If you stop using the shower does the leak stop? If you turn off the water does the leak stop? Have a look at the shower valve to see if there are any leaks. You might have to cut a hole in the drywall behind the shower to do a full inspection.

Thank you. There are built-in shelves on the other side of the shower, which will make cutting an inspection hole very challenging. Yes, if the shower water is shut off, the leak stops.

Sounds like plumbing, disassemble the value and have a feel around/look inside. Do you have a shower head that screws into a pipe behind the wall? Check that it isn't loose.

It "could" be the plumbing and the mixing valve would be the first thing to check. Investigate as Jeff says. Is it wet? That being said, it's often NOT the plumbing (you're lucky if it is). You could do a flood test on the pan. Google flood test shower pan for correct method. This would help you determine if the pan is leaking. I assume there is no evidence of water in ceiling below that room? First line of defense for leaky shower: Your first line of defense should be to have your plumber check for leaks in the:

· shower control valve and/or pipe to the shower head

· water supply lines

· drain and trap

Specifically related to the evidence you've described IF not the plumbing:

The walls outside my shower are damp and spongy, drywall is bubbling, or damaged. There is evidence of water or moisture damage and mold/mildew on the wall(s) on the other side of the shower.

Potential Causes

· The shower pan liner was not correctly installed

· The shower drain was not properly installed

· The shower drain weep holes are clogged

· Improper or lack of waterproofing on walls up to the shower head

· Fasteners were placed through the tiled surface and not sealed

One way to determine whether it's the plumbing or a problem with the construction of the shower is to run the shower for several minutes but route the water elsewhere. You can take off the shower head and slip a garden hose over the pipe and run the water to another drain or out a window. If you still have a leak, it's the plumbing; if it's dry, you have a problem with the shower construction.

Thanks to all above. Some weekend work for sure...