We had some plumbing issues in a bathroom that required us to open up the wall for the shower. That's been fixed but now I'm wondering how to seal the replacement piece of backer board. It'll be screwed into the wall studs so I'm NOT worried about it moving. I just want to protect it against water/moisture, etc.

What should I use? I assume a normal silicon won't bond to concrete.

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Detail the existing backer board so there is no thinset left at all, you will need that room for the new thinset. Tape off the areas you don't want caulk on and use any caulk that will bridge the gap in the sheets, and cover with a liquid applied membrane meant for showers. Set your tile.

  • Any caulk that will bridge the gap? I don't have to use a specific one?
    – Mike B
    Oct 18 '15 at 18:16
  • In my opinion, no you only need something that will bridge the gap and withstand the pressure needed to apply the liquid applied membrane over it. That is the main requirement for the caulk, the membrane is the work horse here. Oh, do not use 100% silicone, latex with silicone is cool, or anything that is water based clean up. Put it only in the joint, if there is a thin smear on the surface, no big deal, you do not want anything blobbed up on the surface. You will want every available means possible to have as much room for the thin set.
    – Jack
    Oct 19 '15 at 0:23

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