Finally figured out why my bathroom remodel smelled like a sewer

I went through almost the exact same thing last summer after we put down new LVP in our master bath. The contractor swore the old flange height was fine, but we kept getting that faint whiff of swamp gas every time the shower steamed up the room. I spent a fortune on those Zip-It tools and enzyme cleaners thinking it was hair buildup in the shower drain.

It wasn't until I sat on the lid one morning and felt a tiny "thunk" that I realized the back left corner had about an 1/8th inch of play. I swapped out the standard wax for a Sani-Seal foam gasket beacuse I was worried about the wax blowing out again if the subfloor flexed. Using those plastic Wobble Wedges to stop the movement made all the difference.

It’s frustrating because you trust the pros to get the leveling right, but if that flange is even slightly recessed below the finished floor height, a standard ring just won't cut it. I always tell people now to check for that tiny shimmy before they start pouring bleach down thier pipes.

In our case, the toilet was rock solid, but we still had that intermittent sulfur smell right after we finished the basement bath. I spent days checking the wax rings and even used a borescope to look for leaks. It turned out the contractor had installed a cheap Oatey Sure-Vent behind the vanity, and the internal spring had failed almost imediately.

Every time we flushed the upstairs toilet, it woud create enought pressure to pull the air out of that basement sink's p-trap becuase that vent wasn't doing its job. We replaced it with a higher-quality Studor Redi-Vent and the smell vanished. If your toilet is level and the wax is good, check under the sink or inside the wall for one of those mechanical vents. They are supposed to be permanent parts but they fail more often than you'd think, especally if they get dusty during the construction phase.

I had a similar nightmare with a basement remodel where the smell whould only kick in when the dryer was running. Turns out the exhaust fan was pulling air right trough a tiny gap in the flange because the bolts weren't tightened down enugh on one side. I didn't bother with a second wax ring; I used a flange extender kit to bring it flush with the new tile. I also switched to those solid brass bolts because the cheap ones that come in the box always seem to strip or bend when you're trying to get a real seal. Once I cranked it down and caulked the base—leaving a small gap in the back to see leaks—the smell vanished for good.

While the focus has been largely on the gasket integrity and the verticality of the flange, I find it imperative to mention that if you’re pulling the fixture anyway, you must verify the structural integrity of the subfloor surrounding the closet bolt penetrations, as even a minor amount of moisture-induced rot will render your shimming efforts moot by allowing the bolts to oscillate within thier mounting slots. I personally prefer a more fastidious approach by applying a bead of 100% silicone around the base—leaving a small weep hole at the rear, naturally—to act as a secondary stabilizer, wich effectively mitigates the lateral shear forces that typically compromise a standard wax seal during frequent usage.

That's a solid catch on the flange leveling, but have you ruled out the possibility that the seal is actualy holding and you're just dealing with a dry floor drain nearby? Sometimes when people remodel, they stop using a secondary drain as much, and the trap primer fails. If you're still getting a whiff during weather changes, I'd double check that it isn't just a venting issue trough the roof stack. It's easy to blame the wax ring becuase it's right there, but sometimes the air pressure just forces sewer gas through the smallest path of least resistance elsewhere.