We bought a 1920 house about a year and a half ago and occasionally we can smell a sewer gas odor and I’ve been trying to figure out what the cause is. Whenever we smell it, it’s coming from either the second floor bathroom or the kitchen below it on the first floor. Sometimes it smells bad in the dish washer when I open it up right after running it.

The house was replumbed by the previous owner. It has those red and blue plastic pipes. The sewer pipes (at least in the basement) appear new as well.

Last night, I figured out something new which may be related: when the tub in the second floor bath is stopped, the toilet does not flush well. We had started smelling the odor last night in the bathroom when the toilet wasn’t flushing. My wife took a shower and realized the tub wasn’t draining well and we figured out that the stopper was engaged. After disengaging the stopper, the toilet would flush fine.

I have been looking for a pattern that can explain every time we smell the odor, but nothing seems to fit. Running water to fill traps doesn’t seem to fix anything. I have a couple theories:

  1. The vents are not done correctly
  2. There’s a siphoning issue
  3. There’s a crack in a vent
  4. Toilet seal is breeched (toilet is slightly wobbly)

Do any of these make sense?

Other notes: the previous owners added 2 bathrooms: one on the first floor and one in the basement that has a pump in the floor to pump waste water up into the sewer line. I don’t know if this is relevant. The kitchen was expanded and the kitchen sink moved. There’s a vent under the sink. I can’t figure out where the vent for the second floor bathroom is. It’s definitely not through the roof and the only think I see outside on the exterior wall is the air vent for the ceiling thing.

Does anyone have an idea what could be going on here? What could be causing the sewer gas smell? And why doesn’t the toilet flush well when the tub is stopped? Is that a telling sign of what the issue is?

  • 3
    Don't tighten the bolts on the toilet to stop the wobbling, but insert plastic wedges specific for this purpose. Use the wedges at two separate locations to both level and stop wobbling. The necessity for having the tub drain open for the toilet to flush well indicates to me that there is no proper vent for the toilet (and tub?). Sounds like the tub drain is acting as the vent for the toilet and so the sewer smell may be coming from the tub drain. Commented Jan 22, 2019 at 11:43
  • Ok, well I just tested what I thought made sense last night. I engaged the stopper for the tub and flushed the toilet. It flushed fine. I also looked and the tub has a trap... so I’m not certain that the toilet is venting through the tub drain. But I don’t know. Maybe it is...
    – hepcat72
    Commented Jan 22, 2019 at 12:30
  • The stopper on the tub doesn’t work well. It’s hard to tell which position is the stopped position. When I flushed last night, there was standing water in the tub. Maybe when I flushed the toilet just now, it siphoned off the tub trap’s water but last night, with the standing water, it couldn’t get the air it needed to flush via siphoning. Is there an easy way to tell if the tub trap has enough water in it to not allow the sewer gas through?
    – hepcat72
    Commented Jan 22, 2019 at 12:35
  • So I just tried to reproduce the poor toilet flushing behavior last night by engaging the tub stop, running the tub so it had standing water like last night, then flushed the toilet 3 times. It flushed fine each time. There must be some other complication or cause.
    – hepcat72
    Commented Jan 22, 2019 at 15:17

2 Answers 2


It turned out to be that there was no wax seal on the toilet. I hired a plumber who noticed that the toilet was wobbly and believed that a bad seal was the problem. It’s been a few weeks since he installed a new seal and we have not noticed the sewer gas smell once, so I think that was it!

There were other clues. The previous owner had caulked the hell out of the bottom of the toilet. The floor has a slight slant to it. After pulling up the toilet, the plumber noted that they’d sort of tried to level it with some sort of cement like substance. He tightened the toilet down well, but since the floor has a bit of a slant there, it may loosen over time and that wobbling would degrade the wax seal and it will eventually all flush away. He expects it will need resealed every year or two. (We plan on redoing the bathroom, including pulling up the floor and sistering the joists so we can put down tile, so that should more permanently solve the problem.)

The plumber also noted that with the oversized tub catch, he didn’t think it would siphon even if there was not a vent.

He explained that we smelled it in the kitchen (below the bathroom) because the sewer gas could freely go under the floor and down the wall and get into the living space below. (It’s an old house and thus pretty leaky and poorly insulated.)

He believed that the smell in the dishwasher was a separate issue. He said it was due to backwash from the disposal under the sink. He pointed out some discoloration in the tube that runs waste water from the dishwasher into the disposal. There wasn’t much, but it shows it has happened a bit. He also said that food remnant in the sewer pipes amplify the sewer gas smell. We may notice a smell in the dishwasher from time to time because of this backwash. We haven’t smelled that in quite some time. We don’t use the disposal much because we compost.

There is a vent on the toilet drain, BTW. I overlooked it before.

Side note: I learned what a loop vent is. He pointed it out in the basement. Pretty cool.

