Leak help

[Glen H]

I had a slow leak coming from under the cultured marble base of a shower. It would not leak with a short shower but would after a long one - about 20 minutes. Water would weep into the bedroom from under a floor stud (one on bottom of the wall that is nailed to the concrete). I positively confirmed with a drain plug that it was not from poor seal (silicone on the joints of the shower). Marble guy and I filled the bottom of the shower up to the curb( about 4” of water) and left it overnight, no leak. To make a long story short, I tore out all the marble. The supply pipes were dry but there was water under the pan. I hired a plumber to come look at it. The first thing he told me was “your water is not coming from your supply pipes”. Our city has between 65 and 90 psi of pressure. He said if they were the source they would be leaking now.
It seems stupid for me to ask but I’ll ask it anyway. Is he right? Is there any way a pipe under pressure would only leak occasionally?
To be brief, the shower valves do not leak and I have through much testing almost positively eliminated the drain as the source. If it’s not the supply pipes it has to be the drain. But I’m in doubt and can’t put up new marble without knowing if the drain is the only source.
Appreciate some help here thanks
I should add that the Sheetrock behind the pipes was black and wet.

 

[frodo]

It could very well be the caulk joint where culture marble wall panel meets the pan.
or it could be the corners. or where the faucet face plate meets the marble

another possibility. is the shower riser pipe
it only has water in it when you take a shower.

another thing. what kind of plug did you use to test the drain?
a screw in type
or a blow up ball PUSHED down into te pipe?

here is why, a no caulk shower drain can leak in TWO spots and the only way to test for both of them is to insert a blow up ball into the pipe
a screw in test plug. or one with the wing nut on top, will only check the drain to marble connection
but you need to also check the drain to pipe connection

the pipe comes up into the drain body. and a rubber gasket slips over the pipe. is compressed by a big brass nut

 

[Diehard]

Since the pipe feeding the shower head typically only has water in it while in use, did you check that pipe for leaks while you ran the shower?

EDIT: Oops, a comment was added while I was typing. Sorry.

 

[Glen H]

Guys, thanks. The riser is not leaking. I ran a garden hose from the shower outlet to the drain and ran it and pinch tested it to increase pressure at the joints. All this after tearing out the marble so I can see the pipes.
The caulk joints weren’t my problem because I filled the bottom up over the caulk joins 4” , no leaks overnight.
The plumber did ask me how I plugged the drain to test it. It was a wingnut expander type. He mentioned that I should have called him before tearing out the marble because he would have used the blow up ball type down in the drain pipe (as Frodo pints out).
I’m concluding that it has to be the drain. Even though it looks so well made and tight.
Assuming supply pipes under pressure cannot leak occasionally.

 

[frodo]

that rubber can be tightened
pull the strainer cover off, look in the hole
see the brass ring with 4 notches in it?
place a screw driver against the notch and tap the screwdriver with a hammer
you are turning the nut that compresses the rubber compression ring

 

[Jeff Handy]

Everything Frodo and Diehard said were good things to check.
Especially the seal around the shower valve trim plate, water gets behind it and runs down the wall.

You say you think the drain is the source of the leak.

But meanwhile, the sheetrock behind the supply pipes was black and wet.

So water from the drain is magically climbing up the supply pipes and soaking the drywall?

Did you try taking off the shower head and capping the shower arm?
Then turn on the shower valves.
This creates more internal pressure in the riser and ell and shower arm, than letting water out through an attached hose.
And you can inspect everything while the whole setup should be dry and quiet, no garden hose gushing water into the drain.

Look closely over every inch of the riser for any slow leaks.

Also, look all over around and behind and under the shower valve body, and move the valve handle or handles through their full range as you watch for water sneaking out from somewhere in there.

BTW, you should cap the shower arm, turn on the shower valves, and allow enough time for any trapped air at the top of the riser to get out through a tiny leak at the ell or shower arm, if there is a leak up there.
You would not notice a tiny air leak, until the water in the riser finally gets all the way out to the cap you installed.

 

[Glen H]

Jeff, thanks! The sheetrock was more damp than wet and mostly on the bottom foot up from the floor. None of the rock was damp above about 18”. The actual “wet” was down on the concrete mostly. There was water specifically around the hot lines coming up from the slab (where they have that foam protectant coming up) - might just be from the drain. I will definitely plug the shower spout and run full pressure and leave it for a while as you recommend. And also run the hot cold valves wide open.
But while I have your ear, is there anyway pressurized supply lines can leak intermittently?
Any other hints would be followed thoroughly, I’m a good listener especially when I don’t know anything about it.

 

[Jeff Handy]

Hi Glen,

First of all, I still don’t picture water climbing uphill, even 18 inches.
So it could still have been from a leak at the valve trim plate, against the marble wall.

I guess a pressurized line could leak intermittently, especially the hot line.
It goes from barely warm to full hot after a long shower, so if there is a slight flaw at a solder joint or pex connection, that flaw could increase a bit from thermal expansion during a long shower.

 

[Glen H]

Well , I have run hot water with a spout on the hose to the drain to simulate shower flow and used a heat gun aimed at the joints and no leaks. Did the same with the cold. I then capped the shower stem and opened the valves wide open and left it under pressure for a few hours. Not a drip.
I’m going to order new marble and see what happens. They’ll install a new drain. The rock on the backside of the supply pipes I will leave off so if the leak shows up from there I can see it and fix it.
Sheetrock will suck up water.
Thanks for the help.