Guide
Using an opening rooflight to ventilate a room
A skylight that opens does a second job besides letting light in: it lets the house breathe. How the air moves, which rooms gain most, how the sash should open and the limits worth knowing before you buy.
Why does an opening rooflight clear a room so well?
Warm air rises and a rooflight sits at the top of the room. Open it and the heat, steam and stale air leave there, drawing cooler air in from lower down.
A side window and a rooflight both open to the outside, but they do not work the same way. A window sits at head height, so air moves through it more or less level. A rooflight sits above you, at the highest point the room has, and that is where the warmest and most used air collects.
The stack effect in a house
Air heated by cooking, bathing, people and sunshine gets lighter and climbs towards the ceiling. When there is a way out at the top, that air leaves and cooler air is pulled in elsewhere to replace it. Builders call this the stack effect. It runs without a fan and without any wind, and it is strongest when there is a big difference between the temperature of the room and the air outside.
What this means in practice
On a still, warm afternoon a room with an opening rooflight and an open door lower down will keep drawing fresh air through, while the same room with only a side window may sit stuffy. In cool weather the effect is stronger and quicker, which is why a small opening is often all that is needed to clear a steamy bathroom.
Where the effect stops
The air has to come from somewhere. If every other door and window is shut, the rooflight has nothing to draw on and the room clears slowly. A gap at the bottom of a door, a window cracked on another floor or a stairwell leading down will help far more than a larger opening at the top.
Which rooms in the house gain most from an opening rooflight?
Rooms that collect steam, heat or still air, such as a windowless bathroom, a kitchen extension, a landing and a loft room, gain most. Rooms with plenty of side windows gain less.
The best rooms for an opening rooflight are the ones where air gets trapped. Some are obvious, and some only show up once the house is lived in.
Bathrooms and utility rooms
An inner bathroom in a terrace may have no outside wall at all. A rooflight over it is a window where none could go, and opening it after a shower clears steam before it settles on the paint. Our guide to sun tunnels covers the light-only alternative, but a tunnel does not open, so the choice between the two is a choice about ventilation.
Kitchen extensions
A kitchen in a glazed extension gets cooking heat, steam and sun together. A rooflight over the hob end lets that rise away instead of building up under a flat ceiling. It will not replace a cooker hood, since grease and smells need dealing with at the source, but it takes the heat off the room as a whole.
Landings and stairwells
The stairwell is the chimney of a house. An opening rooflight at the top of it pulls air up from the ground floor, through open doors, and out. That can cool the whole house on a warm night with very little effort.
Loft rooms
Rooms in the roof run warm in summer because the sun heats the roof itself. An opening roof window at the high end of the room lets heat out, and a second opening lower on the slope lets cooler air in. Together they ventilate far better than either alone.
| Room | Air problem | Helpful arrangement |
|---|---|---|
| Inner bathroom | Steam with no outside wall | Small opening, left ajar after bathing |
| Kitchen extension | Cooking heat plus sun | Opening over the working end |
| Landing | Warm air collecting at the top | High opening with doors left open below |
| Loft room | Roof-heated, still air | Opening at the top, another low on the slope |
How should the sash open for ventilation?
Top-hung, centre-pivot, raised-hinge and sliding rooflights all vent differently. Pick on how wide you need the gap, how the weather reaches it, and who can reach the control.
The type of opening decides how much air you move and how much weather comes with it. A few hinge arrangements cover most of what is fitted.
Hinged at the top
A top-hung sash swings out from the lower edge, like a shop awning. Rain on a pitched slope runs past the gap, and the sash itself shelters it, so it can be left slightly open in light weather. The opening is widest at the bottom, which suits letting warm air out and cool air in at the same gap.
Centre-pivot
A centre-pivot sash rotates about a middle axis, with the top going out and the bottom coming in. It gives a large clear gap, which is useful for cleaning the outside glass from indoors, but at the lower edge it brings the sash into the room. Furniture beneath needs to leave space for it. The trade-offs are weighed up when comparing centre-pivot with top-hung roof windows.
Raised and sliding units
On a flat roof, an opening rooflight may lift its whole sash on arms, giving a gap around the entire edge, or slide one pane over another. The lifted version vents quickly and the sliding version stays low. A lifted sash on a flat roof also stands further above the upstand, which can affect the planning height on a house.
Restrictors and security
A restrictor stops the sash opening further than set, which is sensible above a stair or over a child's bed. It also lets you leave a rooflight ajar overnight without leaving an opening big enough for anyone to get through.
What about rain, noise and heat gain?
An open rooflight lets in weather and sound. Rain sensors close it automatically, a lowered gap keeps noise down, and solar control glass reduces the sun's heat while it is shut.
The same opening that clears a room also lets the outside in. The trick is to choose features that let you take the good without the rest.
