Failure to clean was a contributing factor in 22 percent of the structure fires U.S. fire departments answered in eating and drinking establishments, about 1,650 fires a year out of 7,410. It’s the single largest preventable cause on the list, and it’s the only one on the list you can fix with a schedule.
Here’s the part that catches owners out. The grease that starts a restaurant fire is almost never the grease you can see.
The film on the cook line, the haze on the hood lip, the shine on the wall behind the fryer: that’s the visible tenth. It matters, and your crew should be removing it nightly. But it isn’t where the fires start, and cleaning it doesn’t tell you anything about the condition of the part that does.
Why Grease Behaves Differently From Every Other Soil in Your Kitchen
Every other soil in a restaurant stays where it lands. Spilled sauce, dropped flour, tracked mud: they fall, they sit, somebody wipes them up.
Grease doesn’t do that. It leaves the pan as vapor.
A charbroiler or a fryer at working temperature sends up a plume of grease laden vapor mixed with steam and smoke. Your exhaust hood is built to capture that plume and pull it out of the building. As the vapor travels up into the hood and along the ductwork, it cools. And when a vapor cools, it condenses back into a liquid and sticks to the nearest surface.
That single fact drives everything else. Grease deposits furthest from where it was made, on the coldest surfaces along the path, which is to say the ones nobody looks at.
The other fact worth knowing is what the deposit actually is once it’s up there. Cooking oils are Class IIIB combustible liquids. Their flash points run roughly 600 to 680 degrees Fahrenheit, and they self ignite somewhere around 685 to 725 degrees with no flame needed at all.
Those sound like impossible temperatures until you remember what happens when fat drips onto a charbroiler grate. A flare up clears 700 degrees in a second, and that flame gets pulled straight up into the hood by the very system that’s supposed to protect you.
Fire needs fuel, oxygen, and heat. A greasy duct supplies the fuel permanently and the oxygen for free. The only variable left is the third one, and a kitchen manufactures heat all day long.
Where the Grease Actually Goes
Follow the air and you’ll find the fuel. It stacks up in six places, in roughly this order:
- The filters, which are doing their job when they’re loaded, which is why they come out and get cleaned on a cadence.
- The hood plenum, the chamber behind the filters. First place the air slows down, first place it drops its load.
- The horizontal duct run, usually above the ceiling. This is the worst of the six: the slowest airflow, the coolest metal, and nobody has laid eyes on it since the building was inspected.
- The vertical riser heading for the roof.
- The rooftop fan housing and the curb it sits on, where grease pools and then overflows, running down the outside of the building in a hard rain.
- The roof itself, where a heavy deposit becomes both a fire load and a landlord conversation.
Notice the pattern. The further the grease travels from the cook line, the less anyone looks at it and the longer a fire in it can run before anybody in the kitchen smells smoke.
That’s the difference between a fire and a total loss. About 59 percent of restaurant fires start in the kitchen or cooking area, and roughly 68 percent of them stay confined to the object where they started. What decides which side of that line you land on is whether your ductwork is a metal tube or a fuel path running through the ceiling.
The Number That Decides Whether Your System Is Legal Tonight
Ask three people how much grease is too much and you’ll get three shrugs. There’s an actual number, and almost nobody outside the trade knows it.
NFPA 96, the standard that governs commercial kitchen ventilation, sets the threshold at 0.002 inches, which is 50 micrometers, roughly the thickness of household plastic wrap. If an inspection finds grease measuring more than that, the entire exhaust system has to be cleaned immediately. Not the section where it was measured. The whole system, hood through fan.
Two more requirements come with it, and both are worth knowing because both are things you can verify yourself:
- The system has to be cleaned to bare metal, with no powders or coatings applied afterward to make it look finished.
- Access panels are required roughly every twelve feet of horizontal duct, and they exist so somebody can physically open the duct and look.
