In our house, we have a partially below grade basement. Occasionally, our basement will smell like sewer gas (farts) but I can not determine where the smell is coming from.

The basement does have sewer clean-out drain in the utility/washroom but our clean-out drain cap is securely tightened. It seems like the smell is coming from the downstairs bathroom which is next to the utility room.

The bathroom downstairs is hardly used but I occasionally flush the toilet and run the shower to keep water from evaporation from the P trap on the shower.

Additionally, it seems that when we use an upstairs bathroom the smell is more present.

Anyone have any ideas on how I can find the smell or what I can do to resolve the smell?

  • 2
    "I can not determine where the smell is coming from" -- Blame the dog, it worked for my stepdad.
    – Alaska Man
    Jul 31, 2020 at 18:53

1 Answer 1


It sounds like a P-trap to me, one thing I learned years ago, water evaporates, thus the reason you have to re fill the P-traps.

Try pouring some vegetable oil in the P-trap at least enough to fully coat the surface this keeps the dry air in the house from pulling the moisture out of the traps, the plumbing side is very humid so it is not taking water out of the traps. I have had the same smells but after adding the vegetable oil to the floor drain, Sink and shower I did not have this problem in that house.

Since then if we have a mud room I put oil in those floor drains , in my last home we had a bathroom that wasn't used except when the family was home that sink and shower got a good shot after the first time we thought something died in the house , opened the bathroom door there was the smell. It never happened again with liquid vegetable oil, never had a bad smell from the oil.

One person, a snow bird I told about that, even used it on her toilets. I have not had an issue with toilets but if I was going to not be there for an extended time I would probably add to the toilet also.

  • Would this be safe to do in a house on a septic system? Jul 31, 2020 at 18:13
  • 1
    Absolutely safe all but 2 of my homes have been on septic, vegetable oil though not automotive. You get a few drops of cooking oil quite often when doing dishes, if someone uses the sink and the soap or toothpaste cuts the oil it is easy to replace, I have not had any problems using this trick since the 80’s.
    – Ed Beal
    Jul 31, 2020 at 18:56
  • 1
    Yeah, it doesn't take much, put some water in a bowl and see how many drops of oil it takes to cover the surface.
    – Jasen
    Aug 1, 2020 at 1:17
  • I agree with @jason but maybe I have used more than a few drops, maybe a 1/2 cup.
    – Ed Beal
    Aug 1, 2020 at 1:38
  • I have added the vegetable oil but it seems not to help. It only occurs when you take a shower in the upstairs bathroom. What else can I try? Aug 7, 2020 at 20:58

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