We have a newer home (~5 years old), since we first moved in we've noticed occasionally a sewer gas smell on the 2nd floor. Over time we've traced it to either the washing machine, or the shower/tub drain (the two share a wall). Plumbers did a smoke test during the first year but found no leaks or anything wrong, and chalked it up to most likely evaporation due to lack of use.

Over time, I just got in the habit of filling the p-trap in the tub, then closing the drain to limit evap. With washer, just running the washing machine if the smell was back.

Recently we've seen a very consistent set of behavior. With the tub drain open, run a bunch of laundry and the next day the sewer gas is at the tub drain. Refill p-trap, run laundry a few days later and smell is back. When I tried blocking the tub drain, running laundry, the smell ended up coming from the laundry room.

It seems like there's some siphoning and self-siphoning going on. I don't know if something was improperly installed, or even where to look. The plumbers do not have any ideas of what could be going on (but haven't been out to review in detail). One possible explanation is a blocked vent stack, I have not checked this in detail but it would've had to have been blocked from day 1 almost (certainly possible). The washing machine being pumped past the shower/tub drain and siphoning that trap makes sense, but doesn't explain why the washing machine trap seems to self-siphon if the shower/tub drain is closed.

I tried to draw up a schematic of what I think the stack looks like. I have a poor resolution photo I found when the walls were opened, but I don't know if the tub/drain actually attaches to the vent stack of the washing machine (seems like it does), which I believe sets up a wet vent (not a plumber)? You can see the tub behind the washing machine discharge assemblage, so attachment to the vent stack if it's there would be below subfloor.

Would very much appreciate any advice on what could be going on here and what my best course of action would be to fix.

it appears as if the washing machine drain ties into the tub vent pipe..
not legal and most likely the washing machine discharge is syphoning the tub trap.
to isolate the tub drain you must also stuff something into the overflow plate.

I'm not a pro plumber, but both fixtures are behaving like they are not properly vented, In the absence of a functioning vent, the washer and tub drains are venting each other resulting in the trap siphoning and the sewer gas smell.

Since both fixtures tie into that same vent stack, a blockage in that vent would affect both fixtures. I would have a plumber get on the roof and see what's going on with that vent.

In new construction like this, the DWV piping is installed with caps/seals at each fixture and vent, and then pressurized to prove to the local inspector that there are no leaks. It's possible that the roof vent in question was never opened up after passing inspection.

Very much appreciated, in scouring posts on the subject I've seen a mix bag of 'allowed' vs 'not-allowed' in terms of wet venting of washing machines. You say it's not legal, is that the case in MO as it is in MA? The fact that it's against code somewhere probably suggest at a minimum it should be avoided if possible.

Good idea, will check on that one.

Haven't gotten on the roof yet but can see the vents from an elevated position behind our house. I counted four black plumbing vents, three of them point directly upwards while one had a u-bend so that the exhaust points down. No idea why, still researching why those might be used versus the direct upward vents. The most likely location of the problem vent is with one that is pointing skyward. I don't know what the caps would look like, but there was nothing obvious from near ground level that it was covered (will try to get a downward look at some point).

as piped...the washer draining could pull the tub trap but the tub draining could never pull the washer trap.

How would the issue be resolved, guessing I won't have much of a leg to stand on with the plumbers if it's not a code violation where I live so I either have to live with it or pay out of pocket to get it addressed. Seems like putting the washer on its own vent line would resolve but would be fairly invasive...

I believe (I am not a pro) not draining a major fixture (washing machine) past a lesser fixture (tub) is code everywhere. Remember the washing machine is discharging a lot of water quickly under pressure (there is a pump inside the washing machine to evacuate the water).

Two possible solutions.

(1) run a vent off the tub line below where the washing machine connects, up past then reconnect back to same vent say 42" or higher.

(2) install a laundry sink and connect the sink drain to where the washing machine drains, and put the washing machine drain into the sink. The washing machine draining under pressure will fill the laundry sink only, the sink then in turn drain slowly under gravity and will not siphon from the tub because the normal gravity flow will not completely fill the entire pipe to cut off air as it drains past the tub connection to cause the siphoning.



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Ideally this is a code violation (from a who is responsible for paying for repairs), but I can't seem to find it. I looked at the MA code as others have noted it is not legal do this there and it's fairly clear you cannot do what I presume is being done. However, the MO one says that wet vents are permitted and ties no limitations to whether a clothes washing machine is used. One small caveat though, it says the dry vent connection to a vertical wet vet "shall be an individual or common vent serving one or two fixtures", I believe, could be wrong, but I believe ours ties into a circuit in the attic that couples up to the rest of the second floor fixtures, which would number greater than two for sure, not sure if that's what the piece is trying to enforce (i.e. a dedicated vent pipe to serve the wet vent)


(MA): A waste and vent that serves a kitchen sink, a garbage disposal, a dish washer, or other fixture installed for culinary use, or one that receives the discharge from a clothes washing machine may not serve as a wet vent for any other fixture.

