Phoenix Journal · Extraction
Brown, spreading stains on a commercial kitchen ceiling mean grease is reaching a surface it should never touch - here is how to tell whether it is escaping your canopy or leaking from the ductwork above.
Kitchen extraction & grease
A ceiling that is slowly turning yellow-brown above your cooking line is telling you something - grease is reaching a surface it was never meant to touch, and the extraction system is not doing the whole job it should.
Those soft, spreading stains are almost never a decorating problem. They are a symptom, and in a commercial kitchen they usually come down to one of two causes - either grease-laden air is escaping the canopy before it can be captured, or grease that is already inside the system is finding its way back out through the ceiling void. The two look similar from below, but they are fixed in very different ways, so it is worth learning to tell them apart.
Below we set out both causes side by side - what points to each, what it does not explain, and the pros and cons of treating your ceiling marks as one problem or the other. Getting the diagnosis right matters for more than appearance - under BESA's TR19® Grease standard, grease build-up in kitchen extract is a recognised fire risk, and around 70% of commercial kitchen fires are estimated to start in the extract ventilation itself.
The first and most common explanation is that your canopy is not capturing everything the cooking line throws up. Hot air rising off a chargrill, fryer or wok forms a fast thermal plume carrying tiny grease droplets. If the canopy cannot catch and hold that plume - the principle engineers call capture and containment - the grease-laden air rolls out around the canopy edge, cools, and settles on the nearest cold surface. That surface is your ceiling.
This is usually a design or balance issue rather than a dirty-system issue. Under the DW/172 kitchen ventilation design method, the canopy should overhang the appliances on every open side, the face velocity across the canopy must be high enough to draw the plume in, and the make-up air feeding the room must be balanced to the volume being extracted. Get any of those wrong - a canopy that is too short, a fan that has lost performance, or replacement air blowing the plume sideways - and grease escapes no matter how hard the fan is working. If you are also seeing droplets forming and falling, that is a related warning sign; our guide on why grease drips from a canopy covers that side of the same fault.
Points to a capture problem
What it does not explain
The upside of a capture problem is that it is fixable and often preventable - adjusting overhang, restoring fan performance, rebalancing make-up air and keeping filters clean can stop new grease reaching the ceiling entirely. The downside is that a survey and possibly some remedial work are needed to correct the design or airflow, so it is rarely a quick wipe-down and done.
The second explanation is that the grease is not coming from the room air at all - it is coming from inside the extract system, above the ceiling. Over months of service, grease condenses and pools inside the ductwork. If joints, seams or access panels are not fully sealed, or if the run is long and poorly graded, that liquid grease can weep out and soak down through the ceiling void, staining the plasterboard from above. The stains then map the route of the duct rather than the position of the canopy.
Condensation makes this worse. Warm, moisture-heavy air from cooking meets cooler duct and ceiling surfaces, and water condenses out carrying grease with it - the same physics behind the moisture you may have noticed lower down, which we cover in why there is condensation under a canopy. In cold weather the effect intensifies, which is why leakage-type ceiling marks often look worse in winter. This cause is strongly linked to how long it has been since the system was properly cleaned - the thicker the internal grease film, the more there is to migrate. TR19® Grease sets a maximum residual film of 50 µm (fifty microns) after cleaning, verified by deposit-thickness testing, precisely because heavy internal build-up is both a fire hazard and a leakage hazard.
Points to a leakage problem
What it does not explain
The advantage of pinning down a leakage problem is that a thorough clean and reseal often stops it at source, and it doubles as fire-risk reduction. The drawback is that it means opening up and inspecting the concealed run, which is more involved than treating the visible canopy - but leaving weeping grease in a ceiling void is exactly the condition insurers and fire risk assessors want to see removed.
In practice, the two causes often overlap - a tired system that both leaks internally and captures poorly. The reliable way to separate them is a proper survey - checking canopy overhang and face velocity against the DW/172 design intent, measuring the internal grease film against the TR19® Grease 50-micron threshold, and following the stain pattern to see whether it tracks the canopy or the ductwork. Once you know which cause dominates, the fix is straightforward - rebalance and restore capture, deep clean and reseal the system, or both.
Grease marks on the ceiling are the visible edge of a system that needs attention - whether the grease is escaping the canopy or escaping the ductwork, the answer is the same in principle - capture it properly, clean it thoroughly, and keep it that way. Treat the stain as a prompt, not just a paint job, and you protect both the look of your kitchen and its fire safety at the same time.
Questions
Indirectly, yes. The marks themselves are a symptom of grease not being fully captured or contained, and the same conditions that stain a ceiling usually mean grease is building up inside the extract system. Since around 70% of commercial kitchen fires are estimated to start in the extract ventilation, staining is a signal worth acting on quickly under the TR19 Grease fire-risk standard.
Only temporarily. Repainting or wiping the ceiling treats the symptom, not the cause. If grease is escaping the canopy or weeping from the ductwork, the marks will simply return. The lasting fix is to survey the system, correct the capture and airflow to the DW/172 design intent, and deep clean the extract so grease is removed at source.
It depends on how hard the kitchen works. Under BESA's TR19 Grease guidance, heavy-use kitchens running 12-16 hours a day are typically cleaned every three months, moderate-use kitchens every six months, and lighter use annually. After cleaning, any residual grease film should measure under 50 microns, verified by deposit-thickness testing and recorded on a certificate.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.