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So our house was flipped about 7 years ago and we bought it. In the last two years we've started smelling what we can only describe as raw sewage small occasionally emanating from the (new) attic bathroom. I feel like it usually happens on hot days but I haven't really kept a tracker. We stopped using the bathroom last year when I felt as though the draining pipes that went down the walls of the second floor were making funny noises. As though maybe water was leaking every time there was a toilet flush. Incidentally those pipes are being moved when we redo our second floor bathroom so at least it that is already in the plan to being addressed. However we aren't sure those pipes are actually messed up and the smell is CLEARLY coming from the floor above. also of note, and to my chagrin, the outflow pipes from the attic bathroom run along all THREE axes of the main bedroom. so we get the surround sound effect when someone flushes the toilet (I hate that bathroom). We don't really know where to start with this. The smell comes and goes and can be overwhelming at times. I don't THINK the pipes on the second floor are the main concern, but hey what do I know. Is this a semi common thing? Does it have a general fix? I thought the u-pipes used in the sink, toilet and shower drain were designed to prevent this?

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  • The P-traps(you call u-pipes) are suppose to prevent this. They trap/hold water to stop gases from coming up. There might be a leak/loose joint in the drain pipe that allows gas to to escape or a problem with the vent system that empties the trap/s part way.
    – crip659
    Jul 30, 2023 at 22:57
  • This sounds like you have a venting problem. If you can draw out what you know of the drain plumbing including where the vent stacks are, we will be more likely to help. Also, if you are not using a bathroom at all, make sure to run water down the sink every few months so it doesn't dry out the trap.
    – KMJ
    Jul 31, 2023 at 3:33

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You wrote --

We stopped using the bathroom last year

That's the problem! Every drain (toilet, sink, shower, tub) must have water in it at all times. The water in the P-trap (as crip659 said) prevents sewer gases from entering the bathroom. If you're not using the plumbing, the water evaporates and sewer gases can come up the drain.

There's a guest bathroom in my house that no one uses unless we have guests. During the pandemic when we had no guests, I had to remember to run some water down all the drains in that bathroom every three months or so, or ewww, that smell would fill the room.

So go upstairs, flush the toilet and run the water in each fixture for a few seconds to fill the trap. Repeat on a schedule of your choice to prevent it happening in the future.

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  • I read an article recently about the problem of HVAC condensate traps drying out. The author claimed that a trap may lose 1-2 inches of water per month (depending on ambient humidity, air circulation, etc). The plumbing drain traps I'm familiar with would cease blocking odors with a loss of just 2 inches of water, so that author's claims match your results somewhat well.
    – Greg Hill
    Jul 31, 2023 at 4:07
  • A bit of oil poured into the drain will float on top of the water and can slow the drying. Cooking oils work but will eventually go rancid and make their own odors. Mineral oil doesn't sour.
    – Greg Hill
    Jul 31, 2023 at 4:08
  • I have a rarely-used bath that needs the trap topped up every few weeks in summer (I use a separate shower) or I can smell the drains. Air flow makes the source of the smell very hard to trace
    – Chris H
    Jul 31, 2023 at 8:41

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