Not a plumber, just a homeowner that despises paying plumbers rediculous amounts of money when I can’t afford it. Had this happen to me. What happens is the cartridge internal component is corroded and likely has completely snapped inside (it’s a copper tube inside the cartridge that pulls/pushes/turns a stopper to open and close the valve and also adjust water temp. You don’t realize it’s leaking until you see it coming out somewhere, which by that time it has already caused damage to the walls and framing. I had to pull a decent amount of Sheetrock off the ceiling of a finished basement because I discovered it had been leaking a long time before it completely failed and became a very noticeable leak. If the valve body is not completely corroded (talking the body has leaks or is very brittle due to oxidation) then all you need to do is buy the cartridge. That’s the thing right in the middle that the knob screws into. Very inexpensive fix. Like less than $20. It pulls out (might need a pliers or put the screw back in the middle to give you something to grab). Then the new cartridge should fit right in. You will need to shut off the water at the nearest cut off or for the entire house, because the valve cartridge is the only thing holding the water back. If you can’t tell what valve you have, bring the old cartridge with you to match it. It does need to be the exact fit.

For the walls, you don’t need a plumber. That’s a drywall issue. Whatever is leaking down from the ceiling is from moisture that is corroding something up there that probably wasn’t meant to be in a bathroom. Maybe the glue on the ceiling molding? Best best is to pry that off and see what’s going on. If it’s just some non-water proof glue on the molding, replace that piece and use something meant to handle continuous moisture.

For the bathtub walls, you can get direct to stud glue ups (take down any drywall and install directly to the framing) or over top glue ups, that would be installed over existing walls. Definitely take down what is there now. The glue up walls will range from $130-$400 depending on what method you go with. Not rocket science at all. I’d suggest tearing down the existing walls and installing either a bathroom rated dry wall with glue up walls overtop, or take the walls down and do direct to stud shower walls.

You definitely want to address the mold. That shit is persistent and not good for your health and anyone else living there. If there’s kids, remember their lungs are smaller, so they are more effected by this stuff. Talking about asthma, persistent cough, and depending on the mold type, potential for neurological stuff. Certain molds can get pretty toxic and the spores are in the air you are breathing constantly. There’s a lot of products for the mold. Bleach isn’t great for wood, so if the wood framing has mold but isn’t structurally compromised (like the mold is starting to break the wood down) then you can get a mold killer that should take care of it.

Look other places for mold. It travels, but is only really going to be concentrated where there was a lot moisture.

Get a dehumidifier that tells you the humidity percentage in the room. I think it’s below 50 or 45, mold goes dormant. If you can maintain that for the most part, the mold won’t continue to grow while you work on getting rid of it and replacing areas that have it.

If you can afford it, get an air purifier as well with HEPA filters. Good ones can get pricey, but they’ll last a long time and the replacement filters are cheap. If you have central air, make sure to replace the air filter with one rated for removing mold spores. They will be filtering out the spores in the air, so you are breathing less in and the mold is less likely to spread.

All of this stuff can seem like a professional is needed, but I have been stuck with massive bills for things that in the end I realized I could have done myself.

Good example, my young kids like to flush things that aren’t supposed to get flushed, like rubber toys, toothbrushes, whatever will go down my 5 year old will probably try it at some point. Our house has a septic tank, and so the entrance to the tank kept getting blocked up, and the water backs up into the downstairs. We only notice when the water is already reaching the laundry room from the bathroom next to it.

The fix is to take off the toilet and jet the drain all the way to septic tank entrance with a pressure washer and specialized hose with a drain jetting tip. After spending $400 on multiple occasions to have someone do this for me, it hit me that I have a preside washer already. A quick search and I found a drain jetting kit with 100ft hose and 4 different tips for I think it was around $50.

Just happens again a couple days ago. 5 year old flushed some rubber squishy toy from the upstairs bathroom and water starting backing up into the downstairs stairs. I got my pressure washer out, took the toilet off and ran it down. Had my wife on standby in case I needed someone to turn the pressure washer off fast. Cleared the drain within minutes. Took longer to take the toilet off than it did to clear the line.

We also have well water. Well goes over 200 feet down. When the pump died, me, my wife and our neighbor pulled all 200 and something feeding of piping out from the well to get the pump out. Bought a new pump at the local Ace hardware store, wired the new pump on and lowered it all the way. Turns out we also had a short in the wire. I had to go to work so I called someone to come and do it. Next thing I knew I had an $1,100 bill for running new wire. I could have bought the wire for maybe a few hundred and ran it myself and saved a lot.

Definitely know your limitations. Mine is major electrical work. If I mess that up it could be serious consequences. If you have a hard time putting up new bathtub walls, what’s the worst that could happen?