u/Aggravating-Bug2032

I have recently discovered some water damage in my walls and I'm mystified as to the source.

It is a townhouse approximately 15-20 years old. I am the third owner and have been here 7 or 8 years.

It is an inside unit with neighbours on either side and behind. It has a flat roof. The roof was replaced three or four years ago.

Two years ago I noticed water damage on the ceiling of the second floor bathroom. After opening up ceilings and walls, we discovered that when the house was built the drywallers had put drywall nails through the drainpipe that runs from the roof through the wall in my closet and bedroom, and down to the second floor. We taped over those holes and everything was fine. We repaired the drywall.

A few weeks ago, I noticed water damage on the new drywall in the closet. The drywall was bubbling on a horizontal line, perpendicular to the floor, aligned with the seams of the drywall replacement from the earlier leak, along with new damage on the ceiling. There were two spots where there was damage, both horizontal seams.

In January we had a massive snow storm with at least four or five feet piled up on the roof and it took weeks to melt. Then in March we had a few days of heavy rain. It was after this I noticed the bubbling and the wall felt damp.

Opened up the wall today and tried to recreate the leak. Flooded my roof, listened to bathtubs worth of water flow down the drain. Everything is dry. No leak. No sign of any water damage except that noticed on the outside of the wall.

It looks like this

closet wall -> |O| <- bedroom wall

the space between the walls is only enough for the drain. There is only water damage on the drywall that forms the closet wall. there is no damage to the bedroom wall, either the exposed side, or the side that is basically touching the pipe.

The plumber said he had never seen anything like this but I'm not so lucky that I would experience something no one else in the world has experienced (though if I was, it would probably be this).

The plumbers best guess is that it was the result of the massive snow and heavy rains, unusual for my area, that caused this.

I tend to agree, but it doesn't explain why just one wall has damage, and pretty significant damage at that.

I'm at a loss as to what to do except repair the drywall and wait to see if it happens again.

Does anyone have any ideas? Thank you.