An AOV rooflight is an automatic opening vent: a rooflight over a stairwell or lobby that opens automatically on a smoke detection signal to clear smoke from an escape route. It is life-safety equipment, not a comfort vent, and its size, control panel and free area are set by the building’s fire strategy.
The letters stand for automatic opening vent. You will also see AOV smoke vent, smoke ventilator and stairwell vent used for the same thing. On domestic work it almost always appears at the head of a stair core, which is why it turns up on loft conversions.
What an AOV actually does
A stairwell in a fire fills with smoke from the bottom up, and smoke, not flame, is what stops people getting out. An AOV sits at the top of that stair core and vents the smoke vertically out of the building, keeping the escape route usable for longer and giving the fire service a cleared route in.
It does that without anyone touching it. A smoke detector on the escape route signals the AOV control panel, the panel drives the actuator, and the vent opens. Nobody has to find a handset, and nobody has to be conscious.
Five things separate an AOV from a normal electric rooflight, and every one of them is a cost item:
- A dedicated control panel. Not the rooflight’s own controller. A separate panel with battery backup, commonly rated for 72 hours of standby, so the vent still works when the fire has taken the power out.
- An interface with the fire alarm system. The vent has to be triggered by detection, which means the alarm contractor and the rooflight have to talk to each other.
- Manual override at ground level. A break-glass unit or key switch on the ground floor so the fire service can open or close the vent from where they are standing.
- A specified geometric free area. Commonly 1m² over a domestic stair core. This is a measured opening area, not the size of the rooflight. A 1.2m by 1.2m unit does not give 1m² of free area, because the sash only opens so far.
- Fail-safe to open. Lose power or lose the detection signal and the vent opens. That is the opposite of a comfort rooflight, and it is the right behaviour for life safety.
The free area point is where specifications most often go wrong. Getting a real 1m² out of a domestic-sized unit usually needs a hinge or spindle actuator with a wide opening angle rather than the chain actuator that comes as standard on a comfort rooflight, which is why an AOV is not simply a normal rooflight with a different label on the box.
Where a fire strategy requires one
An AOV is required because a fire strategy says so, not because a rooflight seemed like a good idea. The situations where one commonly appears on work we survey around Ipswich, Suffolk and north Essex:
| Situation | Why it comes up |
|---|---|
| Two-storey house converted to three storeys | A loft conversion adds a storey. Part B generally requires a protected stairway to a final exit, and smoke ventilation at the head of the stair is a standard part of how that is achieved. |
| New three-storey or taller dwelling | Designed in from the outset by the architect, with the vent sized in the fire strategy. |
| Flat conversion with a common stair | Shared escape routes carry their own smoke control requirements, usually stricter than a single dwelling. |
| Basement conversion with a single stair | A single escape route out of a basement often needs smoke clearance designed for it. |
| Inner rooms or long internal corridors | Where the escape route passes through space with no external wall to open. |
None of that is a checklist you can apply to your own house from a web page. It is a list of the circumstances in which somebody with the relevant qualification will need to look at your drawings.
We install AOVs. We do not specify them. The fire strategy for your building sets the free area, the control arrangement, the detection interface and the override position. That strategy comes from a fire engineer, the designer or Building Control. It does not come from us, and it does not come from your builder’s preference. This page tells you what an AOV is. It is not a substitute for a fire engineer’s design, and nothing on it should be used to size or omit one.
What that means if you are planning a loft conversion
Ask the question at drawing stage. If your loft conversion takes the house to three storeys and the drawings say nothing about smoke ventilation, that is worth raising with your designer before anything is ordered, because retrofitting a vent, its panel, its cabling and its alarm interface after the plasterboard is on is a genuinely miserable job.
Lead times matter too. A standard electric rooflight runs three to five weeks. An AOV unit with a control panel and a fire alarm interface runs five to eight weeks, and it needs the alarm contractor available for commissioning, so it wants ordering early rather than at the point the roof is opened.
On coastal roofs at Felixstowe, Harwich or Aldeburgh the actuator and its bracketry get the same marine treatment as any other unit we fit: 316L stainless throughout rather than 304, sealed cable entries, and the control panel sited inside the warm envelope rather than in a cold roof void where it will condense. A life-safety device is the last thing that should be corroding quietly in a loft.
Common questions
Is an AOV the same as an opening rooflight?
No. They can look similar from the ground, but an AOV has a dedicated control panel with battery backup, an interface with the fire alarm, a ground-level manual override and a specified measured free area. A comfort rooflight has none of those. An AOV also fails open on power loss, where a mains comfort unit fails safe in whatever position it is in.
Can an AOV be used for everyday ventilation?
Some systems allow a comfort-open function alongside the smoke function, but it has to be part of the specified design and it must never compromise the life-safety operation. Whether it is permitted on your building is a question for whoever wrote the fire strategy, not for us.
Next steps
Our AOV smoke ventilation rooflight installation page covers the panel, the detection interface, commissioning and free area calculation in full detail, and what an AOV smoke vent costs to install gives published figures. For the wider picture, see electric skylight installation, and for how ordinary units behave, read what happens in a power cut and what controls come with an electric rooflight.
If a designer or Building Control has specified an AOV for your project, send us the free area and the control requirement and we will price the installation. 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.