A 4 inch pvc elbow with a cap on it in the corner of my basement had a crack in it. I started to excavate and then realized that there wasn't a pipe connected to the other side of the elbow. I then broke off the top to see what was inside (hoping it was treasure). Instead, I found a 2 inch wet corrugated pipe.

Any idea what this thing is?

Thanks for looking.

Adding: I have the PVC elbow off now. The corrugated pipe has slits down the side. I don't know if its purpose is to let something in or out though.

Also, outside, my gutters drain in to 4" pvc that is routed to the storm sewers.

Edit: Adding second picture with the PVC elbow removed. You can see the 2 inch corrugated pipe with slots now. The pipe seems to bend about 2 inches below the surface and travel level with the floor.

Edit: I excavated further. The third picture shows how it looks now. You can now see the flashing over the footer.

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  • Do you have a sump pump?
    – Steven
    Commented Dec 19, 2015 at 22:22
  • No sump. I finally got the entire elbow out and can now see the corrugated pipe clearly. It has slits in it, so I was thinking maybe it's a french drain (all but the sump) or a preinstalled radon system (without the ventilator). I just don't know how to confirm any of those. I'm guessing 2" is small for a french drain?
    – thatguy
    Commented Dec 19, 2015 at 23:23

2 Answers 2


Radon system, or "radon system if one happened to be needed, which it evidently wasn't" - had it been needed, the pipe was in place ready to hook up to a fan...

Far cheaper than finishing the basement and finding that you need to slice up your slab to put in a radon abatement system after it's done.

As for @ojait's objection to it being a radon system because it's corrugated pipe, I'd suggest considering on what side of the slab the perforated corrugated pipe is. The point of a radon system is to have a very leaky pipe below the slab connected to a solid pipe above the slab connected to a fan to suck the radon from the soil to the leaky pipe, then through the non-leaky pipe (still under suction when in living area in case of leaks) and out of the house. Which is what we have here.

2" is too small for drainage, 4" would point to drainage. Barring "custom rat-a-trail because _The_Rats_from_NIMH_ is not fiction" there's just not much other reason to run small-diameter perforated pipe under a slab.

  • Thanks Ecnerwal. Is there a way to confirm this? If it is for radon, then the water inside it suggests that I need to install a French drain.
    – thatguy
    Commented Dec 21, 2015 at 3:15
  • You have not mentioned water in your basement, so I don't see that you would have to do anything! Presumably the radon test passed, so you don't need a radon system, either. Let it be, or explain further why you can't. I'm seeing "I tore up my basement floor for no good reason" so far...
    – Ecnerwal
    Commented Dec 21, 2015 at 3:21
  • The water is inside the corrugated pipe, but it evidently made it to the top of the pipe at some point recently; you can see the reflection of the water in the top picture. That's above the level of my slab, so it's a matter of time before that water finds a way in. The pvc elbow cracked sometime prior to my efforts. I discovered the crack while investigating a musty odor. I'll read up on how to fix things, but knowing the purpose of the corrugated pipe will determine my next steps. Your explanation is reasonable; I'd like to confirm it because I've heard from others that it's a French drain.
    – thatguy
    Commented Dec 21, 2015 at 5:07
  • Musty odor == cracked cap/pipe that leads to the land of underslab, most likely. The wetness in the first picture does not appear to be standing water (it has not gone out of the perforated tube into the PVC pipe), and msy just be condensate.
    – Ecnerwal
    Commented Dec 21, 2015 at 5:11
  • I finally acquired the plans for myself and my home has an exterior french drain I didn't know about. This pipe isn't listed on the plans, but your assessment seems very logical. Thank you.
    – thatguy
    Commented Dec 25, 2015 at 2:32

Some people consider corrugated pipe treasure! Corrugated pipe IS usually used as drain line for leading run off water away from the building. But from the photo you posted I'd be hesitant to say that with assurity. It could be a jury rigged floor drain that some one slapped together. Have you poured water into the corrugated pipe?

  • Ha. I'll trust you on that. My gutters go in to 4" pvc that drains to the storm sewer. I haven't poured anything in to the corrugated, but it does appear to be wet. There is a floor drain 8 feet from this creation. There's also a full bath 10-15 feet the other direction. I haven't run water in that bathroom in over a month. The bathroom was an add on and never inspected based on the quality of their work, but I believe that this corrugated pipe is original to the house because concrete was poured up against the elbow and the slab looked intact until I started on it.
    – thatguy
    Commented Dec 19, 2015 at 23:25
  • The PVC is a tee old French drain may have terminated here at one time then retrofit captured the old corrugated line needed to drain further away in solid pipe. Find the other end of PVC SOLID LINE IT IS LIKLEY BLOCJED WITH LEAVES IR SOMETHING SLOWING THE FLOW clean out the end good and see if water level goes down. Get this flowing asap. foundation drainage could be compromised and during heavy rain you could get real trouble
    – Kris
    Commented Dec 20, 2015 at 0:13
  • Hi Pam. The PVC didn't go anywhere. It was literally an elbow with a cap on it. The other end of it had this corrugated pipe coming in to it. The elbow was 1/2 filled with concrete.
    – thatguy
    Commented Dec 20, 2015 at 0:24
  • If you put a garden hose into the corrugated pipe and open the hose valve to allow a moderate amount of water to flow you could than listen at each of the other 2 drains for water movement. BTW was the PVC cap glued to the 90?
    – ojait
    Commented Dec 20, 2015 at 3:11
  • Thanks Ojait. I'll give that a try. Yes, it was glued. I put a camera 3 feet down the corrugated and didn't see anything of interest. I was hoping to identify a turn. The hose I can see is way above the waterline in the trap of the basement drain nearby. I really don't think I will find it's connected to the drains. My current theories are preplumbing for radon or a French drain. Is there a way to tell them apart?
    – thatguy
    Commented Dec 20, 2015 at 4:13

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