Have a black spot around the base of the toilet. Seems to be getting larger. I assumed this was caused from a water leak of some kind around the toilet. Had the plumber out. He said the toilet was firm, not moving and there was no staining on the ceiling in the room below. Plumber said he didn't think we had any kind of water leak and he felt it was the adhesive from the floor coming through (so he didn't pull the toilet and check). He said flooring folks use a lot of adhesive by the toilet. Huh?? I thought this type of staining was always caused by water. The toilet is caulked all around so I don't think water from condensation got in. Is this type of stain possible from something other than water like the plumber said?

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I thought this type of staining was always caused by water

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It is. Black spot is mold. Probably caused by partially blocked drain. Pull toilet, snake drain to insure that it is clear. Kill mold with diluted bleach. Reset toilet with new wax ring. Do not replace caulking.

This is the reason I do not caulk around toilets. If the drain backs up and water comes out under the toilet I want it to run out from underneath the toilet so I can see it. Depending on how long this has been going on you may find floor damage.

Find different plumber.

The International Plumbing Code requires all fixtures to be sealed where they come in contact with walls or floors. That includes tubs, mop basins, lavatories, sinks, shower bases, urinals, and toilets. Properly sealed fixtures will prevent some of the nastiest germs / bacteria from having a place to hide and grow in the event of an overflow.
A properly prepared bathroom floor, and, a correctly installed toilet, will sit leak proof, and, odor free for decades.Attachment 46735

Don't care what plumbing code says, if my toilet drain backs up I want to know about it.
Please reduce the size of your picture, it makes the post to hard to read.

Mike can you please quote a section number from the code that directs toilets to be caulked? I have never caulked atoilet and neither has Harold for one simple reason, all wax rings eventually fail. When they fail there will be significant damage if the toilet is caulked. The picture does not show caulking the toilet to the floor, it shows caulking the flange to the floor, may be are talking about two different things

I apologize for the poor quality photo, the picture shows a flange that I repaired; we always seal all exposed wood near the flange with thin set mortar. I will try to post a " before " photo that shows the same flange set below the floor level. The previous plumber tried to stack several wax gaskets to make up for the low flange; stacked gaskets can never provide a sanitary seal - they will break down due to probing by a closet auger / snake, erosion from water flow, and, uncompressed wax gaskets will shrink.
A correctly installed wax gasket, on a properly prepared, stable floor and flange, will never touch water unless there is a blockage and backup in the pipe it is connected to - and, a correctly installed wax ring will withstand the very slight pressure imposed upon it by an overflowing toilet bowl...
Section 405.5 of the 2012 International Plumbing Code ( both the IRC & IPC have had this requirement for decades ) states : " Water tight joints - joints formed where fixtures come in contact with walls and floors shall be sealed ".
The Plumbing Codes are the result of very careful consideration, experiences of illness & death, and the potential for unhealthy, risky practices & occurrences. The codes intent is to describe the BEST possible methods for providing, and, maintaining sanitation - these are a Plumbers most important duties..........

Before photoAttachment 46739

Not a licensed plumber but put into quite a few. I think Harold and I both never consider a toilet sitting on a floor to be "Water tight joints - joints formed" Since there should not be water on the floor under the toilet we would assume it is not a water tight seal. My maintenance crew interprts he code the same way. Tom Speedball should pick up on this and chime in maybe early tomorrow.

As said above few times: moisture leak under the vinyl. Replace wax ring.

Milo