Slow drain after tub-to-shower conversion

by traff_ss   Last Updated January 23, 2018 21:21 PM

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.



Answers 1


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.

Josh King
Josh King
January 23, 2018 21:19 PM

Related Questions



Adding shower to half bath drain waste vent help

Updated November 14, 2018 18:21 PM


Fixing Shower Without P-Trap

Updated June 30, 2018 21:21 PM

Plumbing replacement - Drains, P-traps, and Vents

Updated September 11, 2018 02:21 AM