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Unread 10-13-2021, 11:13 AM   #1
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Bonding foam shower pan to subfloor

I just watched a video of a guy slapping thinset on plywood subfloor then installing the foam right on that. It was by not-the-manufacturer but someone who claims to work closely with them, is blessed, etc. That's a topic in itself, but let's assume he was following the protocol (will follow up on that later).

It is my understanding that on plywood, we want to staple down wire lath or something of that sort to bond the receptor to the wood floor. Concrete, thinset, etc. don't bond well. And yet there he was, spreading thinset right on the wood with no other mechanical fastening scheme. There was no cleavage membrane either, which is something I have seen more often when lath is put down.

So, without knowing the manufacturer, cuz I'm not tellin' (but it wasn't orange), does this seem wrong to anyone? And, aside from perhaps not being the mfr recommendation -- instant disqualification, I know, I know -- would it be a mistake to put down a cleavage membrane, lath, a thin scratch coat of mud, THEN put down the foam pan? Actually, there's no reason to be coy, it was Laticrete's pan. No dings on the product, it looks pretty fun, I am just wondering about the installation method since it seems to go against convention.

I know that using lath and a thin mud bed will build it up the floor a bit but that is outside the scope of the inquiry here. In any case, it made me cringe to see thinset going right onto plywood. It was the mfr's super-dooper sticky LFT stuff, but I don't see that sticking for too long unless it has polymers that are capable of really bonding to wood nowadays. Or, maybe the probable natural cleavage is intended and expected, and the thinset is more of a gap filler/leveler than an actual bonding interface?
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Unread 10-13-2021, 11:23 AM   #2
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wooden subfloor




a foam shower base


adequate water proofing (water proofing fabric with seams and corners enveloped)


a shower base that will hold and will protect the wooden floor from damage.

you can go over kill if you want and prep with concrete board or lathe and parge but that is entirely up to the installer. a personal preference.

products like the schluter shower system are intended to simplify tile installations. my advice keep it simple and focus on your slopes.

if you are pouring a shower base or using drypack it is slighty different. but dont overcomplicate.
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Unread 10-13-2021, 11:44 AM   #3
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Gerry, the method of installing a foam shower floor is entirely up to the product manufacturer. There is no industry standard for that. There is no industry standard for foam shower floors.

There is certainly nothing wrong with bonding something directly to a plywood subfloor, so long as the thinset mortar (which I think you meant, rather than thinset) meets the requirements of ANSI A118.11, which is the standard "for EGP (Exterior Glue Plywood)Latex-portland Cement Mortar."

You'll find that Schluter, which you indicate you're not talking about, specifies the use of an unmodified thinset mortar, A118.1, for installing their foam trays to a plywood subfloor, in direct opposition to the industry standard. Why? Because they say it will work and it saves the installer from having to use two different thinset mortars to install their shower system.

Manufacturer can specify any installation method they see fit.

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Unread 10-15-2021, 09:22 AM   #4
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First of all, I've been in the tile business for 50-odd years, and from the beginning we routinely called thinset mortar "thinset."

Second of all, I don't agree with Schluter in the use of unmodified thinset to bond anything to plywood.
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Unread 10-15-2021, 10:55 AM   #5
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Me, too, but I think it important for the visitors to know the correct industry terminology so they might be less confused.

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Unread 10-16-2021, 03:07 PM   #6
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I think most brands want their foam pans installed with thinset mortar. I've only installed Wedi and Schluter before and both of those were thinset.

ANSI 118.11 grading is specifically for bonding to plywood
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