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I'm getting ready to finish waterproofing my new shower before tiling it and am wondering if I should waterproof the mud pan on the floor? My construction methods are as follows from bottom to top:

  1. Wooden subfloor
  2. pre-sloped mud pan (with tar paper & metal lathe between the subfloor)
  3. pvc shower pan liner 6" up the studs & over curb
  4. Concrete board on walls down to bottom of liner
  5. Final ~1/2-3/4" mortar bed over the liner and mortared shower curb

I'm going to paint a few layers of liquid waterproofing membrane over the concrete board walls (and should I do the ceiling since I'll be tiling that as well?). Does it matter if I also waterproof the mud floor? I know it's not necessary since that's what the liner is for, but would it hurt anything? I'm mainly interested if there's a concern for hydrostatic pressure buildup between the impermeable PVC liner and the liquid membrane if water were to get in between them somehow.

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Any shower I have done in the past 5 years has been done with both the PVC liner over a sloped bed and the liquid applied membrane. I coat all the corners and the drain 1 coat, then paint 2 coats over everything up to the splash line of about 5 1/2 feet. No need to do the ceiling.

If you have not done it already, let me suggest this.... I use a modified thin set and fiberglass tape to cover all the joints before applying the liquid membrane. It strengthens the joints where the cement boards meet. Also bridges gaps for the membrane.

  • Thanks jack. I'll just go ahead and coat it all. I'm taping and thinsetting the joints today – tbox Sep 8 '16 at 16:08
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Do NOT use a topical waterproofing treatment on the floor if you already have a shower pan liner. Two impermeable layers creates a "mold sandwich". See http://floorelf.com/answers under "Membranes".

Go ahead and apply the topical waterproofing to the ceiling in addition to the walls. You may not need as many coats there as on the walls, since you don't expect water to come into contact directly (read the directions for the number of coats required).

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"I'm going to paint a few layers of liquid waterproofing membrane over the concrete board walls"

  • The concrete board walls are designed to be the substrate to which you bond tiles with mortar, the waterproofing membrane (often tar paper) needs to be under the concrete board walls and overlapping the vinyl pan at the bottom, so moisture that might penetrate flows down the waterproofing membrane into/onto the shower pan, then flow to the drain weep holes. Also, I cannot imagine that tile would adhere properly to concrete board with a waterproof coating on it.

"Does it matter if I also waterproof the mud floor?"

  • It might. The vinyl liner is your primary waterproof membrane and is supposed to terminate such that moisture penetration of the tile/grout/final mud layer can flow to weep holes in the shower drain. If you somehow block that function you could end up with pooled water under the tiles, not good. Again, the final mud layer is the ideal substrate for tile adhesion with mortar, how to you intend to affix the tiles directly to a waterproof surface?

image stolen from internet

  • Reading my answer I realize it might help to be distinctly clear: NO, do not apply waterproofing liquid to the top mud layer. NO, do not apply waterproofing liquid to the cement board tile substrate on the walls. YES, do ensure that you have properly waterproofed below both of those materials. – Jimmy Fix-it Sep 8 '16 at 4:47
  • I think you may be misinformed about the liquid waterproofing membrane. Unless you can find some sources that say otherwise, I'll go by the manufacturers instructions which say to coat the walls with membrane and apply tile directly to it. The image you posted is indeed one way to make a shower (of which there are many) but I'm taking a slightly different route – tbox Sep 8 '16 at 16:08
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    OK, good luck with it. Normally a no-no to hot mop (or modern equivalent) over hot mop. I hope it's not a recipe for trapped moisture. – Jimmy Fix-it Sep 9 '16 at 1:14

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