Rain
A rooflight left slightly open can get wet in a driving shower. An electric unit that carries a rain sensor shuts itself, and a manual one can be fitted with a restrictor so that only a small gap is ever open. We explain the sensor in our guide to rain sensors and controls.
Noise
Sound travels through an open window straight into the room. A rooflight above a bedroom on a busy road, near a school or under a flight path will let all that in when opened. A narrow gap lets less in than a full opening, and a closed rooflight with good glazing keeps most of it out. If sound is a worry, test it before relying on the rooflight for night ventilation.
Sun and heat
An opening rooflight clears hot air, but glass on a south or west slope heats a room quickly when the sun is on it. Solar control glazing cuts that heat when the sash is closed. For a rooflight over a room used by day, the pairing of coated glass and an opening sash gives you the controls you need in both seasons. Rooflights over rooms normally have laminated inner panes, which also soften sound.
Winter use
In cold weather a short burst of ventilation does more good than a long, draughty one. Open wide for ten minutes, close fully, and the room keeps most of its heat.
Does an opening rooflight change any of the rules?
On a house the same 150mm projection and highest-point limits apply, and a side-elevation slope has extra conditions. A flat of any kind has to apply to the council for a new rooflight.
Choosing a sash that opens does not change who decides whether it can go in. It adds a few points to check.
On a house
A rooflight on a house is permitted development under Schedule 2 Part 1 Class C of the GPDO 2015 if it projects no more than 150mm beyond the plane of the roof slope and sits no higher than the highest part of the roof. A sash that opens upward on arms is measured as installed, so ask for the closed height over the roof. Where the slope faces the side of the house, the window needs obscure glass throughout, and it cannot open unless every opening part is more than 1.7 metres above the room's floor.
On a flat
Flats and maisonettes have no householder permitted development. That includes a Tyneside flat on either floor: a new rooflight there needs the council's planning permission. Your title or lease names who has the roof, so check it and ask the neighbouring owner first.
Other things that can change it
An Article 4 direction or a condition on an earlier planning permission can remove Class C from a house. Living in a conservation area does not by itself take Class C away from a house. A listed house keeps Class C on the planning side, but almost any rooflight on it needs listed building consent, and that consent decides the job. Our planning guide covers each case.
Building Control
Cutting a new opening is building work. It is notified to Building Control, and we handle that. Under Part L for existing dwellings in England, the limiting U-value is 2.2 W/m²K for a rooflight and 1.4 W/m²K (or WER Band B) for a roof window.
Our view: choose the opening for the room's air, then check the height, the roof owner and the council before ordering.
What should you ask before choosing an opening rooflight?
Ask where the replacement air will come from, how far the sash opens, how it is operated and who can reach it. The best opening is the one you will actually use.
A rooflight that is awkward to open will stay shut, and then it is only a window. A few questions at the start save disappointment later.
Can you reach it?
Manual rooflights need a pole, a handle or a cord, and a high stairwell is a long reach. If you cannot get at the control without a stool, an electric motor is worth considering. Our hub on electric and opening rooflights shows the options side by side.
Where does the cool air come from?
Walk the house in your head with the rooflight open. Is there a door or window low down that can be left ajar? If the room is closed off, ask whether a small vent or a gap under the door is possible, and whether the room beneath is warm enough for you to want to pull air through it.
What is the sash for?
If the main aim is to get a bathroom clear of steam, a small hinged opening is enough. If it is to cool a whole upstairs on summer nights, you want a wide opening and a clear path for air below. Say which job matters when we visit, because the right unit follows from it.
Setting the price
No price is given until the free survey has been done, and the quote that follows is fixed and in writing. Unit type, size, glazing, controls and access all shift the figure, as the costs page explains.
Fresh air and opening skylights: common queries
Can an opening rooflight replace an extractor fan?
Not entirely. A rooflight clears steam and heat when it is open, but it does nothing when it is shut and it cannot filter or duct away cooking smells at source. For a bathroom with no outside wall, the two together work well.
Is it safe to leave a rooflight open overnight?
With a restrictor that limits the gap, yes. Check that the restricted opening is too small for anyone to climb through, and that the sash locks properly when closed. A rain sensor adds protection if the weather turns.
Will an opening rooflight make a summer loft room cooler?
It helps a great deal when paired with a second opening lower on the slope or a window below, so air can pass through. Solar control glass cuts the heat coming in while the sash is shut, and the opening takes out what builds up.
Do I need an electric unit to ventilate a high ceiling?
Not strictly, since poles and winding handles work, but a rooflight above a stairwell or double-height space is out of ordinary reach. An electric motor makes sure it is used. The hub page covers how electric systems work.
Does opening a rooflight lose a lot of heat in winter?
A short, wide burst clears damp air quickly and loses less heat than a window left cracked for hours. Close it fully once the air has changed. The rooflight's own insulation when shut is covered by Part L limits.