I recently hired a plumber for a tub-to-shower conversion into a first floor (slab) master bathroom. I had already removed tub and demo'd the walls, so the scope of work involved:

  1. Extending existing copper lines up into exposed wall for shower valve/head
  2. Add 2" PVC drain/p-trap in center of shower (including slab work and new concrete), coupling to existing 1.5" drain inside slab (using Fernco coupling)

Number 1 was pretty straight forward. Number 2 was also straight forward, although I wasn't sure about the Fernco to adapt the 2" PVC to the existing 1.5", but he assured me this was standard practice: Closeup of Fernco coupling in slab Overview of slab cut-out, with PVC sitting to right

I was happy with how everything turned out initially, and after tiling we started to use the shower. But we noticed something strange: for the first minute or two, the water would not drain fast, and would start to puddle (~1"), but then all of a sudden all the water would drain and it would stay that way for the duration of the shower (ie, no puddling at all near the drain). It has been about 4 months and now the 'puddling phase' lasts more like 4-5 minutes and gets deeper, but then, like magic, it all suddenly drains. We used the bathtub as a shower for years and never had any drain issues.

Other things to mention:

  • There was no roof vent for the bathtub, but it did have the standard stopper assembly so this could have acted like a vent?
  • The original drain in the slab is about 48" from main sewer line (toilet), which has a 4" roof vent.

My main questions are:

  1. Why does it drain slow at first, then suddenly drain normally?
  2. Did the plumber use proper technique, ie, the Fernco?
  3. Is this a vent (lack of) issue? (If so, I'm screwed)

Thanks for reading.

UPDATE (01/25/18): I have contacted the plumber who did the work. I told him about the "double trap" theory, which totally makes sense to me. However, he is not yet convinced as he says he "has done hundreds of tub-to-bath conversions and never had any issues if there was a tub trap in the slab." He thinks it is some type of obstruction in the line, and will come out and run the line to try and push out whatever might be in there. My engineering/physics mind makes me less optimistic than he is, but I hold out hope since it is, by far, the easiest solution. Fingers crossed, and I will update soon.

Another UPDATE: The plumber finally came out (he was battling flu). He contended that there should be no problem with having a new trap as part of the shower drain which feeds into the slab where there is (presumably) another trap...as long as there is a down-stream vent, which there is. However, I was really skeptical since what Josh said below made sense from a physics point-of-view. His thought was that there was simply some blockage somewhere; which again didn't make sense since the water drains well after running for a few minutes. He proceeded to use his auger to try and clear out the blockage. Well, we were all very surprised when he pulled out a 4" piece of 1/2" copper all wrapped up in a big hair-mess. WTF? It must have fallen in the drain when he did the original work, and the only explanation to why it drains well after a few minutes is that it somehow "floats to the top" or whatever and gets out of the way. Not sure we'll ever really know why, but now the system drains well, no problems at all. I don't really care since I won't have to redo things to fix it. :) Just thought I'd post in case anyone comes across this and wonders about this same type of problem.

  • Did you pull out a trap section from the tub or is this in the concrete below the section shown?
    – Josh King
    Commented Jan 23, 2018 at 20:55
  • Josh: There was no trap when I pulled out the tub, just the 1.5" drain as shown in the pic (tub drained directly into pipe). So it must be in the slab?
    – traff_ss
    Commented Jan 23, 2018 at 21:07
  • Is it common to bury a trap so deep? In the UK the trap would always be directly under.
    – handyman
    Commented Jan 23, 2018 at 21:36

3 Answers 3


You should have a trap generally the trap would be with in 8 to 12 inches of the drain. The trap prevents critters (at least partly) and sewer gases from entering the home from shower drain.it sounds like you have an AIR Bubble and until that gets filled - it acts like a blockage. Next time get a bucket of water and force it into the drain and then see how fast shower drains.

It is possible it could be a venting issue you are not up the creek. You mention the Toilet being close by and I would guess the two drains are connected and the vent for the toilet exists, so you might need to enlarge it or see if by chance it has become clogged.

Also given that you dug all of that out check that pipe for clogs and also your venting pipe where it meets the toilet drain it is very possible you have a piece of debris stuck and it rolls around a bit and out of the way after it 'floats' (moves).

  • You were correct about a piece of debris; read my update for the full story. Thanks.
    – traff_ss
    Commented Mar 1, 2018 at 21:08
  • @traff_ss Nice to know your problem was resolved and I could be of at least a little help. Big piece of copper wow.. but you know without notice things can fall in. If you could mark this as the answer by clicking the Check box next to it - I would appreciate it.
    – Ken
    Commented Mar 2, 2018 at 15:49

If you didn't take out a trap it is likely in the ground, and the shown section added a second trap, having two traps in line is the problem, it is called double trapping.

The air in between the two traps is stuck and requires some pressure to clear and start the water flowing, provided by your build up of water in the shower. This region between the traps is also more likely to accumulate material and clog, which is likely why it is getting worse.

It can be fixed by venting between the traps, or removing one of them, but since they are both in concrete it is not an easy fix. Short term you could try and snake the drain or use other methods to try and clear debris to improve drainage, but it is going to be a problem.

  • I did not catch that part that the op added a trap. Why do this - if the tub is there and drains fine all these years and no sewer gases etc. If it ain't broke don't fix it - connect and go. I think you hit the nail on the head. Op will be redoing a shower floor if he wants to fix it.
    – Ken
    Commented Jan 24, 2018 at 6:59

Did the plumber use the correct device fernco coupler, Yes fernco couplers are very common for pipe repairs and modifications. Why is the drain running slow , the tub drained fine, I think you guessed this one correctly that the drain did have a vent because of the tub, now the syphon needs to get started then it drains even 1 inch higher would make a big difference and tubs are usually set slightly higher than shower pans.but if double trapped it makes sence to me that it has a tougher time getting started each 90 creates resistance to flow and the trapped air in between is the problem unroll the syphon starts.

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