A restaurant floor drain backing up has one of five causes: a blockage in that drain or its trap, grease built up in the line downstream, a restriction in a branch line shared with other fixtures, a full or failed grease interceptor, or a problem in the main sewer. The floor drain is usually where it shows first because it is the lowest opening in the kitchen, not because the fault is there.
Which of the five you have decides the fix, and you can narrow it down in a few minutes by looking at which drains are affected and when. Start with the steps below if water is on the floor now.
What to do first
- Stop sending water down the drains. Turn off the dish machine, stop pre-rinsing, and hold off on mopping.
- Keep staff and food away from the water. Treat it as contaminated, and clean and sanitize anything it touched.
- Do not serve from an affected area. If wastewater is in a food prep or storage area, stop using it until it is cleaned.
- Note which drains are backing up and what was running when it started.
- Call a commercial plumber, and tell them how many drains are involved.
- Call your grease hauler if the interceptor is due or overdue for pumping.
A sewage backup into a food facility is a health matter. Under California’s retail food rules, a facility with a sewage backup in food areas is generally expected to stop the affected operations until it is corrected, so ask your environmental health agency what it requires if you are unsure.
Read the pattern before you call
The pattern of the backup tells a plumber where to start. These observations take no tools and involve no dismantling.
| What you see | What it suggests | Where the restriction probably is |
|---|---|---|
| One floor drain is slow, the others are fine | A local blockage | That drain’s strainer, trap or short branch |
| Floor drains back up when the three-compartment sink is emptied | The shared branch cannot carry the surge | The kitchen grease line |
| Several kitchen drains back up together | A restriction after they join | The grease line or the interceptor |
| Kitchen and restroom drains back up together | A problem past the point where both lines meet | The building drain or main sewer |
| Water rises with nothing running in your unit | Another tenant’s discharge, or the main | Shared building drain, or the public sewer |
| It gets worse during service and clears overnight | A partial restriction that normal flow can pass | Grease narrowing the line |
| Dark, greasy water with food solids | Kitchen waste | Grease line or interceptor |
| Sewage, toilet paper or sewer odor | Sanitary waste | Main sewer |
Two of these observations matter most.
One drain or many. A single slow drain is a local problem. Several drains at once means the blockage is downstream of where they connect.
Kitchen only, or restrooms too. In most restaurants the kitchen grease waste runs through the interceptor on a separate line from the restroom waste. If only kitchen fixtures are affected, look at the grease side. If toilets are gurgling or backing up as well, the restriction is in the main.
The five causes, in detail
1. A blockage at the drain itself
Floor drains collect whatever is washed across the floor: food scraps, labels, mop strings, broken glass and grit. The strainer or basket catches some of it. The trap below catches the rest.
This is the simplest case. The drain is slow or standing, nothing else is affected, and the debris is within reach of the strainer. Clearing the strainer and basket is a staff job. Anything in the trap or beyond is a plumber’s job.
A commercial kitchen floor drain clogged at the trap can also smell. If the drain smells and drains normally, the trap may have dried out and lost its water seal, which is a different problem with a different fix.
2. Grease in the line downstream
This is the most common cause in a working kitchen. Fats and oils go down the drain as liquid in hot water, cool in the pipe, and harden on the wall. The line narrows a little every day.
A narrowed line handles a trickle and fails under a surge. So the backup arrives when a sink full of water is released or the dish machine discharges, typically in the middle of service. It then drains away slowly, which makes it tempting to ignore.
A cable punches a hole through grease that closes again within weeks. Removing the coating takes hydro jetting. We explain the difference in hydro jetting vs snaking.
3. A full, undersized or failed interceptor
If the grease trap or interceptor is full, it stops separating and can restrict flow. Everything upstream of it backs up, and the lowest drain overflows first.
Check the service record. If the unit is past its pumping date, that may be the whole answer. The City of Sacramento’s guidance for food businesses says to have an interceptor pumped out completely at least once every three months. Small indoor traps need cleaning far more often than that.
A unit can also fail mechanically. Baffles break or go missing, the outlet tee falls off, or the flow control is removed. A unit that was sized for the last tenant’s menu may be too small for yours. Our guide on grease trap vs grease interceptor covers how each type is sized and maintained.
4. A shared branch line that cannot keep up
Sometimes the pipe is clear and the problem is how much water is being sent down it at once.