(MO-adopted from IRC): A combination of fixtures located on the same floor level shall be permitted to be vented by a vertical wet vent. The vertical wet vent shall be considered the vent for the fixtures and shall extend from the connection of the dry vent down to the lowest fixture drain connection. Each wet-vented fixture shall connect independently to the vertical wet vent. All water closet drains shall connect at the same elevation. Other fixture drains shall connect above or at the same elevation as the water closet fixture drains. The dry vent connection to the vertical wet vent shall be an individual or common vent serving one or two fixtures.

i just re-read your post.
this statement throws everything out the window.
" but I don't know if the tub/drain actually attaches to the vent stack of the washing machine (seems like it does)"
no one here can offer any help based on your assumptions.
obviously something is wrong but until we know exactly how the configuration is piped, we are all in the dark.

Right, I'll try to confirm with the plumbers. I did a test yesterday using some cling wrap around the tub overflow and tub drain, when the washing machine first pumped out flow the cling wrap flexed and moved, after that it didn't really do anything although I could see what appeared to be some air 'pulsing' a bit.

Not overly scientific but appears to confirm there's a relationship between the washer and the drain, but that's been pretty obvious from our previous 'run laundry and get tub drain sewer gas' experiences.

Guess it's possible there's some indirect coupling outside of my presumed wet vent. I might get an endoscope/boroscope and see if I can see the water actually clearing the trap to be 100% sure, or maybe it's oscillating instead of clearing out, etc...

Appreciate all the help and advice thus far,

I'll leave it be and try to coordinate with the plumbers.



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when you do your tests, make sure you do a wash with detergent and not just an empty load. The detergent will create suds which causes a suds pressure zone along your piping above and below where your washing machine connects, the suds can seal off air in piping and blow out trap seals as well. Of course a lot of that also depends on how much detergent you use, whether those detergent is HE (high efficiency), the load being normal, heavy, light etc...all come into play. With washing machines, not only can it siphon when passing a lesser fixture, the suds effect can also cause positive pressure to blow out trap seals.



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code is the MINIMUM

individual fixture vents are good/best practice





sounds like maybe your waste arms are too long for wet venting to satisfactorily function or the plumbers used wyes instead san Tees



you can try a 'studor vent' to solve the problem

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

So a quick update. I got an endoscope to watch what happens to the water in the tub drain when the washing machine is running/pumping out fluid. What's interesting is I see water in the drain move upwards during the initial surge of flow out of the washing machine, not rapidly so, just like it's filling up. It then drops back down slowly and as the washing machine continues to 'sound like' it's pumping out water, I see maybe some movement of the water in the tub drain but not a lot.

So I did not see the trap suddenly get sucked clear as best I could tell, my thought was maybe the oscillatory motion might be enough to open a gap. When the sewer gas smell returned and I checked with the scope, I still saw water, and as best I could tell there was no visible gap. As I stuck my head closer that's when I realized the smell was coming from the tub overflow. I tried sticking my nose right up to the drain and didn't detect anything (although it's tough as the sewer gas smell was everywhere at that point), but when I put my nose to the overflow it was definitely overpowering.

Most diagrams I see have the overflow tapped into the tub drain BEFORE the p-trap. The fact that I'm getting sewer gas from the overflow but not the drain suggest maybe there's two p-traps, one for the tub drain, and then maybe one downstream of both the tub drain and overflow I guess. It's possible that's the p-trap that's being pulled dry. When I scoped the overflow and ran the tub at the same time, I can see water splashing but can't get the scope to make the turn cleanly to see what's beyond that. I'd like to get to the trap that's is getting cleared and actually witness it, The plumbers want to do a smoke test but based on the behavior of my system I'm not sure what that would tell us. I don't think it's a leak, I think it's a vent arrangement problem, plus we did a smoke test years ago for this same issue and it told us nothing then.

Anyways, slightly more information than before but only raised more questions for me.

There's really not room for a second trap just for the vent, see pic below of standard tub trap setup. The manufacturer of the tub drain assembly makes it to go together like this and have a single exit to the trap. In addition, two traps in series would cause an air bound situation that simulates a blockage (I had to solve this issue on another house.)



I would remove the plate covering the vent on the tub and use your endoscope to look down at the trap through the vent and see what you can see when the washer runs.



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