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Unread 04-03-2021, 10:11 AM   #1
ericz
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Would using styrofoam as preslope with PVC liner work?

Hello! I am new here but I have searched and read a lot on this forum. Learned a lot. So thank you!

Background: I live in north california and I am currently rebuilding a shower.
The old shower had server water damage to the frame and subfloor. THe homeowner spent a shocking amount of money to fix it, put on new subfloor etc. So his priority is the protect the wood frame and subfloor at any cost.

Now here is the problem:
here people typically do a preslope with deck mud and then put a PVC liner on top of it. ANd then tile above the liner. But since the home owner wants to put redguard on his subfloor, that would create a "moisture sandwich", which is bad. So how about putting some styrofoam preslope and then cut it to fit, and fill in everywhere that it doesn't fit or doesn't cover with thinset. After that I plan to cover it with traditionally PVC liner.

The question is: Is there any serious problem with this design/method?

Any input is greatly appreciated. Thank you.
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Unread 04-03-2021, 10:36 AM   #2
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HI Eric

Yes with the traditional clamping drain system you are supposed to do a preslope under the liner. Whether the floor has redguard on it or not moisture should never get through it because the liner will sit atop. They make styrofoam preslope that makes it easy.

Ironically every shower I have torn out, I am in PA near Hershey I have never seen a preslope under a liner. The water sits in there, weep holes clog up and the water wicks out. I have seen some major damage to the extent of the joists rotted.

I hope that helps ya.
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Unread 04-03-2021, 10:38 AM   #3
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just caught this yes they tile above the liner, but over the mud bed over the liner just to be clear not sure what you meant they don't tile right onto the liner. You may have meant that but I never assume
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Unread 04-03-2021, 10:48 AM   #4
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Welcome, Eric.

There is no standard in the tile industry for the pre-slope in a traditional shower receptor construction. That pre-slope is part of the plumbing of your shower.

But the tile industry standards do call for the liner to be properly sloped, as does the plumbing code, and the tile industry suggests it be constructed of deck-mud type mortar. No reason at all, though, that you can't make it of something other than mortar. And, as Jon points out, at least one manufacturer (Noble Company) makes a foam pre-slope for the application.

Still easier (no requirement for a level subfloor) and a lot less expensive to do the deck mud method.

If I understand your description, your customer wants a second waterproofing membrane on top of the top mortar bed in a traditional receptor construction because he thinks that gives him added protection against leaks. In fact, he'd be creating another problem by trapping moisture in his top mud bed with no way for it to evaporate up through the floor tile surface. And he will get absolutely no additional waterproofing as there is no way to properly attach the RedGard to the clamping drain used with the receptor liner. Just a bad idea all around.

I recommend you tell him to chose a receptor construction method and do it correctly and that it will last 50 years. If he wants it to last even longer than that, chose a method and do it even more carefully. The traditional shower receptor construction is a wheel that needs no reinvention at all.

[Edit] And in re-reading your post, it now appears your customer wants to put RedGard on the wood subfloor under the pre-slope instead of what I said earlier. You can certainly do that if you want. The industry standards call for a cleavage membrane (poly sheeting or roofing felt) under the expanded metal lath you'll install before placing your mortar pre-slope. While not specified, the RedGard would serve the same purpose, which is just to keep the wood subfloor from leaching moisture out of your pre-slope mortar. has nothing at all to do with the waterproofing of your shower receptor.

I see Jon is re-reading as well and maybe between us we've answered the question?

My opinion; worth price charged.
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Unread 04-03-2021, 10:55 AM   #5
ericz
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Thanks!!

The home owner means this

from lower to higher

Subfloor plywood
A layer of redguard or some water proof material (so even if everything above failed, the plywood is still good. That is what he cares about because previous failure just costed him too much money.
styroform preslope. Due to irregular size, I think I have to use deck mud to "fill in the gaps" but 95% will be manufactured preslope. and maybe 5% mud here and there.
PVC liner.
thinset
Tile.

He plan to use a typical 3 piece drain. but make it linear drain
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Unread 04-03-2021, 10:58 AM   #6
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jondon: Thanks!

How much a mud bed above the liner it would need? I am afraid there could be too much of a buildup.
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Unread 04-03-2021, 11:00 AM   #7
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Quote:
Subfloor plywood
A layer of redguard or some water proof material (so even if everything above failed, the plywood is still good. That is what he cares about because previous failure just costed him too much money.
styroform preslope. Due to irregular size, I think I have to use deck mud to "fill in the gaps" but 95% will be manufactured preslope. and maybe 5% mud here and there.
PVC liner.
thinset
Tile.
Eric, I can't tell if you're new to this or not, but you cannot tile directly over the liner material in a traditional shower receptor construction. You must have a second mortar bed of a minimum 1 1/2" thickness over the liner upon which you bond your floor tile.

And, again, there is absolutely no need for painting waterproofing on the subfloor. In a properly constructed shower receptor that subfloor will never see any water other than that potentially splashed out of the shower during use. If he wants to waterproof that floor he'd need to continue the waterproofing up the walls a bit and put a dam across the entry door, eh?

My opinion; worth price charged.
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Unread 04-03-2021, 11:01 AM   #8
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CX: Understood. Thanks a lot!!
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Unread 04-03-2021, 02:13 PM   #9
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Eric I would rethink this what you are doing, there is a much easier way than what you are attempting to do. As CX mentions there is no benefit to putting a liquid on the subfloor plywood. If a shower receptor failed I wouldn't want that water trapped up in there is would be nasty and mold would form. I think you may have mentioned at the beginning of a moisture sandwich and correct you don't want moisture trapped between two waterproof layers.

If you are limited on how much of a mud bed you can do I would scrap the liner and simply your life on this. You can put in a kerdi drain, sits at 3/4" off the plywood and 1/4" per foot for slope so if you were in the middle of a 5" wide shower with the drain in the middle you will be at roughly 1 1/2" in height at the perimeter.

You can do a foam pan or mud. If you put in a foam pan they normally have the waterproofing fabric already on it. If you do a mud pan or bed you can waterproof over the mud with that regard after 3 days of curing time to be safe. With surface waterproofing water should never get beyond the surface if done right. If he has had problems in the past it wasn't done right. Do it right and it will not leak. I think from what you are saying you are trying to go about it backwards.
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