A dish machine, a three-compartment sink and a prep sink discharging together can overwhelm a branch that handled each one separately. This often appears after a remodel or an equipment change: a bigger dish machine, an added sink, or a fixture moved to a different line.
A pipe with poor slope or a sag behaves the same way. Solids settle in the low spot and the line clogs repeatedly at the same point. A camera inspection shows it.
5. The main sewer
If restroom fixtures are affected too, or sewage is coming up with nothing running in your kitchen, the restriction is in the building drain, the lateral or the public sewer.
Causes here include grease that has travelled past the interceptor, tree roots, a broken or sagging pipe, and blockages in the public main. In a multi-tenant building, another tenant’s discharge can come up through your floor drain because yours is the lowest opening.
If you suspect the public sewer, report it to the sewer agency. In the City of Sacramento that is 311 or 916-264-5011.
What the plumber will do
The work normally goes in this order.
- Ask about the pattern: which drains, when, what was running, and when the interceptor was last pumped.
- Check the interceptor: how full it is, and whether the baffles and outlet are intact.
- Restore flow: open the line with a cable or jetter from the nearest cleanout.
- Clean the line: jet the grease line so the pipe wall is clear, not just the center.
- Inspect with a camera: confirm the line is clean and look for sags, breaks or roots.
- Report what was found and recommend a cleaning interval.
The city’s guidance makes a point that is often missed: after an overflow, clean the sewer line from the interceptor outlet to the main as well as the line from the building to the interceptor. Grease that has passed through the unit sits in that second section.
What drives the price of that work is set out in how much commercial hydro jetting costs.
What staff can safely do, and what they should not
Staff can clear the strainer and basket with gloves on, stop water use, mop up with a sanitizer approved for the kitchen, and record what happened.
Staff should not remove the trap, push a cable into the line, or open the interceptor. A buried interceptor is a confined space that can hold dangerous gases.
Chemical drain openers are a poor idea in a kitchen. They do little against a long grease coating, they sit in standing water where staff then work, and they can injure the plumber who opens the line afterward. Many sewer agencies also restrict what may be discharged. If one has already been used, tell the plumber before work starts.
Boiling water and degreasers push grease further down the line, where it sets again. They move the problem. They do not remove it.
Keeping it from happening again
Most kitchen backups are predictable, because grease builds at a steady rate.
Scrape and dry-wipe before washing. The less grease that reaches the sink, the less reaches the pipe.
Keep strainers in every floor drain and sink, and empty them through the day.
Pump the interceptor on schedule, and keep the manifests. An inspector will ask for them.
Put the grease line on a jetting schedule based on how fast it builds up. A camera check at each visit tells you whether the interval is right.
Review the plumbing after an equipment change. A new dish machine or an extra sink may need a larger line or a larger interceptor.
Keep floor drain traps wet. Drains that rarely see water dry out and let sewer gas in. A trap primer, or a routine of running water into them, prevents it.
Frequently asked questions
Why is my restaurant floor drain backing up? Usually because of grease in the line downstream, a full interceptor, or a blockage at the drain itself. If restroom drains are affected too, the restriction is in the main sewer. Which drains are involved tells you where to look.
Why does the floor drain back up when I drain the sink? The sink and the floor drain share a line that is partly blocked. The surge of water cannot get through fast enough, so it comes out at the lowest opening, which is the floor drain.
Can I use drain cleaner in a commercial kitchen floor drain? It is not recommended. Chemical cleaners do little against a long grease buildup, they are hazardous in standing water, and local rules may restrict them. Mechanical cleaning or hydro jetting removes the cause.
How often should restaurant drain lines be cleaned? It depends on volume, menu and how well the interceptor is maintained. Many kitchens are on a one to three month jetting cycle. Set the interval from what a camera shows at each visit.
Do I have to close if the floor drain backs up? If wastewater is in a food preparation, storage or service area, you generally need to stop using that area until it is cleaned, sanitized and the cause corrected. Ask your environmental health agency what it requires.
Count the drains before you call
One drain is a local clog. Several kitchen drains are the grease line or the interceptor. Kitchen and restrooms together are the main. If you can tell the plumber which of those it is and when the interceptor was last pumped, the visit starts at the right cleanout.
We clear, jet and camera kitchen lines and repair grease traps and interceptors for restaurants across the Sacramento area, and we can work between closing and prep. See commercial kitchen plumbing services, or call (866) 981-1310 and tell us how many drains are backing up.