Phoenix Journal · Ductwork
Getting greasy extract air from a ground or podium kitchen safely out above a tall building is one of the hardest jobs in commercial ventilation. The route you choose - up an internal riser or up the outside wall - shapes fire risk, cleaning access and cost for the life of the building.
Kitchen extraction · High-rise buildings
When a commercial kitchen sits at the base of a tower, its extract air has to travel a long way up before it can safely leave the building - and every metre of that journey is a decision about fire risk, cleaning access and cost.
Approved Document B and BS 9999 both expect a commercial kitchen to have its own independent extract system that discharges to open air, and it is good practice to release that air at least 2m vertically above the highest point of the roof so grease-laden vapour cannot ignite the structure or drift back into other services. In a two-storey unit that is simple. In a fifteen-storey mixed-use block - flats over restaurants, a hotel over a lobby cafe - the duct may climb 40m or more through occupied floors before it reaches daylight.
There are broadly two ways to make that climb: send the duct up an internal fire-rated riser inside the building, or run it up the outside face of the wall to a rooftop fan. Both are used across the UK, both can be made compliant, and both have real trade-offs. Under the Building Safety Act, any building of at least 18m or seven storeys with two or more residential units is a higher-risk building, so on many towers this is not just an engineering choice - it is a decision the Building Safety Regulator and your insurer will scrutinise. Here is how the two routes compare.
Here the extract duct runs vertically inside the building within a dedicated, fire-rated shaft, passing through each floor's compartment line on its way to a rooftop plant area and discharge fan. Because the duct crosses fire compartments, it must maintain that separation - typically fire-rated ductwork tested to BS EN 1366-1 and classified under BS EN 13501, giving stability, integrity and insulation for a stated period. Fire dampers are not used in kitchen extract runs, so the fire rating is carried by the duct construction and enclosure themselves.
The alternative takes the duct out of the building near the kitchen and runs it up the outside face of the wall - often up a rear elevation or a service core - to a fan and terminal at roof level. Because the air is outside the compartmented interior for most of its climb, the fire-separation demands on the duct itself change, though the penetration where it leaves the building, the fixings, weatherproofing and the discharge point all still have to be engineered properly.
Whatever path the duct takes, the fire risk is the same physics. During cooking, grease-laden vapour leaves combustible deposits along the whole run from canopy to atmosphere, and in a tall riser those deposits collect exactly where they are hardest to reach. TR19® Grease, the BESA specification insurers and building owners now expect, treats the entire extraction route as a single system and grades risk by measured grease thickness - broadly, under 200 microns is well maintained, while 1,000 microns or more is a high fire risk warranting immediate cleaning. To keep grease moving up rather than settling, designers aim for a transport velocity in the region of 8-12 m/s, and the cleaning itself should be carried out and signed off by a technician holding the BESA Grease Hygiene Technician qualification.
The single biggest predictor of whether a high-rise system stays compliant is not the route - it is whether cleaning access was planned at the design stage. Retrofitting panels into a 40m riser or onto a finished facade is slow, costly and disruptive. Get the access right early and both routes can be kept demonstrably clean for the life of the building.
Questions
No. Guidance such as BS 9999 is clear that fire dampers should not be fitted in commercial kitchen extract ductwork, because grease build-up would stop them closing and the damper itself becomes a fire trap. Instead, where the duct passes through fire compartments in a tall building, fire resistance is provided by fire-rated ductwork tested to BS EN 1366-1 and classified under BS EN 13501. The kitchen should also have its own independent extract system that discharges to open air.
Cleaning frequency follows the grease risk, not the calendar alone. Under TR19 Grease, heavily used systems are typically cleaned every three months, moderate use every six, and light use annually, with the frequency confirmed by measuring grease thickness along the run. In a tall riser the deposits collect where access is hardest, so a documented clean of the whole canopy-to-atmosphere route, signed off by a BESA Grease Hygiene Technician, is what insurers expect to see.
Often, yes. External ductwork changes the appearance of the elevation and commonly requires planning consent, which can be difficult on prominent, conservation-area or listed frontages. It is worth resolving this early, because a refusal can force you back to an internal riser after the kitchen layout is already fixed. An internal route avoids the planning question but consumes lettable floor area and is harder to clean, so the trade-off should be weighed at the design stage.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.