Soffit and Tile Vent Terminals for Cooker Hood Extraction: Positioning and What to Specify
A UK guide to soffit vent terminals and roof tile vents for kitchen extraction: when to use each, 150mm duct compatibility, non-return requirements, frost protection, and what to buy.
The cooker hood extraction duct has to exit the building somewhere. Through the external wall is the most common route. Through the soffit, into the roof void and out via a tile vent, is the route when the hob sits on a wall below the eaves. Getting this termination point right determines whether the extraction works effectively for the life of the kitchen. A poorly chosen terminal lets rain in, allows backdraughts on windy days, and gives nesting birds or insects a way into the duct. A correctly specified terminal with a non-return flap does none of these. It costs a few pounds and takes ten minutes to fit, but it is the single component people skip or get wrong most often, and the consequences only show up six months after the plasterers have left.
What a vent terminal actually is
A vent terminal is the fitting that covers the exit point of an extraction duct where it penetrates the building envelope. It does three jobs. It provides a weatherproof seal around the duct penetration so wind-driven rain cannot track back into the structure. It lets extracted air escape freely when the hood is running. And it stops backdraughts, rain, and wildlife getting into the duct when the hood is off.
In the UK, Building Regulations Approved Document F sets the rules for mechanical extract ventilation, which includes cooker hood extraction. The duct must terminate at a wall, soffit, or roof position to outside air. It must never discharge into a roof void, because the warm, moisture-laden air condenses on the cold roof timbers and causes rot, staining, and eventually mould in the insulation. And it must never connect into a shared flue or chimney stack serving a boiler or open fire, because flue gases can be drawn back into the kitchen.
So the choice is not whether to fit a terminal. It is which type, and where.
The soffit vent terminal
The soffit vent is the most common termination for a kitchen extension duct that runs horizontally from the hood and exits through the soffit board at the eaves. A 150mm circular knockout is cut in the UPVC soffit board, and the terminal is pushed through from below and secured with clips or screws. Most soffit vent terminals have a round spigot on the internal face that connects to the 150mm duct, a grille or louvre on the external face, and an internal non-return flap that opens under positive airflow and closes by gravity when the fan stops.
What to look for when specifying a soffit terminal:
- 150mm bore, to match standard kitchen extraction duct
- An integrated non-return flap, gravity or spring return
- White or brown UPVC to match the soffit colour
- A wall-thickness tolerance that suits your soffit board. Most UPVC soffit is 8mm to 10mm thick. Check the terminal's clamping range covers up to at least 12mm so it grips the board cleanly without rattling.
The soffit run is the cleaner option where the geometry allows it. The duct stays inside the warm envelope for most of its length, the terminal is easy to reach for cleaning, and there is no roof penetration to weatherproof.
The tile vent terminal
Where the duct route travels up through the ceiling and into the roof void, it has to exit through a tile vent fitted between the roof tiles. A tile vent is a purpose-made moulded plastic terminal that replaces one or two roof tiles. It has a central duct spigot and a weatherproof skirt that integrates with the surrounding tile profile so water runs over and around it rather than under it.
The critical specification point is profile matching. The vent must match the roof tile profile exactly. Most UK roofing suppliers carry tile vents for the common concrete and clay profiles such as Marley Modern, Redland 49, and the various smooth concrete patterns. A tile vent made for the wrong profile will not sit flush with the tiles around it, and that gap is a direct water ingress point. Take a photo and the tile name to the merchant, or pull a spare tile if you have one, before you order.
The duct enters the tile vent from below, routed up through the roof void. Where it passes through a fire-resisting ceiling it must be sealed with a fire collar to maintain the fire rating of that ceiling. The tile vent itself should have an integrated non-return flap, the same as the soffit type.
The non-return flap is not optional
Every extraction terminal must include a non-return flap, sometimes called a backdraught shutter. Without one, wind pressure on a gusty day forces air back down the duct and into the kitchen. You get cold draughts when the hood is off and reduced extraction efficiency when it is on, because the fan is fighting the incoming air.
