What Ventilation Do Building Regulations Require in an Extension?
Building Regulations Part F requires a habitable extension to have both purge ventilation and background ventilation. Purge ventilation clears pollutants and moisture quickly and needs an openable area of at least 5 per cent of the room’s floor area, provided by windows, doors or rooflights. Background ventilation is a continuous fresh-air trickle from vents or a mechanical system. An openable rooflight counts directly towards the purge requirement, and because it sits at ceiling level it is the most effective openable area you can have for clearing warm, moist air.
The two duties Part F sets
Ventilation is not one number. Part F asks for two different things, and one does not substitute for the other.
| Duty | Purpose | How it is measured | How a rooflight helps |
|---|---|---|---|
| Purge | Rapid clearing of moisture and pollutants | Openable area, at least 5% of floor area | An opening rooflight counts directly |
| Background | Continuous low-rate fresh air | Equivalent area in mm² per room | Trickle vents or mechanical system, not the rooflight |
Purge ventilation
The working requirement is an openable area of at least 5 per cent of the floor area of the room. For a 20 square metre kitchen extension that is 1 square metre of openable area, provided by any combination of windows, doors and rooflights. This is the calculation an opening rooflight contributes to directly.
Background ventilation
This is the continuous trickle, measured as an equivalent area in square millimetres and provided by trickle vents in frames or a mechanical ventilation system. An opening rooflight left shut does nothing for background air, and a trickle vent cannot clear a moisture spike, which is why the two are separate. We set out why one does not replace the other on the electric rooflight versus trickle ventilation answer.
How the rooflight earns its place in the calculation
In a side-return kitchen extension the only external wall is often mostly glazed doors, and those doors nominally provide plenty of openable area. But they only ventilate the garden end of the room, and few people open them in a Suffolk winter. The deep part of the plan, where the hob usually sits, gets no air movement. An opening rooflight over that deep end adds usable purge area exactly where the doors cannot reach, and it is frequently what gets the room over the line on paper and in practice. The wider problem of sealed extensions is covered on the ventilation in sealed extensions page.
Position beats size for ventilation. Warm, moist air stratifies at ceiling level, so an opening at the high point of the room removes it directly rather than letting it mix back down. A rooflight opening 300mm at ceiling height does more useful work than a window of the same free area at 1200mm. Paired with a low inlet, it drives the stack effect with no fan and no power.
Overheating, under Part O
Part F is not the only regulation ventilation touches. New dwellings, and some larger extension and conversion work, are assessed for overheating risk under Part O. Removable heat is credited from openable area, and an opening rooflight at the highest point of the room is the most effective openable area for the job, because it vents the hot air where it collects. If overheating is your main concern, glazing does at least as much as ventilation: solar control glass with a g-value around 0.3 stops the heat arriving in the first place, which the energy efficient skylight installation page covers.
Getting it right on your project
The exact figures for your room come from your Building Control body or your designer against the current Approved Document, and the openable area an opening rooflight contributes depends on how far the sash opens, which is where the actuator choice matters. The main electric skylight installation page covers actuators and opening area, and the when to choose electric answer helps decide whether the ventilating unit needs to be motorised, which it usually does when it sits high on a sloping ceiling.
Common questions
Does an opening rooflight count towards Building Regulations ventilation?
Yes, towards purge ventilation. Part F asks for an openable area of at least 5 per cent of the room’s floor area, and a rooflight counts alongside windows and doors. In a side-return kitchen where the only external wall is glazed doors, an opening rooflight over the deep end is often what gets that part of the room over the line. It does not count towards background ventilation, which comes from trickle vents or a mechanical system.
How much openable area does my extension need?
For purge ventilation, at least 5 per cent of the room’s floor area, so a 20 square metre room needs about 1 square metre of openable area from windows, doors and rooflights combined. Background ventilation is separate, measured as an equivalent area in square millimetres from trickle vents. Your Building Control body or designer confirms the exact figures against the current Approved Document for your specific room.
Can a rooflight replace trickle vents?
No. A rooflight provides purge ventilation, the rapid clearing of moisture and pollutants, while trickle vents provide background ventilation, the continuous low-rate fresh air. The two are separate duties under Part F and one does not substitute for the other. A rooflight left shut does nothing for background air, and a trickle vent cannot clear a moisture spike from cooking. You need both.
Get a fixed price for compliant ventilation
Tell us the room size and layout and where the doors sit. We advise where an opening rooflight best contributes to your purge ventilation, and give you one written price covering the unit, the controls and the electrical connection, fixed after survey with a 10-year workmanship guarantee.
Request a fixed quote for an opening rooflight to meet extension ventilation requirements across Ipswich, Suffolk and north Essex.
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