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I smell sewer gas every other day or so in my basement bathroom which has a sink, toilet, and shower. The p-trap on all of them are full and when I get down and smell the drain of the shower, it has a slight stench of the sewer.

Is something wrong? How might the gas be getting into the room if the p-trap is full?

My home was built in the 60s and has cast iron drains under the house. The angle of the shower p-trap is too sharp to scope but I had a plumber scope the drain of the toilet next to it and he found no evidence of a leak or larger issue. Water from the toilet and shower seemed to flow easily. But we technically could not see precisely under the shower.

The plumber did replace the wax seal on the toilet and mentioned the seal there is still not the best but when I lean down there, I don't smell a thing.

He also scoped the sewer vents on top of my house and said they were clear.

When I cover the shower drain with a plastic lid, the smell goes away and does not come back so I am pretty sure it is the culprit - but I am trying to keep an open mind as this does not make a lot of sense to the plumber.

I filled the shower drain with vinegar and then got no smell the next day. Then I added baking soda which foamed and the room filled with sewer gas - I assume because the foaming allowed the gas to flow into the house. Does this mean anything?

My only other thing to add is my Radon mitigation. There is a crawlspace near the bathroom, about 12 feet away which has a large plastic tarp over all the dirt and a radon system pulling air up and out. Any chance this is creating enough pressure to occasionally pull sewer gas out of my shower? It sounds silly but I have no idea what it could be.

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  • Can you tell us how you know the shower trap is full and effective? It's not easy to see. A siphon could pull out just enough water to leave a millimeter of air open in the trap, which is enough to cause your issues. You should clean the drain as suggeted by RMDMan to rule that out, but I'd be surprised if the buildup on the sides of a shower drain, maybe one or two inches above the trap, could fill a room with stink. I'd much more suspect the trap is not doing its job.
    – jay613
    Apr 6, 2023 at 15:19
  • I have a flashlight and can clearly see about 6 inches of pipe and then the top of the water level. The plumber who investigated this issue looked at it and also said it's full.
    – Justin
    Apr 6, 2023 at 15:28
  • Sanity check: Is the vent for that drain stack plugged, perhaps?
    – keshlam
    Apr 6, 2023 at 22:24
  • We ran a scope down the vent from the roof all the way to the sewer line and didn't find any obstructions.
    – Justin
    Apr 7, 2023 at 14:45

1 Answer 1

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What you are smelling is most likely not sewer gas but the smell of the icky stuff on the walls of the shower drain.

it doesn't mean you are dirty, quite the opposite. Taking showers puts all sorts of dirt and debris and soap down the drain that, over time, collect on the walls of the drain.

Remove the strainer and use one of the drain cleaning tools that have the Velcro type surface to grab the hair and icky stuff. Then tape a rag to a long screwdriver and use it to clean the sides of the drain pipe. ( Tape it well, you don't want to lose it down the drain.)

I've had the same type of problem. This cured it and made my wife happier.

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  • Rather than taping a rag to a screwdriver I would think a cylindrical brush on a twisted wire shaft would be better. Taping a rag onto a screwdriver sounds like too much risk the rag would come loose. justmanbrush.com/twisted-in-wire-floor-drain-brushes Apr 6, 2023 at 16:51
  • I used drain cleaner and a brush to clean above the water line. If I need to clean further in... I'd probably need a specialized tool. I would say it's been sanitized between the pure vinegar soak overnight and the drain cleaner today. I'll let you know if it got any better.
    – Justin
    Apr 6, 2023 at 19:55
  • Seems to have worked!
    – Justin
    Apr 12, 2023 at 19:02

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