My wife & I renovated our master shower a couple years ago. About three months ago, we noticed the baseboards in the guest bathroom (on the other side of the wall from the master bath) were getting water damage (similar to how they were getting damaged a couple years ago, thus the shower remodel). I tore out the baseboards & sure enough, water was showing up just like it did a couple years ago. I called my contractor friend out to look (he had spear-headed the original remodel) & his thoughts were that it wasn't leaking thru the tile/hot-mop since none of our tiles were displaced or coming off. So I tested - I plugged the drain & filled the shower with water. It held water for 6-8 hours without any showing up in the guest bathroom, but as soon as I took a shower, there was the water! This made me think it was the drain under our slab, but how was the water getting up?

Here is what I think is happening, but I would like some thoughts on how to remedy it without tearing up my newly-remodeled shower. I think the Fernco adapter mating the original cast iron pipe with the new PVC pipe is leaking. Of all of the pictures I took during the remodel, one of them showed the plumber didn't clean up the cast iron pipe before attaching the Fernco adapter. I believe this joint is leaking, the soil around this joint is now saturated and the water is being forced up thru the slab at the point where the vent pipe goes thru the slab. I called a company that does pipe relining, but they wanted $1800 for this three foot run and wouldn't guarantee a seal since they were getting to the problem area thru the shower trap. I do need to get a couple more estimates, but at this price it might just be cheaper to tear up the shower floor & fix it correctly, thereby confirming the problem & guaranteeing a fix.

Does anyone have any other ideas as to how to fix it? I'd appreciate any thoughts.

  • How far is it from the drain location to the PVC->Cast Iron union? Feb 14, 2011 at 18:10
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    Test to make sure it's not the pressurized water lines feeding the shower by getting the whole place dry and then running water into the shower via a garden hose from outside. If water shows up, you know it's the drain.
    – Michael
    Feb 14, 2011 at 19:24
  • James - about a foot, definitely less than two.
    – Dave Mc
    Feb 14, 2011 at 19:56
  • Michael - gotcha. That's a pretty good idea. I'll try that in the next day or two, but we haven't showered in this shower for a month or more now, so it might not do it right away if the ground has dried out some (my thought here is that the ground is so saturated, the add'l water from when we take a shower is forced UP in the gap between the vent pipe & the slab). Maybe I'll start using it again & try your test.
    – Dave Mc
    Feb 14, 2011 at 19:59
  • Would love to see some pics of the shower, the affected area, etc to help clarify this in my head. Oct 12, 2011 at 15:48

4 Answers 4


It's possible the water is getting around your shower, possibly where the tile meets the tub or pan (if you have one), or behind the fixtures, or even around the door (if any).

I would check all of these and recaulk where appropriate before going in for wholesale destruction. Try splashing water by hand/bucket on areas you might suspect and see if that leads to leaks.

Or maybe you even have leaky plumbing in the wall. Do you have access to see in there?

  • 1
    Good point on the plumbing: it might be a good idea to cut a hole in the wall of the guest room above where the water shows up, and see if you can see the plumbing itself leaking or some other sort of damage there. Especially since you just remodeled, having to patch a piece of drywall is a small price to pay to possibly avoid tearing out the whole shower floor.
    – gregmac
    Feb 14, 2011 at 18:51
  • As I said, I plugged the drain & filled the shower basin with water & it held water for 6-8 hours without a leak, so I don't think it's the tile/grout/hot mop. The water supply is on the other side of the shower & I'd see damage between there (baseboards, dam, etc) & everything look good between here & there.
    – Dave Mc
    Feb 14, 2011 at 20:01
  • @Dave well, let us know when you figure out what it was! Feb 15, 2011 at 13:52

If your shower isn't sealed behind the tiles the water can leak past the tile and grout down into the space. Very common as most builders only use green board and put tile up over the top. This is asking for troubles down the road.

The drain could also be leaking as you believe. In that case plug the drain at the surface and take a shower. If you see water then you know it's coming from the walls and tile leaking. May be good to open up the wall so you can see the water seeping. If no leak from the shower - empty the pan by removing the drain plug. If it now leaks you know the source of the water.


Obviously it is a leak where the drain opening and tile meet, the seal is somehow bad. I say this because I just had this happen to myself, after renovation. I realize your situation is resolved, and you don't need input, but those who still Google this problem may benefit.

I discounted a leaky source and drain pipe all in one move. I took the drain cover off, put the garden hose down the pipe past the opening, and no leak. I put the hose outside the drain and there was a leak.

Time for a new coupling/grommet.


Nice one, Dave! It is good that you detected the leak earlier before it caused more damage. Many homeowners don't realize they have a potential slab leak situation until some major damage occurs to the home, like floor damage and heating problems. One of the causes of slab leak is hard water - a combination of water, lime, calcium and iron in the water and pinhole leak of pipes under the slab. To prevent the leak, fix the pipes under the concrete slab and replace rusting pipes. Seek the help of a professional plumber to assess the gravity of the slab leak. Good luck!

  • How would you diagnose if there is a leak in the pipes under the slab?
    – Niall C.
    Oct 19, 2012 at 23:15

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