Mystery Pipe In Basement (Leaking)

Mystery Pipe In Basement (Leaking)

I'm a 'learn as I go' do it yourselfer, but this pipe has me totally confused. I don't know if it's a sewer cleanout pipe or what. It's PVC, so I'm guessing it's a drainpipe of some kind, all I know is that it's leaking water very slowly.

Can anyone give me a definitive answer? I'd like to fix it myself, but first I have to know what I'm dealing with.


*The pipe extends from the wall, just below the basement ceiling, and it has a yellow cap*.

only one truth only one truth
Jul '22

Um, is that *masking tape* holding the cap on?

Where does the pipe seem to come from? Can you remove the insulation around it?

3wbdwnj 3wbdwnj
Jul '22


djdowd djdowd
Jul '22

Definitive is not something you can count on from a few forum comments. Get a plumber to come in if you want that.

The PVC as well as the size of the pipe both are clues that it's outgoing drain, not anything in coming. Pipe is capped off like that when it's no longer in use. So the question becomes what used to be there? Any chance either there was septic and it's now town sewer? Or maybe the septic field was redone?

Either way, something capped off shouldn't have water there. If it's just a drop or two of water then it should be enough to look like a "leak". It might be something was capped off but they forget to reroute other unused pipe leading into it. That seems unlikely if it's so close to going outside. The more likely scenario is something outside is coming in. That's could be a much bigger problem, a leak in outside lines or old septic that wasn't dealt with. Not to mention that doesn't look like a good capping job.

Either way the cap probably needs to be opened to figure out what's coming from where. Could well be more than just a DIY job.

Call a plumber. What’s directly above the pipe? Is there a bathroom and toilet directly above it?

Jul '22

It appears to exit the structure above grade as the top plates on the foundation are visible. That would rule out septic I would think. Could be an abandoned direct vent for an appliance that has either been moved or removed. The water collecting could be from the outside if the fittings are not tight or it was not turned 90 degrees to the ground.

Sewer line

Been there Been there
Jul '22

That appears to be exiting the exterior wall and is above grade (higher than the sill plate) so it wouldn’t be a septic line…

What PVC is visible on the outside of your house?
Is it an an old vent that is now collecting rain water?

Unless that piece on the far right is a 90 degree elbow and it turns up through the floor? Then the question is easy… what’s right above it? If it was a toilet I’d say that would be quite a bit more than a dripping leak should you flush it!

Mark Mc. Mark Mc.
Jul '22

I think Greg and I had the same thoughts but I was just slower to type on my phone haha

Mark Mc. Mark Mc.
Jul '22

Is there a T connection from above? If nothing is connected to it, must be an old pipe no longer used. Perhaps you can see where it going to on the outside. You can properly cap it by using solvent and an actual pvc cap.

Not today Not today
Jul '22

Some newer homes come pre plumed for radon. That could be a radon pipe. If a radon system was needed having the pipe already in place makes install easy.

My guess is the pipe goes to daylight on the roof. I would open that cap and suck out all the water with a shop vac. Use a fish tape to confirm the exit point. And if it is the roof, put a 90 degrees bend on The end to prevent rain from coming in again.

Sacks cousin
Jul '22

I thought about radon as well. Usually they go straight down a wall to the sub-slab level. But that could have been just a long stub for later use.

We do need an update though! LOL

Radon pipes are smaller diameter, and unless the pipe is underneath the slab it's useless. That's going outside instead where a pre-made radon would be inside to underneath the slap, for example down to a sump pump. Then it would be up to the attic before going outside. Definitely not a radon pipe.

If that pipe went up to something it's all the wrong fittings, you would see the right ones.Chances are the pipe was much longer where the fittings into it were. But then all that was removed whenever it was.

Re: Mystery Pipe In Basement (Leaking)

Right. The radon system I had installed here in the pic uses a 3" line, sub-slab then out into the garage then the roof. You can see the 4" septic line to the right. It does looks as if the photo from the OP is a 4" line.

If it does run through the roof you can have a dry rotted rubber collar that’s letting rain water through. It would drop all the way down to the lowest point (the bottom of the pipe).

Jul '22

Thanks everyone,

I guess I should have been more specific...

Looking at the picture, the pipe extends out from the top of the concrete wall that adjoins the townhouse next door.

There are no bathroom or kitchen fixtures directly above it, and we do not have a septic tank.

It is still leaking slowly as I type this, although we haven't had significant rain for a while. However, I'm thinking that it might be retaining water from the last hard rain we had a couple of weeks ago.