That last one is your leverage. If you’ve ever wondered whether the exhaust cleaning you paid for actually happened, you don’t need to take anybody’s word for it. Ask the technician to open a panel and show you before they leave. Bare metal looks like bare metal.
How Often, and What the 2025 Rule Changed
NFPA 96 sets the inspection and cleaning interval by what you cook and how long you cook it, not by the size of your dining room:
| Operation type | Interval | Typical example |
|---|---|---|
| Solid fuel cooking | Monthly | Wood fired oven, charcoal grill |
| High volume, 12 to 24 hours a day | Quarterly | Charbroiling, wok stations, 24 hour kitchens |
| Moderate volume, 6 to 12 hours a day | Semiannually | Most full service restaurants |
| Low volume, under 6 hours a day | Annually | Seasonal kitchens, church halls, day camps |
Two things about that table trip people up.
The 2025 revision added a rule that any operation running more than 16 hours a day goes to monthly regardless of what fuel it burns. Extended hours pile up grease whether or not there’s wood in the firebox, and the standard now says so.
And your highest risk equipment sets the schedule for the whole system, not just for itself. A semiannual kitchen that adds one charbroiler becomes a quarterly kitchen, hood to fan, the day that charbroiler is installed.
There’s a distinction hiding in the word “interval” too. The schedule tells you when the system must be inspected. If the inspector finds contamination, cleaning follows regardless of when the last one was. The calendar is a floor, not a ceiling.
Get that schedule wrong and it’s not just a fire risk sitting over your ceiling. If a fire does start, the first document anybody asks for isn’t the fire report.
Which Grease Is Your Cleaning Crew’s, and Which Is Not
This is where owners lose the most money, usually by assuming one contract covers both halves of the problem. It doesn’t, and it can’t.
Your janitorial crew owns the visible half:
- The cook line, equipment exteriors, and the grease that runs down onto the legs and casters
- The hood’s exterior surfaces and lip, the backsplash, and the wall and ceiling surfaces around the cooking suite
- Floors, especially the grease that gets walked out of the kitchen into the dining room
- Pulling and running the filters on whatever cadence the kitchen sets
The certified exhaust trade owns the concealed half: the plenum behind the filters, the duct run, the riser, and the rooftop fan, cleaned to bare metal and signed off with a certificate. That work involves confined space entry, roof access, and a document a fire marshal and an insurer will both ask to see. It’s a licensed specialty for good reason.
We’ll say plainly what a lot of pages won’t: Excellence Janitorial does not certify hoods and exhaust systems. We clean the half of your kitchen that gets dirty every night, and we’ll tell you when the other half is overdue.
The two jobs feed each other, which is the practical reason to care. A hood cleaned to bare metal loads back up fast in a kitchen where nightly degreasing has slipped, and a well maintained system takes 30 to 40 percent less labor to clean than a neglected one. That shows up directly on the exhaust contractor’s invoice.
If you’re unclear which obligations sit with you and which move to a contractor, who is responsible for cleaning compliance walks the whole split.
If a Fire Starts, the Adjuster Reads Your Records
That’s the document. Not the fire report, your cleaning records.
A fire claim investigation on a restaurant routinely includes an examination of the exhaust system and a review of the service history behind it. Many commercial policies carry maintenance or protective safeguards language that references NFPA 96 by name, and if the last documented cleaning falls outside the interval for your operation type, the carrier has a written basis to deny on maintenance grounds no matter how the fire actually started.
Which makes the paperwork NFPA 96 already requires worth more than most owners realize. After a service, the contractor owes you three things:
- A certificate kept on the premises showing the date of the inspection or cleaning
- A label posted in the kitchen naming the date, the servicing company, and its contact information
- A written statement of which areas were not cleaned and why
That third item is the one to read twice. It’s your contractor telling you, in writing, exactly where the fuel still is. Inaccessible duct sections, a fan that couldn’t be reached safely, a run with no access panel: whatever’s on that line is the part of your system nobody touched.
