I tapped into a 2" ABS pipe that runs underneath my concrete basement floor, and presently joined it using a Fernco coupling allegedly rated for underground use. (Unclear if this can be used indoors though? I was surprised the shielded ones I saw in stock at Home Depot didn't appear to be rated for underground use).

I had my rough-in inspected, though the inspector didn't say anything about it, and was mostly concerned that it wasn't leaking (it's not, and it's been there for about a year). But there's a bit of space between the pipes (perhaps 1/2") and I'm concerned they could come out of alignment (and I think you can see already the alignment is not quite perfect, though it was previously a continuous pipe; I could definitely force an alignment to make a rigid connection). The earthquake risk here is pretty minimal, climate zone 2, dirt is super rocky, some box elder trees in the yard not particularly close but with spreading root systems.

  • Is this the main risk, that the pipes settle/shift sufficiently divergently to sever the connection? (And this risk is non-trivial?)
  • Should I redo it so the pipes meet perfectly (no 1/2" gap)? (I would probably have to remove more of the concrete floor and replace that section of pipe to the right of the joint)
  • More generally, what's the best thing I can do to improve and/or bomb-proof this junction (without tearing up the entire floor :)? Would a shielded coupler be a good upgrade?

Wye visible on the left to accommodate the drain I added; before I cut in, the ABS pipe was continuous

Wye visible on the left to accommodate the drain I added; before I cut in, the ABS pipe was continuous. The entire length of pipe is underground below the concrete floor.

  • Why wouldn't you use a proper cement joint there? I've only used Fernco couplers for dissimilar pipe connections. I'd use a shielded coupler (even if not rated for underground) before I'd trust bare rubber.
    – isherwood
    Mar 26, 2020 at 14:45
  • Well the two halves of the pipe I'm joining are both buried underneath the concrete; I didn't think a cement joint was an option if I can't move one of the pipes.
    – Tahlor
    Mar 26, 2020 at 15:59
  • 1
    I believe there are slip-couplers for that scenario. A pair of mild-angle bends and one of those should get you there.
    – isherwood
    Mar 26, 2020 at 16:07

2 Answers 2


Be quite difficult to use a normal abs coupling here. The couplings have a stop built into them and you typically need flexibility in the two ends being joined. These look concreted in. If the connection is going to receive concrete on top of it I don't see any risk of this separating. Rubber does tend to age, get brittle and crack over time. I'd probably just embedded it in concrete on all sides, if the rubber does die overtime at least you still effectively have a concrete pipe directing the effluent into the other portion of pipe.

One thing I've done in the past is to take a hole saw and drill out the stop in a standard abs coupling, then slide the coupling onto one side of the pipe, abs glue the two sides and slide the coupling over top the two pieces. The sides need to be quite clean and I did this with 2" pvc last time. I am not sure how much force would be required on a 3 or 4" abs to pull the coupling over the glue. I don't think I'd do that in this case.

  • 3
    You don't need to manufacture your own slip couplings.
    – isherwood
    Mar 26, 2020 at 16:08
  • @isherwood - Thanks! Last time it was a DB2 pvc connector for low voltage conduit. Good to know I won't always have to make them ! Mar 26, 2020 at 22:02

Misalignment or size change would be a reason to to use a Fernco. I have also used them to cut an existing pipe and add an additional drain; this may be the reason it was used with a wye to the left. If the pipes are in the ground or under footings this is the way to make a connection as there is probably not enough flex to add a glued fitting--we cannot tell with the limited photo.

Would I try to make the joint perfect? Probably not it comes down to if it’s not broke don’t fix it. I have found much worse alignments due to tree roots and settling.

I have not had problems with Ferncos passing inspection when converting sections of cast to plastic both inside and outside.

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