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Unread 10-05-2020, 02:45 PM   #1
trickyrick
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Install Kerdi shower tray

I have a bathroom I've removed the old tiles and thinset from the plywood sub floor. The builder installed two layers of 1/2 inch plywood on joists. There seemed to be no grout cracking. My question is the shower has no curb the tray is counter sunk into floor. I was going to use the kerdi membrane to extend the waterproofing about 12 inches past the actual shower tray onto the plywood subfloor. I called Schluter and they said that I cant install the membrane over plywood. I would have to put down a thin layer of thinset first. What are my options
Also I have heard that thinset can wick up water so Im going to cover everything (all seams and shower tray) with Ardex 8+9 (sound good) never used the product
Thanks
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Unread 10-06-2020, 06:14 PM   #2
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Rick,

Tell us more about the 2 layers of half inch plywood.
Are they glued together with a full spread of glue?
What is the joist spacing?
Have you checked the deflection?
Is there a potential height issue with the flooring outside the bathroom?
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Unread 10-08-2020, 08:05 AM   #3
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The ply wood is not glued together its screwed every 8 inches the floor joist are on 16 inch centers and are 2 by 10s the span is only about 15 feet. I did the glass of water test on the floor and there is no deflection. As it stands no there is a about 1/2 inch difference between the two floors
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Unread 10-08-2020, 12:20 PM   #4
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You might want to run the "Deflecto" tool, Rick, the link to which is in the dark blue bar above. I'm pretty sure 2X10's at 15 feet are over spanned.
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Unread 10-08-2020, 12:32 PM   #5
trickyrick
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Ok I got a tape out and measured the span and its only 14 feet so the deflection tool says Im good. But this does not answer my other question. How do I put the Kerdi membrane over the plywood floor
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Unread 10-08-2020, 12:52 PM   #6
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This curbless shower tray, Rick, is it new or was it already there? Is the edge of it even with the plywood? What is it made of? What kind of drain does it have? What is directly under the tray?

Photos would probably be helpful.
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Unread 10-08-2020, 01:03 PM   #7
trickyrick
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Its all new. The top of the Kerdi shower base is level with the top of the plywood
This is what Im using
https://www.lowes.ca/product/shower-...ge-2-in-872888
Under the tray is 1/2 inch plywood.
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Unread 10-08-2020, 01:33 PM   #8
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Short answer is, you don't put kerdi membrane directly on plywood. You would need to cover the plywood with something....cement board, drywall, etc.
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Unread 10-08-2020, 01:43 PM   #9
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Something's not adding up in my head, Rick.

The perimeter of that shower tray has to be at least 1" thick. If it is resting on 1/2" ply the the total height will be at least 1.5".

If the floor outside the shower is 2 layers of 1/2" ply.....well like I said, something isn't adding up. Only thing I can think of is that the joists under the pan were cut down to accommodate the 1/2" ply. And if that's the case they have been weakened unless reinforced in some way.

Regardless, as Laz says, Kerdi doesn't go on plywood as far as I know. But do check out the Kerdi installation instructions for verification.
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Unread 10-08-2020, 01:52 PM   #10
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OK I cut out the plywood under the shower base and cut strips of plywood to go between the floor joist so really the tray rests on the joists (sorry about that should have said that).
I called Kerdi and they said the same thing. Cant put Kerdi on top of plywood
Can I put a very thin layer of thinset on top of the plywood where the kerdi is going to go. Let it dry than apply the Kedi to that
Also has anyone used the Ardex 8+9
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Unread 10-09-2020, 09:20 PM   #11
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I’d recommend a layer of schluter ditra on the floor. I believe it’s only 1/8” thick. Then you can extend the membrane over the ditra no problem. You don’t want to put tile right on plywood anyway, and ditra is the thinnest underlayment you’ll find.
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Unread 10-10-2020, 10:11 AM   #12
trickyrick
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Thanks for the info. The house is about 12 years old we ripped up ceramic tile that was on the plywood and there was no cracks in the tile or the grout is there a reason that I can't put the tile on top of the plywood
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Unread 10-10-2020, 10:28 AM   #13
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I'll hafta take issue with Charlie's statement there, Rick. Ditra is not the thinnest tile substrate membrane available.

Your subfloor structure does not qualify for a direct to plywood installation, but you can, of course, tile over anything you want. But if you were to tile direct to the bathroom plywood, how would you provide any waterproofing outside your curbless shower?

My opinion; worth price charged.
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Unread 10-10-2020, 11:47 AM   #14
trickyrick
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Ok thanks. Sorry with me I like to know y as well. So it's not that the tile will now adhere to plywood it's the water proofing that's a concern.
A few years ago I put a shower in my basement and I used the kerdi membrane right over OSB plywood. I now know that's a no no. But I'm not sure y.
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Unread 10-11-2020, 12:44 PM   #15
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As has been pointed out, Schluter doesn't want their membrane installed directly over plywood. However, it seems most commenters are interested in telling you what won't work rather than how to solve your problem.

To answer your question: 8+9 should work. I've never used it but are you thinking of spreading the 8+9 over the outside of the shower and then installing the Kerdi over it while it's still wet? Or maybe letting it dry and installing the Kerdi over it?

Another way to go would be to use another manufacturer's membrane that will allow for an installation over plywood and use that outside the shower then seam the two membranes together. Something like Noble CIS would work. ValueSeal may work also and it's a lot thinner. Both are waterproofing products.

You could also use ValueSeal over the entire floor and shower pan and not install the Kerdi at all on the shower tray. Obviously, Schluter wouldn't approve of this but you're trying to solve a problem, right?
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