Leaking from both upstairs' bathrooms

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My home was built in 1978 and recently I found out that each time the toilets are flushed or water running in the sinks or showers from either bathroom (have two) upstairs, water is leaking down to the first floor. After finding watermarks on a wall downstairs inside my laundry closet and opening it up, I discovered a water soaked floor and mold on the inside of the drywall along with a black plastic wet pipe (drainpipe?). I was told a plumber would charge a lot of money if he had to look for the leak and tear out drywall so trying to find the leak myself. Another DIY site I posted on received responses that it was a toilet leaking problem or sink drainage as ABS pipes don't leak but that makes no sense as all the sinks, toilets and showers when drained cause water to run down this vertical pipe to the first floor which I have watched happen. There are no other signs of leakage on the ceiling or walls except for the place where the pipe sat next to the wall before I removed the drywall. I suspect it is leaking from where the vertical pipe and horizontal pipe are joined.

Does this sound like a possibility, leaking at the joining of the pipes? Is pretty much my only option to cut out the curved pipe and replace it with new curved pipe and glue or is there a way to seal it without removing the curve if the glue has deteriorated? If the place where they join is not where the leak is any ideas where to look next or will I need to start tearing up the floor or shell out hundreds for the plumber to find it, maybe with a camera? I have attached a rough sketch of the layout of the problem area. I have done lots of "plumbing jobs" with PVC piping such as replacing steel rusted pipes with PVC pipes for sinks and extending and shortening PVC pipes but once I find the leak would it be better to have a plumber do the repair or can a person with some experience do it?


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The Plumbing Wizard
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Bothell, Washington
I would pick up a moisture meter and do some detective work.

You want to find out where the leak starts from, not where it all went. Sometimes the leak is a crack in a ABS fitting, often caused by compression of the home as the wood framing dries and shortens which puts pressure on the pipes and fittings. And then sometimes they just crack for no reason. Fun isn't it.

A drywall jab saw is good for removing drywall for access to the pipes. If the fitting is cracked, it has to be removed and replaced. There is no repair for that.

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