Our house is in Toronto, Canada and was built in the late 1960's. We have lived here 5 years and currently rent out the basement suite. A few days ago, our tenants complained of a sewer smell throughout the appartment. Apparently it has been progressively getting worse over the course of the last week and a half. It was very noticeable when I went down to investigate. The smell appears to be coming from the laundry sink, the floor drain, or both. Both drains have a noticeable sewer odour if you smell them from a foot away. I cleaned out the P-trap for the laundry sink and ran some fresh water into it yesterday. This seems to have improved the overall situation as the larger suite no longer smells, or maybe it is just a fluctuation as apparently there are ok days and bad.

However, the floor drain is baffling. It is filled with water and now that I think about it, it was filled with water when I looked at it a few years ago. This time I noticed that there is a pipe, maybe 3/8" in diameter running under the slab into the drain. There is also a steady trickle of water flowing from the hole beside this pipe, basically under the slab, into the floor drain. I have no idea what this 3/8" pipe would be, why it would be located where it is, or what the source of the trickling water is.

Perhaps the 3/8" pipe carries domestic water and it has burst, resulting in the trickling water (though I have no idea why a domestic water line would be located there). Or, maybe the trickling water is ground water that has found its way into the hole made for this pipe, which happens to run into the floor drain. I have no idea.

I can't clear the floor drain of water fast enough (with a sponge) to look deep into it, but the water level remains relatively consistent - never overflowing the drain. I can't tell if the water level is maintained by the trickling water source, or if it is coming back up through the main floor drain piping. When I feel inside the pipe, I can tell that the floor drain piping heads toward the stack a about 4 feet away, but I can't tell if it has a P-trap. I also sopped up some of the trickling water and found that it had a slight, but noticeable sewer odour. I feel like the floor drain is the source of the larger odour problem our tenants are having.

Does anyone have any idea what could be going on with our basement floor drain? What could the 3/8" pipe be? What could be the source of the trickling water? The water may have been trickling like this below the drain cover for years for all I know.

I have attached a few photos for reference. Thanks in advance for any help/advice!

enter image description here enter image description here enter image description here enter image description here

  • FWIW: I had a mystery pipe in my floor drain too, but mine was dry. Eventually we figured out that it was just the water heater overflow pipe from back in the day (now that would be against code I think)
    – freshop
    Feb 8, 2018 at 20:58
  • If you have an AC unit it may also be a condensate line, I have seen this done above traps in the past and if there mold growing in the pan it can have a fowl order.
    – Ed Beal
    Sep 20, 2018 at 16:48

1 Answer 1


The 3/8” pipe is a “primer” for the sewer line. We use primers when we’re afraid the “P-trap” will dry-out and allow smells to infiltrate up through the sewer line and into the house.

The 3/8” line is a little large, (we use 1/4” where I live) and it’s connected to a cold water line by a sink, toilet, etc. close to the drain. If you can determine where it’s connected, you could put a valve on it and slow the water flow...being careful not to turn it down too much and allow the drain to dry out.

The smell could be from wet/damp soil around the drain and seeping into the house through cracks between the drain and concrete.

  • 1
    Ah, that makes sense; thank you! Considering water is trickling out the hole in the concrete from which the primer line connects into the drain, would you expect the primer line is cracked somewhere below the slab? I would think there should be water exiting the primer line, but not coming directly out the concrete hole.
    – Erik
    Feb 8, 2018 at 4:56
  • Yes, it’s potable water so it should all be “in” the pipe. You can have your water tested by a local medical lab for contaminates. If you don’t know where (or how) to take a “sample”, call a local plumber. They do it frequently.
    – Lee Sam
    Feb 8, 2018 at 5:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.