Shower waterproofing for marble slab walls and pan?

[vipkl]

Hi all, I'm very new at this and need help! I'm remodeling 2 showers, upstairs and downstairs (slab on grade foundation as is the norm in Houston). Will do full tear out down to the studs including the shower pans.

Now I realize this is a tiling forum, but we're actually wanting to do away with grout. Looking to replace with composite stone slab walls made of cultured marble/cultured granite (mold with gel coat), solid surface (uncoated homogeneous), or engineered stone/quartz (hard slabs). With matching composite stone shower pans (one shower can have a one-piece molded pan while the other may need a custom slab base with separate curb piece).

My question is how (or if?) I should waterproof the slab shower backing and have a proper channel drainage system, just like with tiled/grouted showers?

I contacted local marble companies, but they've just been putting greenboard drywall on the bare studs, then the stone slabs on the greenboard. They said the slabs are non-porous and they install with very tight seams using thin silicone caulking. But I'm still concerned with leaks from silicone failure over time (at the wall/floor joints, recessed niches, trims, etc)?

I started asking if they could waterproof the wall backer, either traditional CBU with moisture liner on the studs, or topical membrane on the board. But their crews might be resistant to the extra work in applying the membrane, or they might not be used to cutting cement boards? Then I started suggesting something like DensShield since it cuts like regular drywall but is factory waterproofed (membrane-faced) -- though they'll still need to seal the screws etc.

But then what about the shower pan?

With a tiled base as I understand, you'd have the underpan liner on a preslope, clamped to the drain flange. Water that seeps behind the walls would roll down the wall membrane, onto the underpan liner and mud bed, where the slope would channel the water towards the drain weep holes under the floor.

But I believe the marble companies said their shower pans are placed on the flat subfloor. Well, I'm still trying to get more details. I think one company told me they do use a pan liner placed flat on the subfloor, then a flat mortar (thinset?) to set the marble pan on. Maybe they were referring to the custom marble shower base vs the pre-molded one-piece marble pan, I'm not sure.

Regardless I'm afraid the drain on these marble pans are sealed with no weep holes? Not to mention the lack of a preslope. So then, how can I line/membrane behind the slab walls if there's no underpan drainage channel (any leaked water rolling down the membrane would have no way out)? And I don't think the wall membrane can channel leaked water back on top of the pan since the wall/floor joints would all be siliconed?

Would it be better to NOT use any membrane for the backing? So any small leak through worn out silicone on the marble slab joints can just spread through more surface area (wall boards, wood studs, plywood subfloor) to dry faster?

[koihito]

Welcome Paul,

You are trying to mix two entirely different systems and you are on the right thought path, they don't mix.

Cultured slab showers can and do leak when the silicone joint fails. The way to prevent that is proper maintenance, which in my experience means cut out the silicone and replace it every 4-5 years.

If your main goal is easier maintenance you could also consider a large format tile with a small grout joint and epoxy grout.

[vipkl]

Thanks Nate!

I haven't looked into it, but I've heard about large format tiles (fewer grout lines), rectified tiles (smaller grout lines), and epoxy grout (supposedly unwieldy to install but a lot more stain/water resistant than even sealed cement grout?). Is the combination of all the above common in shower tile installs, or impractical?

So.. I guess there's no way to set up a waterproofing envelope for slab showers? That means people with slabs can only (pre-emptively) replace all the silicone every few years and hope nothing leaks prematurely. I wonder if it would make things *worse* to just put waterproofed backing behind the slabs.


What about showers with tiled walls but a molded pan that installs flat on the subfloor? I suppose the wall backing membrane/liner must "lap" or go over the pan parameter lip/flange so that leaked water drains out on top of the pan (instead of under the pan). But what about the grout at the wall/floor joint, is it porous enough to let the leaked water seep back out to reach the top of the pan? Or must the installer leave holes along the bottom grout line?

Hmm, could I try the same with slab walls? Install membrane/liner behind the slab walls which laps over the perimeter of the pan, AND leave holes in the wall/floor silicone joint for ingress drainage on top of the pan? That means the marble installer must hang the slabs a little bit upward, with a gap from the pan?

[vipkl]

BTW for one of my showers, the stone fabricator companies said to upgrade to a custom shower base -- a custom-cut stone floor slab that meets the walls without any raised lip (the front curb would be a separate piece). They said it'll give more space since a molded pan has the thick curb/lip on all 4 sides that your toes can stub on.

But with the custom pan, the wall/floor silicone joint would be right at floor level and would have to hold back standing water on the floor (and the silicone would be the only line of defense if there's no waterproofing envelope to fall back on). Whereas a molded pan holds the floor water with its raised curb/lip which is all one piece.

Should I be scared of this?

I'll have to check if their reason for a custom base is due to my existing drain location. If so, is it better to relocate the drain and try to get a one-piece molded pan? Pricing is not too different so not an issue...

[Kman]

A waterproofing system consists of the drain which is connected to a membrane that goes at a slope to the perimeter of the shower and then up the walls several feet.

How would you tie the membrane on the floor to a drain that is made for a cultured marble shower? And how would you slope the membrane to direct water to the drain? If you waterproof the walls but not the floor, what value would that have? Any water that gets to the walls simply runs to the floor and causes damage.

I think Nate is correct, you'll have to go with one or the other.

[vipkl]

That makes sense, thanks.

Switching gears then, how can the stone slabs be adhered to the wall/backer?

One slab salesman said they squirt beads of silicone glue behind the slabs (will check with the others). I also read online that a coat of epoxy adhesive can be painted behind the slabs instead -- which is better? And what about the option to trowel thinset mortar? Though I believe silicone and epoxy adhesives create a waterproof bond unlike thinset. (There's also mastic tile adhesive made out of organic resin, but supposedly it's even worse with water and mold resistance.)

What if the slab installer just pumps gobs of continuous silicone between and behind the slabs (around corners/joints, niches, trims). So it wouldn't just be a thin layer of topical caulk that protects from water. Even if a strip of front caulking gets torn off, the gap between joints would still be fully sealed with silicone all the way to the back of the slabs. Would this negate the need for any additional waterproofing envelope?

Questions:

- What about the stone shower pan, is it OK to silicone it directly on the subfloor, or better to set the pan on a mortar bed for leveling?

- Is silicone adhesive not designed to be painted on as a full coat edge-to-edge (perhaps too viscous or dries too fast)?

- How do silicone vs epoxy vs thinset adhere to paper-faced drywall (greenboard), paperless fibermat drywall with acrylic membrane coating (DensShield), or cement/fiber-cement board?


P.S. It seems stone fabricators in other cities also install shower slabs direct to greenboard-on-bare-studs as well (see TerryLove plumbing forum for example). I was also told the slab installers don't even tape the backer joints. So I guess there's no benefit to using DensShield over greenboard in this scenario.

[Cglaser]

Hi Paul,

I just stumbled across your post here from I’m long ago. We are currently getting ready to install large format stone slabs in our shower. What waterproofing strategy did you end up using for your bathrooms? How have they held up over time? Thanks for any advice you can provide!

[Kman]

Hello, Carolyn.

According to his profile, Paul has not been on the site in about 6 years. Unless he happens to have notifications to his email, I doubt he'll see your question.

Feel free to start your own thread (if you haven't already) and ask whatever questions you may have. You may also do a search of the site for threads related to your situation.