Commercial Kitchen Owners Beware: Grease Build-Up Can Lead to Fire

Grease accumulation within a commercial kitchen exhaust system is one of the most common but underestimated fire risks in food businesses. It builds up gradually, often out of sight, and can become a significant fuel load if not removed often enough. This case study highlights how a fire can develop within an exhaust system and why regular cleaning must be based on the actual use of the kitchen, not just a routine habit.

In this matter, the fire occurred in a single-level commercial food premises after the business had closed for the night. There was smoke and heat damage throughout the property, with some impact to neighbouring tenancies, but the most significant damage was concentrated in the kitchen and exhaust system.

Our investigation identified the area of origin within the exhaust duct above the grease filters. Inside the duct were heavy grease deposits, intense oxidation, and white ash-like clumps consistent with burnt grease residue. The grease filters themselves had been heavily damaged, with one filter having failed altogether. Further along the exhaust duct, thick clump-like grease deposits were also present, showing that combustible grease contamination had built up throughout the system rather than being limited to one small section.

The fire patterns showed that the fire had developed through the exhaust system. There was evidence of heat and smoke damage extending toward the roof fan, and the ceiling lining above the duct had failed in a near-circular pattern, exposing burnt insulation and charred roof framing. These findings were consistent with concentrated fire activity within the ductwork.

Based on the physical evidence, our investigation concluded that the first fuel involved was the exhaust system itself, specifically grease deposits within the duct. The likely ignition mechanism was the ignition of accumulated grease by heat generated during normal cooking operations.

This case is a strong reminder that grease is not simply a cleaning issue. In a commercial kitchen, grease inside an exhaust system is fuel. Once sufficient build-up is present, the normal heat associated with cooking can become a competent ignition source.

Importantly, commercial kitchen exhaust systems are not meant to be cleaned only when visibly dirty or on a set schedule that does not reflect actual usage. Cleaning intervals should be determined by how often the kitchen is used, what type of cooking is carried out and how much grease-laden vapour is being produced. A business with heavy or high-fat cooking may require more frequent cleaning than a lower-use premises, even if both follow the same nominal schedule.

In this case, information available at the time indicated that the grease filters were cleaned weekly and the exhaust system was cleaned every three months, with the system due for cleaning shortly before the fire occurred. While that may appear reasonable on paper, the physical evidence showed that significant grease deposits had still accumulated within the exhaust. That is the key lesson: compliance in practice is not just about having a schedule, but about ensuring the cleaning frequency is sufficient for the level of use.

View of the interior of the exhaust hood. Burnt deposits can be seen on the right of the image.

For commercial kitchen owners, this means maintenance of the exhaust system should be treated as an active fire prevention measure. Filters, canopy areas, ductwork and fans all need to be considered. If the kitchen is busy, runs for long hours or produces high levels of grease, the cleaning frequency should be increased accordingly.

Fires like this can spread rapidly through concealed ductwork and into the building structure. They can damage not only the business itself, but also adjoining tenancies and shared building elements. The cost of more frequent cleaning is minor compared to the potential costs of a fire, business interruption, and uninsured loss.

Key takeaway: if your kitchen is used regularly, your exhaust system must be cleaned regularly and that cleaning schedule must reflect the intensity of actual use, not just what has always been done.