Why It Smells Like A Sewer In Your Restaurant


QUESTION From Anthony & Mike in CT:

“One of our bathrooms smells like sewage. We can’t tell if it’s coming from the toilets, the vents or through the damn walls but it’s bad. Two plumbers have been here so far and both of them told us the best thing to do is to try to cover up the smell with air fresheners. Are you f-ing kidding me!!?

Landlord hasn’t gotten back to us since June and now we’re starting to smell the same smell or something damn close behind the bar.

Drains are cleaned and bleached every night. but after a day or so it smells like an open grease trap again.

At this point we’re willing to try anything before calling the next plumber so hit me up with any ideas at all!”


Ahhhh….the smells of a restaurant. From the occasional waft of garlic butter to the faint hint of a dead rodent entombed in the base of a booth somewhere between tables 16 and 19.

The 4 most common reasons for the type of olfactory situation you’re describing:

  1. Vent pipes not capped and/or are venting too close to the HVAC intakes / ac units. (get on the roof or side of your building and check it out)

  2. P trap issue / not filled with enough liquid. (most common - see below)

  3. Broken seals around your urinals and/or toilets (easily fixed by replacing the wax or other synthetic gasket ring under the toilets.)

  4. Leaking / broken vent and/or drain pipes (not the easiest fix so better left last / rule out all others first because the problem could be behind sheet rock or under the floor)

In my experience, the most common result of sewer smell in restrooms / behind bars is that the negative pressure in the floor drain line is pulling air backwards (venting into the bathroom through the floor drain) due to either excessive negative pressure or a dried out p-trap.

If it’s the latter, a gallon of water and a couple quarts of vegetable oil (which doesn’t evaporate) can rebalance the negative draw from a dried out trap.

Very often this is more of a maintenance issue than design flaw since building maintenance should be on top of this / checking the box regularly.

Proper janitorial services would prevent this from happening if they really provided a full / recommended service.

It’s usually (hopefully) just a matter of getting “enough” liquid in the drain to actually fill the trap. A little bleach once in a while doesn’t hurt, and will sometimes cure the problem if you add enough...but only temporarily since the bleach usually evaporates quickly.

The trap needs to stay full of liquid in order to keep the gases from flowing backwards through the line...which is why frequent / full floor cleaning with a full bucket of soap and water is so important.

If it’s not #s 1 - 3, you may have a broken vent and/or drain pipe which really requires a master plumber with commercial experience. A quick way to vet would be to ask if they’ve ever had to trace vents or drains to find a negative pressure causing crack or break and if they have, ask if you can call (or if they’d give your number to the owner and ask them to call you) for a reference.

Let me know what it was / what worked,