I have a rental unit where the apartment shares a common wall with another unit, the two units have identical, but mirror image floor plans. So across my tub is their tub, across my kitchen sink is their etc...

My tenant told me recently that there is an uptick in roach appearance lately, and it appears they are always found near the common wall where both apartments have kitchens and bathrooms. I went there today for a quick 30 minute inspection along that entire wall, pulled out stove, refrigerator, looked under the sink etc...and I suspect there is a leak behind the kitchen.

A related (or unrelated story), the owner of the other side a few weeks ago asked me if I am aware of any leak on my side. I said I haven't heard any complaints, but he said his tenant told him the wall feels wet and spongy. A few days later I asked him whether the issue was resolved he said yes, and it was a broken leaking dishwasher drain hose that ran behind the wall. No idea how long it has been leaking, and sending food debris into the common space.

Back to my end, I discovered there is some deformation of the cabinet back and bottom. The back feels wet at the bottom. I am reasonably sure it wasn't like this before.

See pictures below.





Of course, this moisture could be from something that leaked inside the sink cabinet, like may be my tenant spilled a jar of bleach or detergent, but being that it seems to be directly below the drain is suspicious to me. It could also be the spilled dishwasher hose from next door, but this is about 4" above the slab I am not so convinced this is related to the dishwasher hose leak.

So my guess is this is probably a kitchen drain leak behind the wall and the water runs back to the back side of the wood and down? The front side is bone dry.

The pipe behind the wall is 1.5" copper drain. What is the least instrusive/destructive way to determine what's going on? Use a hole saw and drill say a 3" hole on the back wall right under the drain stub out? And if there is nothing wrong then use the cut out circle to patch it back? Or to remove the entire cabinet back since it's warped anyway?

You can start with a hole saw and see what the drywall looks like. If the whole back panel is warped, I would think that's because it's all had a lot of moisture and probably best to just replace the whole back of it and also check for the roach infestation.

If you needed a larger inspection hole could you just leave a few inches around each edge of the back panel and replace when done with a sheet of "marlite" or whatever it's called in your area...(the thin fiberglass like plastic panel that's often used in commercial/public bathroom walls). would be cleanable and impervious to any future water leaks? either glue (permanent) or screw (removable) to the edges left around the cutout?

As you can imagine Mold is a dangerous health hazard. You must use caution and remove any material containing moisture and dry out the general area thoroughly no matter the cost.