Under-slab leak?

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Altstuder

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May 27, 2021
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New Mexico
Hi guys, I've got a small leak welling up out of the slab from a cold supply pipe goes down into the concrete. I'm guessing it's a small pin-hole because water only appears when after running a cold tap. If I run the kitchen tap a very small amount appears. If I run the bathroom tap, much more appears so I figure that's the bathroom supply. Luckily, it's only 4 feet to the bathroom sink. Questions: does that make sense that water only appears if you run the tap? Why doesn't it leak when the tap isn't running? If the water is coming up from the penetration in the concrete wouldn't you think the leak is IN the concrete not below, so the water is forced up instead of down below the slab? There are four supply lines going into the slab and water only comes up from 3rd pipe. Should I try and chip down a ways and see if I can find it? If it's not just below the surface was thinking to try and push 1/2" pex through the 3/4" copper. Any thoughts? thanks
 
Sounds like your drain line is broken under the slab. If it was supply piping it would always be leaking. Most likely the drain line is clogged too causing the water to drain under the slab through the break
 
Since the leak doesn't occur unless the cold water is running, how could this be a drain leak?
 
It has to be a drain. If it was on the water line you'd actually less water when leaking when you ran the tap. When you shut the tap there is more pressure on the line and it would be continuously leaking.

The hot vs cold thing I think is a red haring the hot water is probably routed differently, as in from the water heater and ran in the walls.
 
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Well, after more time with it, it seems like it is weeping more consistently and stops when I shut off the main, so I'm pretty sure it's supply. I'm going to cap the suspicious supply pipe and see it that stops the water from coming up. It's pretty mysterious, half the evidence suggests drain, half suggests supply. How could I verify it is a drain problem?
 
Pipes.jpg So, here's what it looks like- not very handsome. From the far right, the supply main which is 1/2 pex sleeved through 3/4 copper, then the next three are the supplies. The leak is coming up through the plastic sleeving of the 3/4 copper, the 3rd pipe from the right in the center of the photo. I started chipping down a bit hoping for a miracle. I've only got 9-10 inches there to work with and I'm hesitating to cut that line as I guess those are all soft copper as they're all wonky and bent. I've never worked on soft copper and if I cut it and cap it to see if that is the leaker and I'm wrong I'm concerned about getting that back together. I bought a bit of 3/4 and some slip couplings to restore it if necessary. I'm looking for advice on where to cut and how little I can possibly cut out. As you can see it starts bending near the lower third. Do you have to be gentler cutting soft to avoid deforming it? Am I heading down the right road? thanks
 
Yeah you don't want to crush it with your tubing cutter. You might be better off cutting all three and valving everything off.
 
Might be not work then you want to do but that setup isn't great.
 
Taking that header apart is pointless unless the leak is 24/7.

if it only leaks when you use water then it’s a drain leak. It’s just finding a way out through the slab penetration for the waterlines
 
It is definitely not a drain problem. The only condition in which it leaks is water supply on. This morning I ran a garden hose into the bathroom sink which is only 4 feet away and ran high volume for several minutes which produced no leaks. Did the same thing to the kitchen sink- nothing. Sorry, I think I threw out some red herrings in trying to diagnose. I guess I'll take the header apart, move it up far enough to put valves in and isolate and cap the 3/4 line. Is there any other way to determine where that 3/4 line goes besides capping it?
 
You could hire a locate service to clip onto it. There are generally private companies who locate for municipalities. They may or may not let you hire them to help you. Other than that probably no. The lines need to be isolated from one another.
 
Altstuder said:
Well, after more time with it, it seems like it is weeping more consistently and stops when I shut off the main, so I'm pretty sure it's supply. I'm going to cap the suspicious supply pipe and see it that stops the water from coming up. It's pretty mysterious, half the evidence suggests drain, half suggests supply. How could I verify it is a drain problem?
Click to expand...
You can put food coloring or tracer dye down the drain of one of the fixtures.
 
If it’s a hot water leak you can turn the hot water off for a while and then turn it back on, the leaking pipe will get hot after a minute or so.
 
Get a water pressure gauge and put it on a hose bib. Shut water off to the house, if the pressure drops the leak is on the supply. Make sure no fixtures in the house are dripping or it will throw off the test.
 
Altstuder said:
It is definitely not a drain problem. The only condition in which it leaks is water supply on. This morning I ran a garden hose into the bathroom sink which is only 4 feet away and ran high volume for several minutes which produced no leaks. Did the same thing to the kitchen sink- nothing. Sorry, I think I threw out some red herrings in trying to diagnose. I guess I'll take the header apart, move it up far enough to put valves in and isolate and cap the 3/4 line. Is there any other way to determine where that 3/4 line goes besides capping it?
Click to expand...
I saw an inexpensive line locator for metal pipe on Amazon a bit back. The price has come down significantly since the days of the $5000 Metrotechs. IIRC, it was in the $60 to $100 range, and some impressive units in the $150 range. sounds like a lot at first blush, but compared to having someone locate a line it is dirt cheap.

My Daddy taught me that for most things, a guy who is handy, can buy the special tools and do the work themselves most of the time.
 
From what I read, it's only the cold line weeping, and only when running, no leaking when the hot is run.

This is condensate, like a cold soda can sweating, small drips running down to the point of penetration at the slab.
 
Duckbutter said:
From what I read, it's only the cold line weeping, and only when running, no leaking when the hot is run.

This is condensate, like a cold soda can sweating, small drips running down to the point of penetration at the slab.
Click to expand...

Yeah, he changed his story as the thread developed.
 
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