I am finishing the relocation of a drain after replacing the tub with a shower base and I will probably need to pour concrete back. The opening I created will be entirely under the shower base that I am installing so my question is this: How thick does the concrete have to be. The shower base is pretty sturdy (Kohler Ballast model 60x32) and heavy and it will be set in mortar as well.

I would avoid pouring a very thick concrete layer if I don't have to as this is my first job of this sort and I would like to be able to easily break the concrete and fix whatever needs to be fixed if anything will ever need fixing. I guess ideally it would be nice to just fill the opening in the floor with sand. :-)

The shower base will be along the right side of the picture with the shower drain in the deeper area that see in the picture

enter image description here

The plan is to set the shower base level and then add self leveling concrete around it so I can install tiles.

  • If you are up for a bit of experimentation before you do the actual job, aircrete or expanded polystyrene concrete [ norishouse.com/archives/560 ] would offer an "easier to dig up again when needed" option, but I'd probably limit that to right where the pipes are.
    – Ecnerwal
    Commented Mar 26, 2021 at 13:35
  • hmm that is interesting but I think @isherwood has a point you do not want the humidity to find its way up above the floor. I think your suggestion allows that even if the shower base will cap the area
    – MiniMe
    Commented Mar 26, 2021 at 14:03

1 Answer 1


The only requirements are that the pipes and shower base be properly supported. To accomplish that I wouldn't go less than 3" thick with the slab. 4" is more standard. It's only going to take a few bags anyway, so there's no good reason to cheap out. I would want a good air/moisture barrier under my shower.

If you add fill soil be sure to use inorganic material (not black dirt). Sand or gravel are good, as is washed rock. I would run some water in there after the plumbing is finished to compact the soil--just enough to saturate it. That will help prevent settling after the concrete is installed.

  • all good suggestions. Is there a way to make the breaking easier in case I need? It was fairly easy to break what you see because it was broken before and filled in. The slab was pretty fragile in some areas poor work by the guy who poured the initial slab. However the concrete that is south of the picture, poured by the guys who installed the tub is very hard to break. Any recommendations for making a less rock solid concrete mix but that would still do the job to keep the pipes in place and insulate I guess I could add plastic underneath)
    – MiniMe
    Commented Mar 26, 2021 at 14:08
  • I'm not sure why you'd need to plan for a second removal. Anything you do in there should last 50 years.
    – isherwood
    Commented Mar 26, 2021 at 15:15
  • my first time doing this ..the only thing I am afraid is that I could have a leak in the piping or in the shower drain under the base
    – MiniMe
    Commented Mar 26, 2021 at 15:47
  • Leaks in PVC are rare if you follow basic technique. If you have a problem with the drain assembly it's above the concrete.
    – isherwood
    Commented Mar 26, 2021 at 15:52
  • It is ABS in my case I am in Canada
    – MiniMe
    Commented Mar 26, 2021 at 15:58

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