I recently purchased this home and am interested in figuring out what these pipes are for, if not a rough-in.

I have a 3" pipe coming from slab at exactly 13" from wall on center.

Then three 1.5" (I believe. Can't measure easily) all between the studs behind it.

If this were for a rough-in, I'd assume another at the floor for shower, plus vent to the ceiling, but I see neither.

  • This is a septic system
  • I haven't tested to see where the 3" goes, however, I believe it may go to a sump in the center of the basement which is where I have softener drain being pumped up into the main. I have a separate foundation sump near the opposite end of the basement which is clearly just for gray water.

If it is for a bathroom, why three pipes on the wall? Why no vent?

photo of roughed-in pipes

  • 1
    I'd run a hose in that 3" and see if water comes out where you expect.
    – Ecnerwal
    Nov 30, 2021 at 2:02

1 Answer 1


It's definitely for a future bathroom. One of the 1.5" is going to be your vanity drain. Usually there's a 2" under the slab that you can bust out of install a shower or bathtub. The 3" is definitely for a toilet. One of the 1.5" pipes is going to be your vent.

  • Right. The tall one behind the toilet would be the vent for both fixtures. I have no idea why it's not already connected through the roof.
    – isherwood
    Nov 29, 2021 at 17:00
  • Appreciate it folks. I'll mark this as the answer. If anyone knows how I'd figure out where the slab may have the shower pipe, I don't see any markings hah. Anyway, very helpful and thanks for confirmation!
    – JSess
    Nov 29, 2021 at 18:04
  • very surprised the "toilet" 3" pipe is not capped, the basement should be smelly with sewer gas smells with an un caped toilet connection in there...? unless its not tied into the main drain.... which would be weird. From the first vent to the left, id say the shower will be in line with that near the wall maybe where that white mark is on the concrete? Try tapping the concrete lightly, if the left a drain ready to find, it might be taped up just below the surface of the concrete slab....
    – mark f
    Nov 29, 2021 at 18:47
  • 1
    @markf look again. It's capped/plugged, just not the way you are expecting.
    – Ecnerwal
    Nov 30, 2021 at 2:03
  • It has a knockout test plug in it. I use them all the time. Way cheaper than a regular cap.
    – DrSparks
    Dec 19, 2021 at 23:42

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