Even with all the posts, I'm just as confused as when I originally posted. I think the most likely possibility is that the pipe is collecting water from the roof vent pipe, which has no cap on it, although I'm obviously not certain of that.

If it is the roof vent pipe, then this gets even more strange because I think they are supposed to be left uncapped.

I'm tempted to remove the cap from the pipe and risk getting drenched, but I'm going to remove the insulation behind it first to see if I can determine where the pipe originates from, as someone suggested here.

Anyway, if all else fails then I can always resort to calling a professional, but I still really think this is a DIY job, if I could just figure it out.

only one truth only one truth
Jul '22

If you are in a townhouse and there is an association - why not ask them---if it's something between two units it might be their responsibility

4catmom 4catmom
Jul '22

Curious for sure! If that is a common wall with another unit perhaps there is a race to the roof and it is a vent stack.. However while not out of the question a roof vent is usually not 3" as far as I know. Plus it would not terminate like that for sure. 4cat has a point, if it is "internal" to the structure the HOA should be responsible or at minimum send a professional out to assess. For laughs, I would check with the neighbor to see if they have the other side of that run or a similar pipe in their unit. Providing of course their lower level is also unfinished like yours.

I had a pipe that went to my dry well that backed up into my basement. I installed a smaller pvc pipe into the backlog pipe and my problem was solved. I would smell the water first. Sewage has a distinct odor. Water doesn't. Don't sweat the small stuff.

Robert Rowe Robert Rowe
Jul '22


I only just discovered this leaking pipe a couple of days ago. There were a bunch of stacked boxes directly below it that I hadn't looked at for ages. This slow leak had created a disgusting spread of black mold that had managed to make its way throughout the majority of those boxes, so of course I had to throw out the boxes and everything in them.

Of course, I then proceeded to clean the entire area with white vinegar and baking soda, making sure I didn't miss a single spot of mold.

No, there is no odor coming from this leak, but the potential for black mold to form from it is cause for concern.

only one truth only one truth
Aug '22


That's a thought, but I'll have to wait until the neighbors are back from vacation.

And I agree that the roof vent wouldn't likely be terminating in the basement. The pipe would have to go through the attic and 2 more floors for it to end there. I'm going to pull out the insulation behind the pipe today to see if it helps determine its origin.

only one truth only one truth
Aug '22

A townhouse?? Never mind, forget everything else.

So that wall the pipe goes through is to a different unit? You already know the pipe goes next door. That's all that matters, the problem is next door. You need to inform the rest of the units down the line and get a call into the HOA.

Could it be to vent radon? If theres a sump pump nearby it could have once extended into the basin, and the water is from rain.

Roywhite Roywhite
Aug '22

Radon pipe would be my guess also. When our house was built almost 30 years ago all the houses in our development had a radon vent put in from the roof the the basement french drain. If needed all that had to be done was add a fan to vent the radon out.

GreatMeadows GreatMeadows
Aug '22

Agree with 4catmom. Call the HOA. Pipes in the shared walls should be their responsibility.

Aug '22

HOA or neighbors. Perhaps neighbors first in case it's a homeowner one-off not allowed by HOA......

Babit Babit
Aug '22

Well - what was the answer??

4catmom 4catmom
Aug '22

Well, it’s not Radon.
Radon code is vented up, then pipe venting above highest floor with living space-OUTSIDE.
Why run gas through neighbors home.
No way JOSE’.
Not code.
Not allowed.
..and I’m allowing myself to get plunked.
And all of you , why the HOA?
Making a hole in a wall to another unit?
Sounds like an unauthorized jury rig ( for who knows what reason)
HOA is not responsible for every stupid thing an owner does.
Positive breaking through a wall to an adjacent unit was not authorized by the HOA.
Your on your own on this one.
Should have shown up on any home inspection, leaking or not.
Stick a fork in me on this scam.

Stymie Stymie
Aug '22


"HOA is not responsible for every stupid thing an owner does."

I'm the homeowner, and I never mentioned anything about HOA, that suggestion was made by some who responded.

Secondly, I'm not the original owner of the townhouse, so any potentially "stupid" thing done, was done prior to me living here. It has yet to be determined just where this pipe originates from, so you're jumping the gun with all of your speculation.

Finally, why the snark? Do you have some kind of persecution complex? I don't need to be told that I'm "on my own on this one", I fully understand that. I was just looking for some clues to resolve the problem.

Consider yourself stuck with that fork you mentioned, thanks for nothing.

only one truth only one truth
Aug '22

Is this for a previous gas furnace that was located in the area?

OOT- what was the outcome of the pipe? We need closure! LOL

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