Leak coming from bathtub

I have just moved into a brand new townhouse. There is a guest bath that adjoins my son's bedroom. An area about the size of a manhole cover has appeared in my son's room that is soaking wet. We do not take showers in this bathroom but I occasionally take a bath. It seems to have appeared after my taking a bath for a couple of nights in a row. (This is the second time this has happened within a month but we got the area completely dried out after the first time and now it has happened again.) The first time, the builder found that some pipe or something behind the drain had been damaged during installation and they repaired it. That's when we got it all dried out and thought the problem was solved. I have not taken a bath in it since until just a few days ago and now it has reappeared. There seems to be no apparent cause for this problem because the water damage is kind of out in the middle of the floor with no obvious path of water leading to it if that makes sense. To me, it seems obvious that whatever is happening is happening after I take a bath but the builder insists that he has filled the tub up several times and let it drain and can't find a thing. He keeps trying to put it off on my son saying he's not pulling the shower curtain closed enough when he takes a shower and the water is running out of the tub, onto the floor, back under the tub and into his room. Impossible since we don't use the shower but he will not listen to me. I need advise cause I am not a plumber and don't understand everything the plumber is claiming but I'm also no dummy and know that he cannot be right. But I don't know how to discuss it with him without knowing more. Please help!

tape the shower head shut for a month or longer

looks like you are in for a long fight.

if you tape the shower head shut, and you tell the builder and his plumber that you are about to do this, it means you are ensuring beyond all doubt that no-one is showering from that shower, even when you are not home. They will understand that they are dealing with someone who is methodical, able to plan a process of elimination, and able to communicate and explain it in advance. This is the best kind of correspondent to be if "they" are honest, and the worst kind of adversary if "they" are dishonest. They may be operating with all the good faith in the world but just not be able to see the leak. It happens.

Either way you win. Eliminate that possibility.

diagnosing plumbing problems means getting down on your knees or belly a lot and getting dirty often. You now have to do it too since they are not coming back until the problem presents more information that will enable an accurate diagnosis. Enjoy the stretching, take it easy, like yoga.

the first time around it is normal that you wouldn't learn all about the "thingie" that had to be repaired... now you have to get into this subject and study what there is there, what connects to what, what was apparently diagnosed, what was changed, what might be leaking, and more.

put a few plastic bags over the shower head and tape each bag shut. Layers of plastic; look very convincing.

post more here about the distances involved.

david

Great idea and I see your point. And I can also see that you're right and I will need to learn a little more about the 'mechanics' of how a tub works. I will begin to arm myself with more knowledge!

As for distance...hmmm...I'm gonna say that it's maybe 3 feet away if you went directly through the wall.

Thank you so much for all of this....you're a great help!

Try to find the plumbing access panel. If there is one, it will be located directly behind the business end of the tub. Remove the panel. Fill up the tub and then drain it while looking around at the drain pipes (including the overflow) with a flashlight to find the source of the leak.

For locating a leak on a tub/shower, there are several steps. We start low and work ourselves higher as we go.

Does the tub hold water? Pouring water in the tub, and then checking for a leak on the lower drain.
Is it the overflow leaking? Sometimes when kids fill the bathtub higher than the overflow, and the gasket there isn't sealing, that can leak below. I say kids because so often when I get that call, I ask if any kids recently took a bath. Oh yes, yep.
Okay, we're checking the overflow.

Some tub spouts are the cause. When the diverter is pulled up on the tub spout, does water leak into the wall? That's also a common one. A new tub spout is often my cure. I favor the Delta spouts with their weird adapter as that let's me use a pipe nipple or solder onto copper. A good slip spout on copper also works. Moen makes a good one.

Okay, maybe not the tub spout. Is it the caulking around the wall plate and tub spout. You shower with your back to the shower head and the water bounces off your body and onto the wall with the controls. Make sure that has some clear caulking. Yes you can use white too, kind of noticeable though. I can see white caulking from the kitchen it's so obvious.

Is it the shower head? Sometimes those crack over time? I had one condo where the tenant on the third floor was showering with a dangling shower head and arm. Replaced the shower arm and it was all good.

I had one where the shower head was spraying sidewise over the curtain. And they didn't even notice. Weird.

And then the was the one where I asked first if the husband was showering with the sliding doors open. Oh no, he wouldn't do that.
So I cut drywall, opened up ceilings and found nothing. I'm telling you "NOTHING".

A year later I go back, walls patched and painted, I'm there for something else. Did you figure out the shower leak?
Yes, my husband was showering with the slider door open. Dooh!!