Can a roof lantern be opened?

Three places a lantern can open

The three routes are not interchangeable, and the right one depends on what you want the opening to do.

A lantern with no opening at all is a legitimate choice too, and on a kitchen with a good extract system and openable doors it is often the right one. Every opening part is a junction that has to be weathered, so if it is not doing work, it is a liability rather than a feature.

A vent in the glazed slope

One of the glass panels is replaced with a hinged, framed opening light, usually top-hung and usually opening outwards. It is the arrangement most people picture when they ask the question.

It works well and it is visually discreet, because from below the vent reads as a slightly different pane rather than as a hatch. The catch is that the opening area is limited by what the frame can carry: a hinged sash of insulated glass is heavy, and hinges, stays and an actuator all have load limits. Expect a single panel rather than several, and expect the opening angle to be modest, often 200 to 300mm at the leading edge rather than a wide swing.

Hot air leaves through it well because it sits high in the room. Rain sits on it as it opens, so the drainage detail at the head of the sash is the thing to ask about.

A vent along the ridge

A ridge vent lifts a section of the ridge itself, opening a continuous slot along the highest point of the lantern. Because warm air collects there, it is the most effective arrangement for shifting heat and steam out of the room per millimetre of opening.

It is also the most discreet from outside, since nothing swings out into view, and the aperture is protected by the geometry of the ridge so it can usually stay open in light rain. It is almost always actuated rather than manual, because there is no way to reach it by hand. The mechanism, the control options and how it is wired are covered on roof lantern ridge vents and how they are operated.

Openings built into the kerb instead

The route people forget. Rather than opening the glazed structure, you put opening lights into the vertical face of the kerb, or raise the kerb deliberately to accommodate them.

The advantages are real. The opening parts are vertical, so rain runs off rather than sitting on them. They are cheaper per square metre of free area than a glazed vent. They can be much larger. And they draw air across the room rather than only upwards, which is what actually cools a space on a still summer evening.

The disadvantages are equally real. The kerb has to be tall enough to take a casement, which makes the whole assembly more prominent from the garden and may take a permitted development extension over its height limit. It also complicates the weathering at every corner. It suits a purpose-built extension where the design allows for it, and rarely suits a retrofit.

Manual, pole-operated or actuated

How you work the opening matters more day to day than which type you chose.

• Manual with a winder or pole. Cheapest and simplest, no power needed, nothing to go wrong. Requires the vent to be within reach of a pole, which on a lantern over a 2.7m ceiling it usually is, and requires somebody willing to fetch the pole.
• Chain or spindle actuator. A small motor on the sash, wired to a switch or a wireless controller. Reliable, quiet, and the standard choice on a glazed vent.
• Linear actuator on a ridge. A concealed ram lifting the ridge section. More capable, more expensive, and it needs a power supply run to the lantern before the ceiling is closed.

The decision to make early is the cable. Running a supply to a lantern after the plastering is finished means lifting a ceiling. If there is any chance of a motor later, run the cable now and cap it off. We cover the electrical side more broadly on opening rooflights electrically.

What an opening vent does for the room, and what it does not

An opening vent in a lantern is very good at removing hot air that has already collected and at clearing steam and cooking smells. It is a stack: warm air leaves at the top and cooler air is drawn in at a door or window lower down. That is genuinely useful in a kitchen.

What it does not do is prevent overheating. Solar gain arrives through the glass regardless of whether a vent is open, and by the time the air is hot enough to need venting, the surfaces in the room have already absorbed heat. If a room is too hot in July, the answer is the glass specification and shading outside it, with ventilation as a support act rather than the fix. That is set out on whether a roof lantern makes a kitchen too hot.

What happens when it rains and nobody is home

Rain sensors exist and they work, within limits. A sensor on the roof detects moisture and drives the vent closed, typically in under a minute.

The limits are worth knowing. A sensor detects rain that has started, not rain that is about to, so a heavy shower puts some water through the opening before it closes. It cannot tell the difference between rain and heavy dew, so it will close on a clear autumn morning. And it needs power, so it does nothing in a cut unless there is a battery backup. Treat it as a device that reduces the consequences of forgetting, not as a device that lets you leave a vent open indefinitely.

Weathering an opening part is the harder detail

Every opening in a lantern is a break in a weathered surface, and it is the place a poorly made lantern shows itself first.

What matters is that the vent is part of a drained system rather than relying on a compression gasket alone: water that gets past the outer seal has to reach a channel and drain out over the kerb, not sit in the frame. Ask how the head of the vent is drained, ask whether the hinges are stainless, and ask what the sash weight limit is against the size you are being sold. Those three answers tell you whether the vent has been engineered or bolted on.

Deciding before the frame is made

The reason this page exists is that none of these choices can be made later. The vent position, the opening type, the actuator and the cable route all have to be settled before the frame goes into production, because each of them changes the sections that get cut.

On the survey we ask what the room is used for, where the extract is, which way the prevailing wind comes from and whether anybody will realistically climb to a pole. Then we specify one vent that will be used rather than three that will not. Fifteen years of doing this in Oxford, Building Control notification handled by us, and ten years on the workmanship.