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katyshuler 11-03-2009 01:11 PM

Ditra & Travertine on Interior-Grade Plywood
Hi all. Here's the deal: we pulled up sheet vinyl from our kitchen floor to find what we think is interior-grade plywood on the floor (2 layers, each about 1/2-5/8" thick). The only reason we think it is interior grade is because it doesn't appear green, as if it was pressure treated. We have no clue if the bottom piece is exterior grade or not. We have already purchased Ditra and 1/2" thick 18x18 travertine tiles. You should know ahead of time that we have a height constraint. I don't know what the joist spacing is either...the house was built in 1988 if that helps. Also, we are located in NC, so there are very few months of cold.

The Ditra needs to be mortared to the subfloor, which is causing us to worry about the glue coming apart on the plywood when it gets wet with thinset. I don't want to remove the layer of plywood and put exterior grade down. 1- it could be glued to the layer below which would require us to replace both layers, 2- it will be a lot of work, and 3- it will cost a lot of money to replace it. I also can't add an additional layer on top of the existing 2 layers because of the height issue.

What are our options? Can I GLUE the Ditra down? Can we SEAL the interior grade plywood with a sealant and proceed with the normal Ditra instructions? Need to start this weekend...please advise! :001_unsure:

R&D Tile 11-03-2009 02:11 PM

You don't want green plywood, that said, what you have is probably fine.

Now, first things first, you don't glue Ditra down, you use a modified thinset to set it over the plywood, no sealers or anything done to the plywood.

Second, you said stone, that means the floor needs to meet specs for stone, need to know the joist size, spacing and length of them unsupported.

katyshuler 11-03-2009 06:44 PM

Yes, I know it is supposed to be applied to the plywood using thinset, but we were trying to avoid getting the plywood wet.

Our measurements are as follows: 2x10 joists, 16" on center, and the joists are 11-13' long. There are some support posts under the floor in the crawlspace, but I don't know exactly where. With the 2 layers of plywood, should I worry about the deflection?

Daniel Holzman 11-03-2009 10:01 PM

Most plywood uses exterior grade glue, however very little plywood is "exterior" plywood as defined by the American Plywood Association (APA). That is because exterior plywood by APA definition requires use of special lumber, and is designed for continuous exposure to the elements. Most plywood sold as "exterior" is actually exposure grade 1, which is fine for tiling. Even interior grade plywood is made using highly water resistant glue, and would be just fine with thinset.

If you mix the thinset properly, there is very little free water in it, and you will not damage the plywood, whether it is interior, exterior, or exposure grade 1. Using Ditra is good, get on the Ditra website and it will explain in great detail exactly how to install the Ditra and stone. Read the site very carefully, as they have explicit recommendations on where to use modified and unmodified thinset in the application. Also, they have specific recommendations on allowable settlement (L/D ratio) for the joists.

R&D Tile 11-04-2009 06:48 PM


With the 2 layers of plywood, should I worry about the deflection?
Yes, you should, the plywood doesn't help the deflection in the joist system.

If those joists are 11 to 13' unsupported, they are not rated for stone, ceramic yes.

Thinsetting the Ditra to the plywood isn't an issue.

JazMan 11-06-2009 08:58 PM


Do you know the species and grade of the joists. Also your 11-13' answer for the span isn't telling us anything. We need it to the inch along with the species and grade. Measure only the UN supported span. In other words the distance from the face of one support to the face of the other.

The good news is that a shot coming along the length of the span isn't as critical as not having enough plywood to stiffen the in-between-the-joists deflection. That is where most of the damage happens.


Bud Cline 11-06-2009 10:44 PM

Jaz and Rd are correct. We need specific measurements and information to be able to assist you.

2X10's spanning at 11' will be marginal in some cases, at 13' it would be a no-go for stone for sure!

Travertine is the "hot ticket buzz word" for now but I can tell you travertine issues are cropping up all over the country due to marginal installations and short cuts.:)

The "green" plywood you talked about is not at all what should be used for a tile installation, in fact, it should be avoided at all costs.:)

katyshuler 11-09-2009 09:09 AM

Project Image
1 Attachment(s)
Hi all. I've attached an image of my project. This is about 1 side of our house. The areas in black are different rooms that are not being tiled. The image has dimensions, the direction of the joists, and even the placement of the tiles. WE ARE NOT TILING UNDER THE CABINETS! The tile will overlap under the cabinets a little bit, but we are not putting full pieces under there. I don't know how much this will alleviate the weight, or if this has any effect on my deflection. Please take a look at it and let me know if I should still be concerned. I am just over the deflection problem by about 1 foot I think. UGH!

We aren't opposed to doing what we need to in the crawl space to fix the deflection issue, I just don't know how... Do I need to put in another perpendicular wall in the crawl space or is it as simple as putting 2x8's in between all the studs. It would not really be feasible for us to put a beam in the crawlspace. Please advise!

R&D Tile 11-09-2009 02:46 PM

I wouldn't install stone unless those joists are sistered or more added between them or the span is cut down.

You can install stone if you like and take a chance, up to you.

JazMan 11-12-2009 09:17 PM


You still haven't answered what species and grade the joists are. Do you think all wood species and grades are the same? :no:


Guru10cea 06-29-2011 12:11 AM


Did you go through with this job? I'm in the exact same predicament... How did it go?

oh'mike 06-29-2011 06:28 AM

Guru---post your floor system specs and someone here will figure the deflection for you.

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