Edit: I forgot to mention that that one vent on the roof (which I’d overlooked because it was on the “side roof” [think pyramid] and was gray plastic that blended in with the sky) is the one vent for the entire house. Anything draining anywhere in the house can push the sewer gas (which is heavier than air - as noted in the other answer) up, which is why it was hard to determine a pattern. But also, that opening was there all the time and he said we should have a fairly constant smell. After I mentioned that to my wife, she claimed that she could faintly smell something all the time (which was news to me). I had not suspected a toilet seal breech simply because there was no water damage or noticeable leaks. The plumber explained that you wouldn’t necessarily see leaks when there’s no seal. The waste water only passes through for a few seconds. The plumber was good. He explained everything well. Seemed very astute. It was Povio Plumbing in central Jersey. I recommend them if you live in the area. Oh and he also said that if there was a crack in the drain, you would see this red rust-looking substance on the outside of the drain in the basement because of the type of metal the drain was and there was no evidence of that. And he said that it would definitely be there because the water swirls down the drain and coats all sides.

ADDENDUM: 8/19/2021 We're in the middle of a remodel of that bathroom and thought I would add that I discovered that when that bathroom was replumbed, they cut through the subfloor(? - the diagonal boards beneath the top floor boards) and left a huge gap beside the toilet - and the boards in front of the toilet weren't resting on the joist on one end. Plus, the gap between the joists was extra wide - almost 18". This is likely the main reason why the toilet wobbled. And incidentally, I'm not sure how to reinforce that gap since one of those joists has 3 PEX tubes attached to it. I may move those PEX tubes so I can install a couple ledgers around the toilet with hangars.

  • I also thought it was interesting that he heated the new seal up in the microwave to soften it up, as it had been in his van and it was cold outside and has thus hardened due to the temperature.
    – hepcat72
    Commented Mar 1, 2019 at 18:52
  • 1
    I suggest you take some pictures of the toilet area/floor/joists and create a new Question to seek suggestions. It's hard to imagine what you are trying to reinforce or why the PEX supply lines are obstructing things. Commented Aug 19, 2021 at 14:39
  • Sorry. Didn't mean to ask a new question. I was just thinking out loud. I'll figure it out. I just haven't gotten that far yet and am in the middle of reinforcing for a claw foot tub.
    – hepcat72
    Commented Aug 19, 2021 at 14:41
  • 1
    Yeah, thanks for the answer update. It might not hurt to post the newly discovered toilet/plumbing issues as a new question even if you don't need help. You can post the question then immediately self-answer to help someone else out.
    – FreeMan
    Commented Aug 19, 2021 at 15:13

It looks to me, based on what you have posted, that you have a blocked, under sized or non-existent vent system. I have seen several times when DIY people cannot understand the purpose and function of the vents and think they are superfluous, so they don't replace them when re-doing the plumbing, or use too small of a vent pipe. The volume of water leaving a toilet all at once can create a lot of vacuum and if the vent is too small, it can become enough vacuum pressure to pull water out of other traps.

You don't have to concern yourself IF there is "enough" water in the trap of your tub, because once there is ANY water in the tub, it will fill the trap. But again, without proper venting for the toilet, it might be sucking the water back OUT of the tub trap when it flushes.

Even if it is all properly designed and installed, you may have a bird nest, rat carcass etc. stuck in the vent pipe. I had one client who redid his entire plumbing system because of poor flushing of his toilets and trap smell issues, only to find that the root cause all along was just a dead rat in the vent pipe...

  • I think you may be right. It potentially explains a couple odd things: 1. A few months ago, I cleaned out the tub trap & the connector cracked when tightening it down. I went to Home Depot to get a replacement & they didn’t have one, as it was non-standard oversized. 2. The kitchen sink has a 1-way vent. I’d never seen one before and had to look it up when I installed my disposal (btw, I know what you’re thinking: we compost and don’t use the disposal much, so it’s very unlikely a clog). I think both of these issues are likely attempts to address a siphoning issue by the previous owner.
    – hepcat72
    Commented Jan 30, 2019 at 15:16
  • Had a plumber come and look today. He didn’t think it was a siphoning issue. Rather, he thinks it’s a broken toilet seal. The toilet wobbles and the previous owner caulked the hell out of it. He thinks everything appears to be properly vented. (There is in fact a vent pipe on the side of the roof I hadn’t noticed, which I now feel dumb to have overlooked.) there still could be a crack in either the vent pipe or drain pipe, but he didn’t think it was likely. There would be signs of water on the pipe in the basement and there’s not and we’d smell it all the time.
    – hepcat72
    Commented Feb 8, 2019 at 16:29
  • Yeah, that could allow sewer gas to infiltrate. But that does not explain the toilet flush and tub drain issue at all.
    – JRaef
    Commented Feb 8, 2019 at 18:06
  • I had described that to him, but since that couldn’t be reproduced, he called it a fluke/coincidence. Also, I recently noticed the smell emanating from a corner of the mud room under the bathroom. I also recently noticed the smell in the bathroom near the toilet when at the same time, there was no odor near the tub and the toilet bowl was full.
    – hepcat72
    Commented Feb 8, 2019 at 18:10
  • Sewer gas is hydrogen sulfide (H2S) that is the result of decomposing 'biomass" in the sewer pipes and the entire sewer system. It is heavier than air and will sink / flow through the walls to various places, coming out in places that have nothing to do with the origin depending on air flows in your walls.
    – JRaef
    Commented Feb 11, 2019 at 22:57

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.