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Unread 07-30-2009, 11:22 AM   #1
Ripp
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New bathroom floor half slab, half joist over basement.

Hi everybody,

Been lurking here for a while and have a problem I figure you guys could help with.

Just like the title says. A new master bath will be built in an area where about half the room is over concrete slab and the other half is over the basement. The area is a bedroom now and carpeted so not sure yet what it looks like under there. You can feel a slight change in elevation from wood to slab. I plan to pull up the top layer of subfloor which I 'think' is particle board and replace with plywood. I can see from the basement the bottom sub is ply.

So my questions would be:

Is plywood alone an acceptable substrate to thinset stone over?

How should I over come any problems with level in terms of the elevation change from wood to slab?

Should I have a moisture barrier over the slab like Redguard or something? Never had any problems with moisture before.

Any do's or dont's from the experts?

Thanks in advance for any help. Great forum I have learned a lot just from reading here.
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Unread 07-30-2009, 11:38 AM   #2
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Welcome, Ripp.
Quote:
Originally Posted by Ripp
Is plywood alone an acceptable substrate to thinset stone over?
The requirements for setting even ceramic tile directly to plywood are very strict and usually not readily achievable in a remodel situation. In any case, we generally recommend pretty strongly against it. I'd not recommend it at all for natural stone.
Quote:
Originally Posted by Ripp
How should I over come any problems with level in terms of the elevation change from wood to slab?
I trust we're talking about flat, rather than level. You will need to honor that joint between the two subfloors all the way up through your tile surface in any case. Dealing with a change in elevation there will depend upon how much change, where it falls in your design, etc. But it'll be a consideration, for sure.
Quote:
Originally Posted by Ripp
Should I have a moisture barrier over the slab like Redguard or something? Never had any problems with moisture before.
Only if you have a problem. But you may want to use an uncoupling membrane over the slab anyway, and that may well serve as a moisture barrier, too.

Tape a couple squares of clear polyethylene sheeting, about two by two feet in size, to the slab for a couple days and see if you observe any visible moisture. If you do, you'll wanna do a more serious test before you proceed. If not, I wouldn't worry about it.

You also need to evaluate the joist structure under the framed portion of the floor. You can enter the necessary information into our (Deflecto) above and get a good initial indication. You must also have two layers of plywood or OSB subflooring, properly installed, for any natural stone installation.

Still a bit of research needed on your part to sort it all out.

My opinion; worth price charged.
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Unread 07-30-2009, 12:05 PM   #3
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Thanks for the reply CX

Ok so no stone over plywood. What should I put down as substrate?
Please keep in mind that if I have 2 layers of ply which bring me roughly to level of slab and I add another layer then I incurr a different problem. What would you do about the height difference then?

So you're saying I should have a tile joint at the same place as the slab/wood joint right? That makes sense not to try to bridge the gap with stone. I had not intended to do that. How likely is a crack in the grout at that point? Anything I might do to mitigate it?

I will test for moisture as you suggested, I think I'm good on that one.

Also I ran the deflecto and came back at 804. Should be good in that area too.

Thanks again for the quick reply.
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Unread 07-30-2009, 01:27 PM   #4
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Ripp, if you have 2 layers of plywood totalling at least 1-1/8" properly glued to the joists (first layer) and screwed every 8" or so (no glue between layers), you can use Ditra over both the concrete and plywood. This has the advantages of maintaining floor heights as well as dealing with some moisture issues in the slab. You still need a movement joint in the tile at the juction between subfloor materials. Grout will crack, so use a urethane type caulk for that joint.
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Unread 07-30-2009, 01:38 PM   #5
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Thanks for the reply Bob!

No dealers with Ditra within 110 miles but I believe HD will special order it.

I am planning a heated floor as well and wonder if Ditra is OK with that?
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Unread 07-30-2009, 01:41 PM   #6
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Order on-line from Tile-Experts.com.

Ditra works well with in-floor heat. Goes on top, between heat mats and tile. I do not recommend trying to install both in one swell foop.
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Unread 07-30-2009, 09:45 PM   #7
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I've bought all my Schluter products from Tile Experts.com since day one, Ripp. Shows up on my doorstep promptly and the lack of driving forty miles and paying sales tax more than makes up for any possible price difference and shipping.

Tell'em we sent ya.

My opinion; worth price charged.
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Unread 07-31-2009, 01:19 PM   #8
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Thanks again for the help.

If I have a flatness issue can I use Ditra on one side(the higher side) and XL on the other side(lower) to make up the difference?

Not sure it will be an issue but nice to know just in case.

Also will I want a joint in the fabric there anyway or should I plan to span the area with Ditra?
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Unread 07-31-2009, 01:57 PM   #9
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Yes, you could do that if you like.



You must honor that joint, Ripp, no matter what substrate(s) you elect to use (post 2).

I recommend you visit the Schluter site, find the Ditra Section, and look at the installation instructions you can download there. Will tell you everything you need to know about that product.

If they missed something, we'll fill in for'em.

My opinion; worth price charged.
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Unread 07-31-2009, 03:47 PM   #10
Ripp
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Thanks for the link CX,

Been reading up on the Ditra. Awesome stuff, should take care of everything I was worried about without adding much height.

I will definately use some urethane caulk at the seam.

Thanks for the advice guys, it is appreciated.
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