I am having a bathroom renovated and a tile shower put in. The contractor installed cement board, but I had a fear that they used regular joint compound rather than something that's waterproof. When I checked, I applied a small amount of water from a spray bottle to a fully dry section and rubbed it with my finger. The compound essentially dissolved away. They want to just apply a layer of Aqua Defense over it and then tile, but from what I've read the joint compound can still degrade over time.

Any suggestions on a fix or should I tell them to remove it and reapply the correct joint compound? They haven't tiled yet thankfully.

  • pretty funny, a few more plusses and using drywall and joint compound instead of cement board and the correct sealants that thin set will properly bond to when tiling will become the new normal.
    – ron
    Jul 19, 2023 at 16:24

1 Answer 1


I have been using joint compound on cement board, then applying a waterproofing like Red Guard, or Aqua Defense just as your contractor has done and plans to do.

I've been doing it that way for over 2 decades. Never had an issue. Did it in my home and family members and a bunch of investment properties.

Your experiment with water and your finger is something that would not be likely to happen without many other adverse things happening first. ( physical damage to the shower.)

The Waterproofing seals the walls so any water that does get through the tile cannot get to the sheathing or the compound. It works very well. Is thinset better? Yes, but not essential.

I remember reading a review from someone that coated the inside of a cardboard box with Red Guard, fill it with water and set it on their deck. Two weeks later they threw it away because they got tired of looking at a box of water for 2 weeks.

Your contractor is doing as many others have. James Hardy even Oks drywall compound. They say that fast drying is recommended but not required.

  • Just to echo, ordinary joint compound is completely standard in my area. I had to google to find a moisture resistant compound. (Certainteed M2Tech, and I have literally never seen it in the wild.) Jul 19, 2023 at 0:35
  • James Hardie OK's drywall compound only at cement board-drywall transitions where there is no tile installed and in dry locations. RedGard is not approved for use over drywall except in DRY locations. Quoting a post at the John Bridges Tile Forum: "there is no industry standard that supports the use of a liquid membrane on drywall in a wet area".
    – kreemoweet
    Jul 19, 2023 at 5:43
  • 1
    @kreemoweet no one has mentioned using drywall.
    – Huesmann
    Jul 19, 2023 at 12:44
  • give it time, some one here probably will mention using drywall behind tile for a shower. give you a dollar if you can find one youtube vid of the thousands out there showing how to tile a shower/bathroom with drywall or joint compound.
    – ron
    Jul 19, 2023 at 16:25
  • Yeah, most of the homes that are more than 25 years old have just the moisture resistant drywall behind shower tile. However Most all are stable and in good shape. The ones that are damaged are the ones that got no care or maintenance, or had damage that was poorly repaired with duct tape and spit, if repaired at all. Thank goodness for cement board.
    – RMDman
    Jul 19, 2023 at 16:34

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