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I have a new-construction bathroom with a shower stall. I am tiling all the walls and using greenboard drywall for the ceiling.

On the wall, the substrate is HardieBacker with Hydrodefense. The instructions from HardieBacker say to use a liquid waterproofing membrane on all edges and joints. I've used this liquid membrane on all of the wall except the inside corners of the wall and ceiling.

But, I don't know how to handle these ceiling inside corners. So far I have applied alkali-resistant glass fiber tape to the corners, as instructed by HardieBacker. Then I applied mortar on the wall part of the corner, and a thin layer of joint compound on the ceiling part.

So my question is, how do I finish the ceiling part? Do I put two more layers of joint compound there to smooth it out? And then liquid membrane on the wall part? Or do I apply the liquid membrane on both the wall and the ceiling? If I put the liquid membrane on the drywall ceiling, will it look bad? Or do I do something else?

I've tried to pinpoint this on Google but I can't find a clear answer. Please see the image below.

enter image description here

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  • So the ceiling is staying drywall with paint? If so you can just fill the corner with liquid membrane or silicone. Then when you tile up to the ceiling you silicone the tile/ceiling transition to keep water out.
    – redlude97
    Commented Jan 12, 2022 at 1:13
  • @redlude97 Yes, the ceiling will be drywall, plus whatever is best to put on it, and then water-resistant paint. Do you mean I should use liquid membrane on the entire inside corner, both on the wall and the ceiling? Feel free to post an answer please.
    – Mark
    Commented Jan 12, 2022 at 3:55
  • @Jasen if you can add to your answer that would be great. Please see my comments on your answers.
    – Mark
    Commented Jan 12, 2022 at 18:55
  • @P2000 if you can add to your answer that would be great. Please see my comments on your answers.
    – Mark
    Commented Jan 12, 2022 at 18:55

2 Answers 2

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You can seal the entire shower stall, walls and ceiling, if you think it is all within the splash zone. It will do no harm.

Usually the ceiling is not sealed. If you seal the ceiling, the seams are of course also sealed, otherwise they don't have to be.

The red HardieBacker itself, which contains a Hydrodefense layer, does not require a membrane, but you do have to seal the screw holes and seams (incl all corners) as you have done. That's all.

As for the ceiling, if you seal the ceiling you should seal the seams. If you are not sealing the ceiling, then seal just the wall/ceiling corners if you have membrane left.

The green membrane you have so far applied does not look thick enough to me. It should be multiple layers to form an impenetrable membrane like 2 or 3mil plastic, not a thin layer as one would apply primer. You can simply apply more membrane over the seams. It is important not to leave pin sized holes or thin spots in the membrane. Your tiles of course will guide most of the water down, but the grout is not impenetrable, and moisture as well as water can get in through cracks and capillary effects.

According to Hardie: "When complete waterproofing is required, seal all joints, edges and fastener penetrations with a liquid waterproofing membrane. Follow the liquid waterproofing membrane manufacturer application instructions."

  1. re-apply a thick layer of liquid membrane over all critical areas: seams, corners, screwheads. Include the ceiling/wall inside cornerif you have membrane left (likely)
  2. Ceiling is optional: seal it if you expect lots of upward splashing.
  3. tile
  4. prime and paint the rest
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  • The red is the substrate, which is HardieBacker. The green is the liquid membrane, of which I put on three coats. Should I apply liquid membrane on the ceiling edge, and then primer and paint over that?
    – Mark
    Commented Jan 12, 2022 at 18:51
  • @Mark Corrected.
    – P2000
    Commented Jan 12, 2022 at 19:30
  • I've applied three coats of the liquid membrane; since the picture here, I put on one more layer. It now has some thickness to it.
    – Mark
    Commented Jan 13, 2022 at 6:47
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If you are not expecting the shower to frequently spray on the ceiling you don't need to seal this joint, just treat it as a regular drywall ceiling joint.

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  • There might be a little splash on the ceiling, since this is in the basement and the ceiling is somewhat low. I'm happy to seal the joint if I have to; I just don't know if it's ok to seal the ceiling's green board with liquid membrane before I prime/paint. Please advise, thanks!
    – Mark
    Commented Jan 12, 2022 at 3:57
  • It's ok to do, but it might not look good when finished.
    – Jasen
    Commented Jan 13, 2022 at 10:26

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