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Question DetailsAsked on 6/13/2017

What might be the cause of smell in 2nd floor bathroom from 3rd floor bathroom above?

When the toilet is used on the third floor, there is a smell in the second floor bathroom directly below. What is the likely cause? Blocked vent? Cracked pipe? I am not sure exactly when this started, but it may correspond with the replacement of the toilet on the second floor (or not).

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With that direct connection between toilet use and smell, I would say the wax seal (the seal between the bottom of the upstairs toilet and the flange on the drain pipe) is leaking - or the toilet is sitting on a thick flooring which goes in under it or the flange was set low, and a normal thickness rather than a "tile floor" seal weas used, so it was not thick enough to form a proper seal. If the toilet is not fastened down securely (snugly, not reefed on as the bolts can break the toilet base) - say because the wax seal compressed during use so now the toilet "rocks", that also can cause leakage. If either of those is the case then toilet would have to be dismounted and the seal issue fixed.
Course, could be a cracked pipe or toilet flange, leaking pipe joint, etc. Also, depending on the exact plumbing and whether it was to code or not, it is possible though not likely that the flushing is causing backup at the downstairs toilet and it is leaking out at that toilet flange/seal, but that is far from the first suspect. Vent - not unless you have a cracked vent pipe, or remotely possibly a stuck air vent under the basin if you have a Studor or air admittance vent under there - in which case the smell would be coming from the vent when the toilet is flushed. Not definitive, but "generally" if a leaking toilet seal upstairs you would be smelling predominately urine smell, if a venting issue or cracked pipe than sewer/hydrogen sulfide smell would be more common.
Generally, using a fiber optic scope (takes about 1/2-3/4" hole to three) to look through the underlying ceiling in under the tub/shower and toilet is the best way to see what is going on - looking for wetness, pooling, water staining, mold, etc. Obviously, do not let this go unfixed, because if this is fluid leaking you will be getting floor and possible subfloor wetting which can eventually lead to rot and fungal growth.Couple of similar situation questions FUYI here:

Answered 3 years ago by LCD

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