The flap comes in two forms. A gravity-operated flap is a plastic or metal plate that opens under positive airflow and closes under its own weight when the fan stops. A spring-loaded flap is lighter and opens at a lower airflow, but the spring can relax over years and needs an occasional check. Gravity flaps are simpler and more reliable over the long term, which is why they are the default on most domestic terminals.
Confirm the flap is built into the terminal you are buying, not sold as a separate accessory you have to remember to order. Some bargain grilles are just that, a grille with no shutter behind it.
Frost protection for roof void runs
Where the duct runs through a cold roof void before it reaches the tile vent, condensation forms inside the duct in winter. Warm wet air from cooking hits the cold duct wall, the moisture condenses, and it runs back down the duct towards the hood. Insulating the duct through the void stops this. Wrap the duct in 25mm to 50mm mineral wool and tape it closed so the duct surface stays warm enough that the air does not condense against it.
In a north-facing roof void, or in a genuinely cold winter climate, this is not a nice-to-have. It is the difference between a dry duct and water dripping back into the extractor. Duct insulation wrap is sold by Screwfix and Toolstation in 1m and 5m lengths.
What to buy
| Type | Supplier/brand | Price approx | Notes |
|---|---|---|---|
| Soffit vent terminal 150mm, white, non-return | Screwfix own-brand or Manrose | £5–12 | Simple, widely stocked, standard installation. The default for an eaves exit. |
| Soffit vent terminal 150mm, brown | Manrose, Titon | £8–15 | Same product in brown UPVC for brown soffit boards. |
| Tile vent 150mm, profile-specific | Brown, Timloc, Manthorpe | £12–30 | Match to your tile profile. Roofing or builder's merchant. Profile compatibility chart from each maker. |
| External wall cowl 150mm | Screwfix or Toolstation | £5–15 | For a direct through-wall exit rather than soffit. Non-return integral. Less common on modern extensions. |
Screwfix and Toolstation stock the soffit terminal and the wall cowl, and you can click-and-collect both same day. Tile vents are more specialised. Buy them from a roofing merchant such as SIG Roofing, Roofdepot, or the roofing section at Travis Perkins, or direct from the tile manufacturer, where you can confirm the profile match against your roof.
Tip
Take a clear photo of your roof tiles and note the tile name if you can find it before ordering a tile vent. The merchant can match the profile from a photo, but a wrong-profile vent that has to be returned can cost you a week of waiting while the scaffold sits idle.
Common mistakes
Terminating the duct into the roof void rather than running it through a terminal to outside. The air does not magically leave the building. It condenses on the cold roof structure, soaks the insulation, and over a winter or two produces staining, rot, and mould. This is the most damaging mistake and the one Building Control will pull you up on.
Using a 100mm terminal on a 150mm duct. Fitting an adaptor down to 100mm and then a 100mm terminal restricts airflow dramatically, cutting extraction by around half. Keep the full 150mm bore all the way to the open air.
Fitting a terminal with no non-return flap, so the kitchen gets backdraughts every windy day.
Choosing a tile vent without checking the tile profile, leaving a gap that lets water track under the surrounding tiles.
Running the duct to a soffit position on the same wall as a boiler flue. Flue gases from the boiler can be drawn back into the extraction duct and into the kitchen. Keep the extraction terminal well clear of any combustion appliance flue.
Warning
A duct that simply ends in the loft is a Building Regulations failure under Approved Document F and a slow-burning damp problem. If a builder tells you the duct will "vent into the loft space", stop them. It must terminate to outside air through a soffit vent, tile vent, or wall cowl.
Where you'll need this
- Extractor and ventilation, where you decide the hood type, airflow rate, and duct route
- Kitchen plumbing provisions, where the duct routing is agreed and run before the walls and ceilings are closed
Decide the duct route and termination point at specification stage and confirm it again at first fix. Once the plasterboard is up and the soffit and roof are finished, moving the exit point means opening up finished work, which is exactly the cost this page is written to help you avoid.