The mold issue is the big problem now. It's in the walls, behind the baseboard, and
most likely on the subfloor under the ceramic tile in the powder room and kitchen.
The moisture detector got readings through the tile. Most likely the tile and the
subfloor will need replacing. What a mess.
That may indicate a far more serious problem. You need to cut out that
section of pipe and see why it is leaking. You might find out there is
significant erosion of all of the pipe. That is somewhat unusual up
there where you have good water but people in the Cape around Wayne's
place have had a lot of serious pin holing problems in copper pipe.
The last I heard it was still in court with the plaintiffs fighting
about whether it was bad pipe, bad water or an electrical problem.
All of that made me a plastic pipe fan although there is some copper
in here that has been OK for 50 years.
That section, about 6" was cut out. The hole in the pipe is a bit less than one mm,
according to my non-calibrated eyeballs. The pipe looks just fine, except where the
leak is. The plumber knew of no reason the pipe would start leaking, other than some
'could be's' as you stated, chemicals, electrical, or a bit of copper on the inside
of the pipe that caused a small dam, catching chemicals, etc.
I'll sure be looking at the walls a lot more closely from now on!
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