Why does water puddles show up on my Bathroom Floor when the Water is Shut-Off to the House?

QUESTION

I remodeled my bathroom a year ago. I did the tiling in the shower and floor myself. I also installed the toilet and made sure no leaks before caulking around the toilet. It’s strange that after 6 months, while going on vacation for a week, I shut off the water supply to the whole house as usual and when returning, I came back with a puddle of water sitting on top of the tiles in the bathroom. I tried looking for the source, but the walls were dry, the tiles in the shower were dry, the caulking around the toilet was dry. It seems like it was coming from the bottom up of the floor tiles since I couldn’t find any leak from the walls, toilet, shower, and sink. I dried up the water and it took about a few weeks for the grout to dry completely with white crusting which is from the water minerals. I cleaned the tiles and grout when it was fully dry and I still see some hard water marks on the tile where the water was. No leakage has occurred since. 6 months later I went on a business trip for a week and turned off the water supply again. Came home and this time there was water on top of the tiles again, but less of a puddle. The puddle was mainly between the 3 rows of 12” tiles next to the shower pan and in front of the toilet. However no signs of water seeping through the caulking from the base of the shower pan and tile and the toilet and tile. No water leaks from all the showers, toilet and sink again. The walls and baseboards are dry and the caulk from the baseboard to the tile shows no leaks or holes. So it has to be coming up from the bottom of the tile. There is concret slab under the tiles and I live on a single story home in Southern California. If it helps any, the water supply shut off line is about 5ft from that bathroom and is where the water source plumbing enters the property from the city. As an engineer, I can’t figure this one out. The only thing that comes to mind is if the water is slowly leaking from between the toilet wax and the concrete floor/mortar that is seeping up, but that would happen every day regardless of the water supply being turned off. And the puddle is not directly under the toilet but 6 inches to the right of it and is up against the base of the shower pan and also the water is a foot to the front of the toilet near the wall and the side of the shower pan. It could be just the way the tiles are slightly tilted to direct water toward the shower. What’s strange is the tiles are not wet anymore after I turn on the water supply and dry the puddle. The grout is still drying after a month I came back and has some water stains marks, but no water coming up. So it seems like the water is slowly drying under the tiles. I tap tiles and they seem to be solid and held in place well by the mortar and the grout is still solid with no deterioration. I hope the two weeks accumulated water exposure has not caused major damage to the adherence of the tiles and grout. I also don’t use that shower at the moment since it was done 1 year ago as I have another shower that I use in the house. So I can’t see how water could be leaking from the pipes down to the shower bade beneath the shower pan onto the sub floor. I even actually made one tile removable at the plumbing area of the shower controls and it is dry. Any help to troubleshoot this strange occurrence is greatly appreciated. At this point, when leaving for a week long trip, I don’t plan on shutting off the water supply anymore to prevent this from happening as it’s the only time it happens.

ANSWER

ANSWER -  It sounds like a real mystery.   The only time you get a puddle of water on your bathroom floor near your toilet is when you shut off the water supply to the house and go on vacation.

If the water supply is off, then the only source of the water has to be someone brought it in from outside, or there is some sort of hydrostatic water condition that forces the water up through the grout joints of the tile.

Problems always seems like a phenomenon until you start to intrusively take things a part to look for the evidence of what caused the problem and where the source of water is coming from.  It is never cheap to go to this effort, but you may never fully understand what is causing this condition without doing that.

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