Phoenix Journal · Ductwork
The metal your extract duct is made from decides how grease-tight it stays, how well it survives a fire and how easily it can be cleaned. Here is how to choose it well.
Material choice
The metal you build a kitchen extract from is not a free choice - it is governed by how much hot grease-laden air the duct has to carry and where it runs through the building.
Commercial kitchen extraction sits in its own category because the air being moved is greasy, hot and, over time, flammable. That is why the trade does not treat it like ordinary ventilation. The governing document in the UK is the Building Engineering Services Association specification DW/172, Specification for Kitchen Ventilation Systems (2nd edition, October 2018, with a later addendum), which sits alongside the general low-pressure ductwork standard DW/144. Between them they set out what a compliant extract should be made from.
The headline rule is straightforward: an extract duct must be built from a non-combustible material. In practice that narrows the field to two metals - stainless steel and galvanised steel (including the tightly-controlled galvanised sheet often called Zintec). Plastic, flexible ducting and combustible liners have no place on a grease-carrying extract. For the canopy itself, DW/172 is stricter still and treats stainless steel as the only suitable material.
So the real decision is rarely "any metal we like". It is a narrower and more useful question: stainless or galvanised, at what thickness, and welded or not. Get that right and the rest of the system - fans, filters, cleaning access - has a sound shell to hang off.
Both metals are non-combustible and both appear in compliant kitchens, so the choice comes down to where the duct runs, how it is joined, and how long you need it to last.
DW/172 draws the important line around grease tightness. Where a duct sits inside the kitchen fire zone and does not discharge straight to atmosphere, the specification calls for it to be fully welded and built from either 1.2mm stainless steel or 1.6mm Zintec galvanised sheet, with welded angle-iron flanges and full-faced gaskets at the joints. That welded, grease-tight construction matters more than the metal itself: a folded-and-sealed duct will weep grease from its seams long before a welded one does.
Where the two metals genuinely differ is durability and cost. Galvanised steel is cheaper and perfectly acceptable for a straight run that exhausts directly outside, and much of the UK's kitchen ductwork is galvanised. Its weakness is corrosion: the zinc coating protects the steel, but constant contact with warm, acidic grease and repeated wash-downs wears it away, and once bare steel is exposed it rusts and pits. Stainless steel costs more up front but shrugs off grease, moisture and cleaning chemicals, holds up to higher temperatures, and gives a smooth internal surface that grease clings to less readily. For heavy-use sites - think high-volume frying, charcoal or wok cooking - the longer service life usually justifies the extra outlay.
A sensible rule of thumb: choose stainless where grease and heat are worst - directly above the cooking line and in any run you cannot easily replace - and reserve galvanised for cooler, accessible sections and short discharges to atmosphere. Whatever you pick, keep to a single specification through a given run rather than mixing thin folded sections into a welded system.
Material choice is not just a construction decision - it shapes how the duct behaves for the rest of its life, and above all how it copes with grease and fire.
Every kitchen extract accumulates grease on its internal walls, and that grease is the fuel for the fires these systems are notorious for. Managing it is the job of the BESA specification TR19 Grease, issued as a standalone document in July 2019, which governs the Fire Risk Management of Grease Accumulation within Kitchen Extraction Systems. TR19 Grease is the benchmark insurers and enforcing authorities expect you to meet, and it depends heavily on the duct being cleanable.
That is where the material and construction feed straight into safety. TR19 Grease expects access panels at roughly 2 metre intervals, and at every bend or change of direction, so that the whole internal surface can be reached, inspected and cleaned. A duct built without that access - or with panels that no longer seal - traps hidden grease that no schedule can remove. A smoother stainless surface tends to shed grease and clean up more readily than pitted, corroded galvanised steel, which is one of the quieter arguments for stainless on a hard-worked system.
The other place material matters is fire compartmentation. A duct that runs straight from the canopy to outside can be standard non-combustible galvanised or stainless steel. But the moment an extract passes out of the kitchen and through other parts of the building, it can carry both fire and grease into voids and rooms far from the cooker - which is exactly how fire spreads through building voids and ductwork. Any run that penetrates a fire compartment must be fire-rated to the relevant standards (tested to BS 476 or BS EN 1366), which is a construction requirement over and above the base metal. This is a point to settle at design stage, not after the ceilings are closed up.
Pulling it together, the right specification falls out of three questions: how hard the kitchen works, where the duct runs, and how you will keep it clean.
Start with the cooking. Light, intermittent cooking on a modest canopy is well served by welded galvanised ductwork discharging to atmosphere. Intensive frying, flame grilling or continuous service pushes you towards stainless steel for its resistance to heat, acidic grease and frequent cleaning. Then look at the route: any section that threads through occupied parts of the building needs fire-rated construction regardless of the metal, and any section you cannot easily reach later is worth over-specifying now. Finally, design in the access - side-mounted panels at sensible intervals - because a duct you cannot clean is a duct that will eventually fail its fire risk assessment.
It also pays to think about the whole life of the system, not just the install cost. Grease-laden extract is one of the biggest hidden influences on the air quality and hygiene of a kitchen, and a corroding, leaking duct undermines both - it is worth understanding how ductwork affects the air your staff breathe before you cut corners on material. A slightly dearer, better-built duct usually repays itself in fewer repairs, easier cleaning and a lower fire risk.
None of this is exotic. It comes down to using a non-combustible metal of the right thickness, welding it grease-tight where the specification demands, fire-rating anything that leaves the kitchen, and building in enough access to keep it clean for the life of the site.
Questions
Not always - both are non-combustible and both are accepted under DW/172, so galvanised steel is perfectly compliant for many runs, especially a straight discharge to atmosphere. Stainless earns its higher cost where grease, heat and frequent cleaning are worst, because it resists corrosion and cleans up more easily. The bigger factor is usually construction: a fully welded, grease-tight duct matters more than the metal you weld. For a heavy-use site, stainless above the cooking line is often the sounder long-term choice.
The cleaning frequency itself is driven by how greasy the system gets and is set out under TR19 Grease, not by the metal. However, material and construction decide how cleanable the duct is - a smooth stainless surface sheds grease more readily than pitted, corroded galvanised steel. What matters most is having access panels at roughly 2 metre intervals and at every change of direction, so the whole internal surface can be reached, inspected and cleaned whatever metal it is made from.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.