This ought to be so simple but its got me stumped. I have a one story home and there is a wet spot in the wall behind the toilet. It appears that this area has been getting wet for a long period and must be a very slow leak. At first I thought it was an issue with the boot around the vent but it is in good shape. The only plumbing in that wall is the vent pipe and the supply line for the toilet. The vent pipe is straight with only one junction that looks well glued. There is no indication that water has been flowing down on the exterior of the vent pipe. There is also a outside faucet connected to the same line as the supply line for the toilet. I've removed the sheetrock from the wall behind the toilet and I don't see any indications of a leak except at the very bottom where it is wet. It has been raining all day today and the vent pipe is dry from where it enters the ceiling to the floor. I put rolled up some paper towels and wrapped them around the bottom of the vent pipe and the supply pipe. Within a few minutes the towels around the vent pipe were soaked while the others remained dry. I'm pulling out what's left of my hair trying to figure out where the water is coming from. Any suggestion would be appreciated.

Could be a crack in a fitting. Is this PVC, ABS??

The vent is PVC. I wrapped paper towels around the vent pipe both above and below the fitting. They have remained dry so I don't think it is the fitting. Thanks for the suggestion however.

If the fitting or piping isn't cracked, and you say the vent stack is dry, then the only other thing in the wall is the supply pipe. What else is there on the wall?

The wall also has a double vanity. I was wondering if perhaps the supply line had broken in the slab. I'm not sure what a failure like that would look like.

Brain asks "the only other thing in the wall is the supply pipe. What else is there?"
Let's run with that and check for a hidden leak, Shut down all the water in the house and check the little pointer in the meter. It should nt be moving at all. If on a pump turn it off and watch the gage, It should hold steady. Let me know the results, Good luck, Tom

Good idea, Tom. I'll give that a try today and let you know the results.

Hmmm, Think I'll try plan B now. I still haven't had time to check for the leak as Tom suggested. Waiting for my wife to finish washing clothes, etc. In the meanwhile I pulled the toilet to get it out of the way. And uh oh, there was water under the toilet. Very wet around the flange. Could it be leaking there and wicking under the tile to the wall? I then knocked out the one row of ceramic tile between the toilet flange and the wall - water under the tile! The flange really didn't look in that good of condition. Here's my plan now. I set up a fan to dry the entire area and I'm not going to reinstall the toilet for now. After it is all dry, then if I have no more water for a few days that will convince me it's the toilet since it's out of the equation for now. Let me know if this makes sense. I'll still check for a leak as soon as I can.

One other quick note. The water in my area is very hard. I noticed a significant amount of calcium like residue along the edge of the wall where the baseboard would normally sit. I took several pieces of the residue and put it into a cup of vinegar. The result was some significant bubbling which pretty much confirmed that is was calcium. If the water were from rainwater in the vent pipe, it would not have contained the calcium. That pretty much eliminates the vent pipe.

Ok - Forget plan B! After removing the toilet and finding water around the flange I was convinced that was the cause of this problem. I assumed the water was migrating from the toilet to the wall. However, I've had a fan running on the area all day and it is still wet mostly around the base of the vent pipe. I wiped up the water around the base of the vent pipe and waited a few minutes and it was wet there again.

So the water seems to have migrated from the wall to the toilet flange. The issue is where is the water coming from. There has been no rain today. For the life of me I cannot see a leak anywhere around the supply line. I watched the small needle on my water meter for several minutes and it never moved. I'm still not discounting that the supply line is leaking however. If it is the supply line leaking then it is a very, very small leak and may not show on the water meter. Now I wonder if the leak is below the surface of the concrete. Any ideas out there? I'm stumped and frustrated!

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The water seems to have migrated from the wall to the toilet flange.

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Is this what you mean or is the water coming from the flange to the wall? Big difference. Let me know, Tom

Tom, sorry for any confusion. It appears the water is migrating from the wall to the flange. The toilet has been off for over a day and fans have been running constantly to dry out the area. However, water is still accumulating in the area around the base of the vent pipe. It is only a few inches from the supply line to the vent pipe so I'm assuming there's a leak in the supply line. I can't see a leak in the supply line at this point so I was going to clean them off a bit to see if that revealed anything. Of course the leak could be beneath the surface of the slab.

Then you've opened up the wall to check? Take your towels and dry the area off. Where is the water seeping in from> Above or below the slab? Let me know, Tom

Tom, I've opened up the wall. When I dry with area with towels the water quickly appears again on the surface in the area at the base of the vent pipe. I can't tell if it is coming from above or below the surface. I haven't been able to locate a leak in the supply line above the slab so I am just assuming the water is coming from inside the slab. I going to clean the exterior of the supply line to see if that reveals anything. If that fails, I guess its time to call in a pro.

My guess is that you have a small under slab leak. There should be Armoflex insulation around the raiser coming out of the slab. Is it wet? Back to you, Tom

Tom, there's no Armoflex around the supply line. This home was built in 1980. Perhaps they didn't use that then.

Our code requires ALL copper pipes coming up through the slab to be insulated. Dry the area off at the base of the supply. If water appears then it's coming up from under the slab. If that's the case the slab will have to come up to locate the source of the leak. Sure beginning to look like a leak under the slab. REgards, Tom

I just got off the phone with my insurance company and fortunately I've got a great policy. They are sending a plumber out to fix the problem and then an adjuster to access the damage. I think you are absolutely correct that the leak is in the slab and that's something I don't want to tackle. In any case thanks for all your help It's been very useful. As soon as I know for sure what the problem is, I'll post an update.

And I'll be waiting to hear. Is there no Armoflex or insolation on any of the copper raising out of the slab? Just curious. Tom

I'm working with the plumbers right now. They think it's a leak in the supply line in the slab. They too can't find any leak topside - darn. The water appears to be coming from inside the slab and percolating around the perimeter of the vent pipe. They're talking to my insurance company before proceeding. There is Armoflex on other copper coming out the slab.

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There is Armoflex on other copper coming out the slab.

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And the lack of Armoflex on the raiser might just be the cause of your leak. Just bask your plumber. Cheers, Tom