The 2025 update also allows digital records, including timestamped before and after photographs, which is the cheapest insurance file you’ll ever build. Our guide to the cleaning records and logs inspectors want to see covers how to keep them so they’re actually usable.
What to Do This Month
Grease risk isn’t a mystery to diagnose. It’s a small number of checks with clear answers.
If you can’t name the date of your last exhaust cleaning, that’s the whole finding. Call the company that did it and ask for the certificate and the areas not cleaned note, and if you can’t produce either, book an inspection this month.
If the certificate exists but the interval doesn’t match your operation, say a semiannual schedule on a kitchen that added a charbroiler two years ago, move to quarterly now and tell your insurance agent you did. The interval is set by what you cook today, not by what the contract said when you signed it.
If nightly degreasing has slipped, the exhaust system is loading faster than its schedule assumes, and the next certified cleaning will cost more and take longer. That half is fixable immediately and it’s the cheaper half.
Three tells are worth walking for right now: a hood lip that’s tacky rather than clean to the touch, filters that stay loaded between cleanings, and any grease staining on the roof or running down the outside wall near the fan. The last one means the system is full enough to overflow.
If you run a kitchen in Northeast Pennsylvania and you’re not confident the nightly half is being held to a real standard, we do this work every night and a free walkthrough will tell you exactly where you stand. The same discipline is what makes professional cleaning help you pass health inspections, because inspectors and fire marshals are reading the same accumulations.
The fuel is already in your building. It arrived one dinner service at a time, and it leaves the same way: on a schedule, by somebody whose job it is.
Frequently Asked Questions
Can grease buildup start a fire on its own?
It needs an ignition source, but a kitchen supplies those constantly. Cooking oils self ignite around 685 to 725 degrees Fahrenheit, and a flare up on a charbroiler or a fryer clears that instantly. The flame is then pulled up into the hood by the exhaust system, straight into whatever has accumulated there. The buildup isn’t the spark, it’s the fuel load that turns a two second flare into a structure fire.
How thick does grease have to be before it’s a problem?
NFPA 96 sets the threshold at 0.002 inches, or 50 micrometers, roughly the thickness of household plastic wrap. If an inspection measures more than that anywhere in the system, the entire exhaust system must be cleaned, not just the section where it was found. Systems have to be cleaned to bare metal, with no coatings or powders applied afterward.
How often does a restaurant hood need to be cleaned?
By operation type: monthly for solid fuel cooking, quarterly for high volume operations running 12 to 24 hours a day, semiannually for moderate volume operations running 6 to 12 hours, and annually for low volume kitchens under 6 hours. A 2025 revision moved any operation running more than 16 hours a day to monthly regardless of fuel. Your highest risk equipment sets the schedule for the whole system.
Does a janitorial company clean the hood and ductwork?
Not the concealed part. A janitorial crew cleans the cook line, hood exteriors, walls, floors, and typically pulls the filters. The plenum, ductwork, riser, and rooftop fan are the certified exhaust cleaning trade’s work, because they require confined space access, roof work, and a certificate a fire marshal and an insurer will ask to see. You need both, and one doesn’t substitute for the other.
Will insurance cover a grease fire?
Usually yes, if your records support it. Claim investigations on restaurant fires routinely include an examination of the exhaust system and its service history, and many commercial policies carry maintenance or protective safeguards conditions referencing NFPA 96. If the last documented cleaning falls outside the required interval for your operation, the carrier has grounds to deny on maintenance failure regardless of the fire’s cause. Keep the certificates.
What are the signs of dangerous grease buildup?
A hood lip that feels tacky rather than clean, filters that stay loaded between cleanings, visible drips or pooling inside the hood, and grease staining on the roof or running down the exterior wall near the exhaust fan. That last sign means the system is full enough to overflow. Any of them is a reason to book an inspection rather than wait for the calendar.