Guide from the team
TR19 Grease works on measurement, not opinion. Here is how the wet film thickness test works and what the 200 micron trigger means for your kitchen.
Published 8 September 2026
Ask whether a kitchen extract duct is dirty and you will get an opinion. Ask how many microns of grease are on the metal and you will get a fact. That difference is the whole point of the way TR19 Grease approaches kitchen extract systems: the decision to clean is driven by measurement, not by how things look through an access hatch.
Here is how the measurement works, in plain terms.
Grease from cooking does not sit in the duct as loose dirt. It condenses out of the hot extract airstream and builds up as a film coating the inside of the metalwork, much like layers of paint building up on a surface. The sensible way to quantify it is therefore to measure the thickness of that film.
That is what a wet film thickness test does. The surveyor uses a small gauge, a comb-like tool with teeth of graduated depths, and presses it into the grease deposit at a test point. Some teeth touch grease, deeper ones do not, and where the change happens tells you how thick the film is at that point. The reading comes out in microns: thousandths of a millimetre. It takes moments, it needs no laboratory, and it turns “that looks bad” into a number that can be written down, compared and checked.
Readings are taken at multiple points through the system, not just where access is easiest, because grease does not deposit evenly. It builds faster near the canopy and in sections where the airstream slows or changes direction, and a single reading by the hatch tells you little about the riser or the run to the fan.
TR19 Grease sets a clear threshold: when the mean grease deposit across the system reaches 200 microns, the system needs cleaning. The word mean matters. It is not a case of one bad patch condemning the system, nor of one clean patch excusing it. The readings from across the system are averaged, and that average is judged against the trigger.
To picture 200 microns: it is roughly a fifth of a millimetre, about the thickness of a couple of sheets of paper. That may sound like very little, and that is precisely the lesson. A grease layer thin enough to look like discolouration to the eye can already be at or past the point where the guidance says it must be removed, because a film of that depth across the internal surface of a full duct run adds up to a serious quantity of fuel. Once ignited, it carries fire along the ductwork and through the building.
The eye fails at this job in both directions. It fails safe-looking systems: a thin, even film reads as “a bit grubby” when it is actually over the trigger. And it fails dirty-looking ones less often but expensively, condemning a system that measurement might show is within limits, buying a clean earlier than needed.
Measurement also removes the argument. When a contractor says your system needs cleaning, you are entitled to ask how they know. “It looked dirty” is not an answer you can put in front of a fire risk assessor. A set of wet film thickness readings against a published trigger is. The same duties that make this matter, the Regulatory Reform (Fire Safety) Order 2005 and your fire risk assessment, are about managing risk on evidence, and TR19 Grease (which is BESA industry guidance rather than law) gives you the evidence in numbers.
The readings do not stay in the surveyor’s notebook. They are recorded in the TR19 Grease report, location by location, before and after the clean: pre-clean readings justify the work, post-clean readings prove the metal came back below the threshold. We set out everything else that document should contain in what is in a TR19 report.
The readings also drive the schedule. How fast a kitchen climbs back towards 200 microns depends on how hard and how it cooks, which is why TR19 Grease links cleaning frequency to daily cooking hours: every 3 months for heavy use of 12-16 hours a day, every 6 months for moderate use of 6-12 hours, every 12 months for light use of 2-6 hours. Measured deposit levels at each visit confirm whether that interval is right for your kitchen or needs tightening.
Because Clean Ducts is a BESCA Vent Hygiene Associate member, our operatives are TR19 trained and our work is independently audited, so the measurements and certificates in our reports are verified rather than self-declared. That matters when documented cleaning is a condition of cover in most UK commercial policies; policies differ, so check the wording of yours. And measurement is not only for kitchens: general ventilation and ductwork cleaning and air handling unit work are assessed and evidenced in the same documented spirit under TR19.
The only way to know your numbers is to take them. Book a free survey through our contact page and we will measure the deposit levels through your extract system and tell you plainly whether it needs cleaning, or request a TR19 quote if you are ready to book. If you have an existing TR19 report, send it over and we will read the measurements in it for you.
It is the recognised method for measuring grease deposits inside kitchen extract ductwork. A graduated gauge is pressed into the grease film at test points through the system, and the depth of grease it registers gives a reading in microns (thousandths of a millimetre). Readings from multiple locations are averaged to give the mean deposit level for the system.
TR19 Grease treats a mean grease deposit of 200 microns, measured by wet film thickness test, as the point at which a kitchen extract system requires cleaning. It is an average across readings taken through the system, not a single worst reading. At roughly a fifth of a millimetre, a film at this level can look like mere discolouration while already representing a serious fire load along the ductwork.
Because the eye is unreliable in both directions: a thin, even film over the trigger level can look like light soiling, while a locally dirty patch can make a compliant system look worse than it is. Measurement replaces opinion with numbers that can be recorded, compared between visits and shown to a fire risk assessor or insurer.
They are recorded in the TR19 Grease report, location by location, alongside before-and-after photographs. Pre-clean readings justify the work and post-clean readings prove the result, and the pattern of readings over successive visits confirms whether your cleaning interval is right for how the kitchen is used. The report and certificate then sit in your fire safety records as evidence for your insurer and fire risk assessor.
Send us your existing report or book a free survey. A named engineer replies within 24 hours.