I recently installed a cement shower pan but I had to use two pvc shower pan liners because of the size of the shower. I used the specified pvc cement to "weld" the two liners together at the seam. However, I'm not entirely convinced this will be 100% waterproof at the seams. Unfortunately I completely forgot to test the shower pan liner after I installed it and I already poured the rest of the deck mud on top to the final height.

So my question is, can I use Redgard over the shower pan at this point (over the top deck mud layer) to make sure I won't have any leaks? I already saw a few questions about this on here and they mentioned a "mold sandwich" due to the cement not being able to drain any liquids if there is a leak in the first layer. However, I don't see this as an issue since the drain has weep holes at the pvc liner.

2 Answers 2


I don't think I'd bother.

pvc glue melts the plastic together. It is used in plumbing with no leak issues.

If you do red guard it, I would not be concerned about a mold sandwich. Even if mold did somehow get enough food and water and did grow in the deck mud, how would mold spores get out of this sandwich?

How would mold growing inside deck mud - under tiles - that receives the same food and water be less worse than a mold sandwich? Is the opinion that it can dry easier make it better?

  • This was my exact thought on the mold sandwich. And plus the water should still be able to drain from the deck mud through the weep holes even if there is a leak through the redgard. Is it really that different from sandwiching it with tiles?
    – Programmer
    Apr 2, 2019 at 13:03

Not only can I cause a mold sandwich if you did not install a pre-slope for your liner and water traps it can also degrade your thin set and make tiles pop

  • 2
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    Dec 6, 2022 at 14:03

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