There is a sewer smell coming from my basement. There are only a couple entries to the sewer in the basement: A laundry sink that acts as the drain for a water softener (which regenerates every few days), and a floor drain.

I suspect that the floor drain is eventually evaporating the water in the p-trap, and I'm getting sewer gasses coming up. I've poured a bucket of water down the drain and it seemed like it helped, but it's difficult to tell, probably because the air in the basement doesn't get replaced much.

Is this a known problem? I can think of a few solution if so.

  1. Pour water down occasionally.
  2. Seal the drain or put a valve in it somehow - although this doesn't seem like a bright idea in case the sewer line between the drain and the city sewer has a blockage.
  3. Cover the drain with something to keep the gasses from coming up, but should the sewer have a blockage, the cover will move out of the way or float.
  • My floor drain has a lead cap on it. I've only opened it once - when the basement flooded. – ssaltman Nov 17 '15 at 22:13
  • How certain are you that it is the floor drain? I had a similar problem because an old wash sink was not vented properly when it was originally installed in my 58 year old house. When you notice the smell, turn on the water for several fixtures upstairs to force more sewer gases through the problem fixture and then smell around both the wash sink and floor drain in the basement to confirm which fixture is giving you trouble before solving a theoretical problem with the floor drain. – statueuphemism Nov 18 '15 at 10:44
  • @statueuphemism This is exactly why I'm asking about this. Thanks for the tips! – Steve Nov 18 '15 at 12:54
up vote 8 down vote accepted

You can use something like the non-toxic RV antifreeze which evaporates more slowly than water; Or wash the basement floor occasionally.

A touch of mineral oil (the stuff sold for putting in people) may help to prevent evaporation by forming a surface film, but don't overdo that.

  • Interesting idea using oil as a barrier. Do you think the basement floor is picking up stink from the drain? Washing the floor = water down the drain, which would resolve the (perceived) issue. The floor is actually pretty clean. – Steve Nov 17 '15 at 23:28
  • If you wash it on a monthly basis (at a guess) the trap doesn't dry out, problem solved. If you have the sewer-stinky-smell going on, using a pine-oil soap (or similar) will certainly help the overall basement odor profile. – Ecnerwal Nov 18 '15 at 2:43
  • Mineral Oil is a great use for this. My grandfather even used this in his cabin's kitchen and bathroom where it might not get use several months out of the year. It always worked pretty well. – BrownRedHawk Nov 18 '15 at 14:50

I'm having a floor drain installed, and the plumber included a "trap primer" which puts a little water into the floor drain trap whenever the water pressure in the water pipes changes. When you flush a toilet or turn on the shower bath, the water pressure changes, and the trap primer adds a bit of water to the trap. Note that waterless urinal fixtures would have a similar problem. With no water entering the trap, the trap is filled with urine, which would smell. The solution is to have a fluid that floats on the urine, blocking smell. Kohler sells waterless urinal sealing fluid. The fluid also slows evaporation from the trap.

If you are not using the drain at all, I've heard of people poring oil down to replace the water as oil dosn't evaporate. Obviously use an oil safe for the system (not motor oil) and something that won't smell when it gets rancid. I suspect the cheapest vegetable oil you can find would work.

https://en.wikipedia.org/wiki/Trap_primer#Other_solutions has some good suggestions.

  • 2
    Vegetable oil gets rancid - which is why USP mineral oil is a better bet. But you don't need an entire-trap-full, just enough to cover the surface of the water. – Ecnerwal Nov 18 '15 at 2:38

I had something similar last year after movie into a new house. Long story short, the smell traced to a dirty trap in an upstairs shower. I used a soapy paint roller to scrub the drain.

The basement drains in my house would also become dry allowing sewer gas to emanate and foul the air. To alleviate the noxious and malodorous gas I have written on my weekly chore list to "run water in all traps". I've found this to be a successful way to stop the stench from the drain.

  • How does this answer the question? – Tester101 Nov 19 '15 at 13:19

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