or motor change into a scaffold job.
- Drainage clearance. Water shed off an open sash lands on the roof beside the upstand. If that point is also where a rainwater outlet has been squeezed in, you get a concentrated flow onto a detail that was designed for a spread one.
Pitched roofs
On a tiled or slated roof the unit sits within the rafter line with a flashing kit dressed into the covering. The clearances that matter are different: the tile batten gauge above and below the opening, the distance to the ridge and to the eaves, and whether the sash at full opening clears the tile course above it. Deep profile concrete tiles need a taller flashing kit than plain tiles or slate, and specifying the wrong one is a leak waiting for the first driven rain off the North Sea.
For loft rooms, mounting height matters as much as the mechanism. A top hung unit at chest height opens by hand perfectly well, which raises the honest question of whether it needs to be electric at all. Our electric or manual rooflight page works through that decision without trying to sell you the motor.
Control layout, planned before the plaster
Handsets run on radio at typically 868MHz, not Wi-Fi, with 20 to 30 metres of range through domestic construction. That much is on the hub page. The part worth planning is where the fixed controls go, because a wireless switch plate can be moved and a hard-wired one cannot.
| Control | Decide by | Consequence of deciding late |
|---|---|---|
| Radio handset | Any time | None, it pairs on the day |
| Wireless wall plate | Any time | None, it is surface fixed and movable |
| Hard-wired wall switch | Before first fix | Chasing a finished wall and redecorating |
| Smart home gateway | Before first fix | Needs mains and network at the gateway position |
| Wind sensor | At order | Some units cannot accept one retrospectively |
| Heated rain sensor pad | At order | Pad type is usually factory fitted |
| Battery backup module | Before commissioning | Needs space beside the control unit inside the warm envelope |
Two rules we apply on every job. Group units that should move together, so a kitchen with three opening rooflights operates as one command rather than three. And put a physical switch by the door people actually use, because a control that requires finding a handset gets used half as often as one on the wall.
What we check at survey, in order
- Room floor area and the Part F purge target it generates.
- Available sash size given rafter or joist positions, and whether trimming or a ring beam is needed.
- Required stroke to hit the free area, and therefore chain or hinge.
- Swing arc against every obstruction within 500mm of the frame line.
- Upstand height achievable at the lowest point of the fall, or flashing kit type for the tile profile.
- Exposure grade, and whether 316L stainless, sealed cable entries and a wind sensor apply.
- Cable route, walked in person, before anything is ordered.
- Control positions marked on the wall in pencil, with you standing there.
Standard electric units run three to five weeks from order. Nothing starts on site until the unit is physically here, because a roof opened around a delivery date is a roof that leaks on the week it slips.
Common questions about electric opening rooflight installation
How far does an electric rooflight actually open?
A chain actuator gives 200mm to 500mm of stroke depending on the model, which on a typical top hung sash works out at roughly 25 to 30 degrees of rotation. A hinge or spindle actuator opens angularly to as much as 60 degrees. The number that matters for Building Regulations is not the stroke but the free area it creates, and on a 1500mm wide sash a 300mm stroke gives roughly 0.6m² of geometric free area.
Can the opening be limited so it does not swing into something?
Yes. A restrictor caps the travel either mechanically or as a programmed stroke limit, and we set it at handover to the actual clearances on your roof rather than leaving the factory default. It is also the right answer on exposed coastal elevations, where capping stroke at 60 or 70 per cent removes most of the wind flutter and costs very little free area. Smoke ventilation units are the exception and must never be restricted.
How high does the upstand need to be for an opening flat rooflight?
A minimum of 150mm above the finished roof surface measured at the lowest point, and 200mm on exposed coastal sites at Felixstowe, Harwich or Aldeburgh. Measuring at the lowest point matters because the roof has a fall and the upstand is level. An opening unit also needs roughly 150mm of clear space beyond the frame on the opening side for the sash to swing through, plus access to the actuator bracket for future service.
Can I add an electric opening unit to a roof lantern?
Yes, and it is usually the cheapest way to get powered ventilation into a lantern, at £700 to £1,300 within the lantern installation. One or two panes of the lantern are specified as opening vents with their own actuator. The free area is smaller than a dedicated opening rooflight would give, so check it against the Part F target for the room before assuming it covers the purge duty on its own.
Get a fixed price for an opening unit
Tell us the room, its floor area, the roof type and roughly where the property sits. We survey the structure, the fall or the pitch, the swing clearances and the cable route, then give you one written price with the actuator type, the stroke, the free area and the glazing specification stated on it. That price is fixed after survey, and every installation carries a 10-year workmanship guarantee.
If you are still weighing the options, what an electric rooflight costs to install sets out the figures, how an electric rooflight opens is the short version of the mechanism, and what controls come with an electric rooflight covers the handset and switch options. When you are ready, request a fixed quote.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.