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Ok here is a doozy we have been dealing with for over a year.

House was a new build 2012 and we had never had any plumbing issues until we had our roof replaced a bit over a year back and around the same time began to experience occasional sewer gas smell from the shower floor drain and also across the house from the washing machine, but only while actively in use draining water. Over time it has progressively become more consistent (it is now daily) and stronger odor. I used a flammable/sewer gas detector and it registered up to 50ppm after the shower ran for approximately 5 minutes, then when shut off returned to zero. In the laundry room the gas odor stays in the washer tub until the room gets sufficiently aired out.

Based on the coincidental roof replacement, I assumed the "upgraded" wrapover roof vent boots somehow were constricting the vents and so we had them all replaced with standard 3 in 1s with new extended PVC. This did nothing to address the issue. Plumbers also ran cameras down the vents to check for obstructions and everything is clear and clean. I used my endoscope and verified both the shower and washing machine traps are holding water and not dry.

We then ran cameras from the rearmost cleanout adjacent to the master bath all the way to the sewer tie and and found no leaks, clogs, or issues. Similarly we checked the front yard main cleanouts both into the house and away towards the sewer and found it to be in nearly new condition with no leaks or clogs. As a test, I uncapped the main cleanouts in the front yard and ran the shower, and still got sewer gas smell coming from the drain (this is what is most confusing). What is also unusual is that if we leave the house for a vacation and return after no plumbing has been used for approximately a week, we do not get any sewer gas for 2 to 3 days before it completely returns.

We next called and had the city perform a sewer pull and degrease/washout which also had no effect.

As of today we can hardly stand taking showers, especially at night. The entire master bath and bedroom is overcome with sewer gas on a daily basis. The washer tub gas is less consistent - some days we can run the washer with no issues and other days it too becomes overwhelming.

Not sure what to do now - none of our other drains have any issues including half bath, 2nd bath with kids shower, kitchen sink, etc. I cant make much sense of what could cause sewer gas to pressure past the shower trap while it is actively draining instead of simply exiting though the roof vent like it presumably had been doing for over the past decade. I even tried installing a backflow preventer in the shower drain but as soon as there is flow through it and into the trap, the gas is seemingly bubbling past and into the shower.

Currently at wits end, my plumber essentially gave up on us. No idea what to check next. Help!

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    sounds like your p-traps are siphoning.... you could have the wrong slope or could be over the critical arm length. you could have also had a sewer backup event and the p-traps are now coated in sewer sludge which when you add water activates the smell post vacation. how large is your p-trap pipe (1.5" pipe, 2")?, how far to the vent(s)? how air sealed is your house, are you highly negatively pressurized / do you have a 600cfm range hood? Sep 20, 2023 at 3:03
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    If it was not there before but it is now happening, it might be just blocked roof vents.
    – Traveler
    Sep 20, 2023 at 3:34
  • Wondering if the roof work loosen up some vent joints. They would only leak air/smells so be harder to see.
    – crip659
    Sep 20, 2023 at 13:16
  • I'm with @FreshCodemonger, sounds like a siphon due to bad or non-existing vent
    – Tiger Guy
    Sep 20, 2023 at 13:22
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    @ChrisFoskey That's a clue: "At times the backflow device totally stops the flow momentarily and standing water appears" can only be true if the air pressure in your drain line beyond the backflow preventer is greater than air pressure in the room plus the standing head of water in the shower drain. That should never happen. Air pressure in all your drains beyond P-traps should be equal to outdoor air pressure. You can measure pressure difference room vs drain vs outdoors with a U-tube manometer and long tubing or find a more technically oriented plumber who can figure this out on site.
    – MTA
    Sep 21, 2023 at 13:37

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You’ve observed that sewer gas enters when water flows down the master bath shower drain or the laundry drain. It would make sense if debris had fallen down the stack vent and obstructs the venting for of these drains. More complex explanations are also possible, like there were two roof vents before and only one now because the roofers only brought one vent cuff so they roofed over the other vent.

All explanations depend on the plumber only scoping the stack itself and not actually scoping each vent line to confirm the shower vent and laundry vent are clear. If those vent lines were actually open then those traps would not be siphoning out and allowing gas to enter one glug at a time.

The building air pressure would result in continuous sewer gas intrusion. It would not be limited to when the laundry or shower was in use.

There’s sure to be a better plumber who can scope the lines, and auger out the debris. Finding that better plumber is your best bet.

Failing that, your odds of directing a scope and cleaning the debris out yourself by directing either an auger or a high pressure water jet into the correct vent branch are low. A number of schemes involving large shop vacs on the stack vent and blowing air into the shower drain to dislodge and remove the debris come to mind, but I’m skeptical any of them will work on heavy roofing debris.

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