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-   -   Ditra over Cement Backer board (https://www.johnbridge.com/vbulletin/showthread.php?t=113036)

awdpsi 06-04-2014 03:21 PM

Ditra over Cement Backer board
First off all, hello and thank you for such a great forum. Next allow me to apologize in advance for any ignorance or lack of knowledge I display. I certainly know what its like reading on forums as a professional (NOT TILE...haha) and shaking my head at people posting.

But with all of that said, I really enjoy this forum. I have been reading on here for a few weeks on random points while I have been preparing to redo my bathroom. I am posting this topic, because some of the searches I came up with that addressed this were much older or had conflicting answers in the thread. If I annoy anyone with the repetition, I sincerely apologize.

So I have a 3/4 inch plank style subfloor laid on a diagonal. When we were down to the subfloor I changed some rotten planks out as a result of previous water damage, rescrewed any boards that were questionable and added wood bracing in between the joists from the basement in any spots that had too much bounce in them (bounce from the planks not from joists). I then used the flexbond thinset on the back of the cement board and screwed down the 1/2" cement backerboard on the floor.

I know this doesn't help with strength but due to the way the planks were laid it REALLY helped tie the floor in together and add rigidity to it. When I do the jump / bounce test the floor it is so rigid now (obviously non scientific). The joists were in really good condition and didn't have bounce noticeable or much deflection. It was in random plank boards where that was felt.

Anyhow that is the point I am at now (cement board thin set on top of 3/4inch plank subfloor). I purchased ditra which I intended to put down over the cement backerboard. For some reason I felt that this was another step in protecting the overall job after reading how good it was. However, after reading more, I feel like people have stated that if there is cement board down, I shouldn't add ditra on top of that.

So my question in short is should I return the Ditra or use it over the cement backerboard?

I am laying 12x24 tiles. Again thank you for entertaining my questions and sorry for any frustration in reading my post. Thanks


jadnashua 06-04-2014 03:40 PM

No cement board manufacturer (cbu) allows installation directly over wooden planks - they all require at least 1/2" (and you'd be better off with 5/8") ply installed over the boards first. The reason is, the solid wood is just too unstable with moisture changes.

So, once you remove the cbu, add some ply, I'd just use the Ditra on top of it.

Richard Tunison 06-04-2014 03:50 PM

Hi Scott and welcome. :)

What Jim said and,,,, run your joist structure through the deflecto in the dark blue bar at the top of the page and see what it says.

awdpsi 06-04-2014 05:44 PM

Thanks for the replies. Well that stinks, bad. I don't have any clue how I'm going to even get that cement board up with the thinset under it and the joints already taped and thinseted. Any suggestions? Haha

The deflecto score is probably just under 377( 11.25) span before metal beam. Plus I also added wood bracing In between the joists in a few spaces.

Peerless Tile 06-04-2014 06:13 PM

Your floor, your dime. Sounds like your in pretty deep at this point. No your flooring structure is not correct, but withstanding what you'd have to go through to back up and start over, you'll have to asses if it's worth risk of continuing with tile from where you are now. Just how big is this area to receive tile? Can you deal/afford with a total or some partial failure (cracks in grout for instance) of this floor. It may also turn out just fine. Like I said before, your floor your dime.

awdpsi 06-04-2014 07:13 PM

Thanks everyone.

Greg, so the area receiving tile is about 74 sqft. Out of which the vanity will be covering about 22 sq ft. So approx 50 sq ft of exposed tile. I understand your point and worse then risking it is the fact that Im anal in doing things the right way. I want to do nothing else but to run in there and rip it out. My largest enemy is the lack of time and the fact that this is our only bathroom with a shower (wife, baby, toddler). Im so sick to my stomach.

Anyhow, I would now like to ask some Ridiculous questions (not because I am looking to hear a particular answer, but because they are in my head and you all are very knowledgeable)

1. Does the age (been down for 50 years) of this 1x4, diagonal subfloor under the cement board, (as in its not new lumber its already seasoned) have any additional issues or less for that matter?

2. Will laying Ditra over the cement board on top of this issue I have created help me, hurt me, or is it just a waste at this point.

3. Does the fact that I keep my basement (where the underside of this wood subfloor is exposed to de humidified year round (have a finished basement) help at all?

4. Last and the most ridiculous of the ridiculous questions. I know this is an impossible question to answer but based on some of experience you guys have are you thinking this is something that would create random cracked tiles, just grout that has to be constantly done (was thinking of running 1/16 spacers) or is this just total devastation. I just don't know what type of failures I am looking at and how soon (I assume this is the million dollar question though). So thats why this is my most ridiculous question, and I understand that.

Again, I can't thank everyone enough for all of the help and patience being extended to me. I am just trying to see how bad I screwed myself and make a judgement call that I know I will have to live with.

Thanks again everyone.

Kman 06-04-2014 07:16 PM

If I had to tear up what you have, I'd use a utility knife and cut the mesh tape at each seam, back the screws out, and remove the CBU. Depending on how long it's been down, you may find that the thinset scrapes up fairly easily with a floor scraper.

As said before, what you have done isn't to manufacturer's specs, so nobody can truly endorse it. If you had T&G planks run perpendicular to the joists, you might have a better shot at it surviving as is, but I have serious doubts based on what you've told us.

A_J 06-04-2014 07:58 PM

I have the same age and type of subfloor as you, and I don't think you will have a problem getting the thinset off. Nothing really sticks to those old, dried out boards. If I were you I would tear it out and redo with 5/8" ply and ditra just so you can sleep easy at night.

awdpsi 06-04-2014 08:34 PM

AJ as of right now I'm leaning that direction. I wish I didn't have to try to take it up, but doesn't appear that the job so far is going to magically become ok. I really hope I can get the stuff up. I just laid it this passed weekend, so on Saturday when I have a chance to go at it again, It will have been down for a week exactly. UGH....

So If I though down plywood, is there any specific type of plywood I need to use or specifically stay away from?

Is this good enough?: http://www.homedepot.com/p/Unbranded...6081/100004472

Also do I just put down some construction adhesive between the subfloor and the plywood and screw the heck out of it?

Kman 06-05-2014 01:50 AM

That's actually the kind of plywood you don't want. Anything with the word "sheathing" on it is no good for tile.

Look for exterior grade exposure 1 plywood with a face grade of A, B, or C. I usually install 3/4" Sturdi-I-floor tongue and groove, but if you're looking for something thinner, just look for something like I mentioned in the desired thickness.

Don't use any construction adhesive, just deck screws that will penetrate through both layers, every 6".

awdpsi 06-05-2014 12:05 PM

ok so pretty much any hardwood plywood like oak, maple or birch. Just nothing like sheathing.

jadnashua 06-05-2014 01:33 PM

No, you do not want a hardwood ply, either (although a good high ply birch plywood can be used)!

You want something that says exposure I or exterior rated glue, underlayment, with the worst side at least grade C (or A or B)...IOW, no D faces.

If you want to know more about this, go to http://www.apawood.org/level_c.cfm?c...ub_ply_libmain
and near the bottom, download the file: Data File: Selection, Installation and Preparation of Plywood Underlayment. It's free, but you have to register on the website to get it.

awdpsi 06-05-2014 02:07 PM

I down loaded the pdf. Thanks. I am just trying to find something I can pickup from my local homedepot. I want to know what I am looking for before I get there because all of their employees are useless for the most part.

Their website doesnt give you alot of information that you mentioned. Something like this says it works as an underlayment, but I am not sure if that meets all the specs it should.


jadnashua 06-05-2014 02:17 PM

On the back side, it will have a stamp with the specific classification of the panel. IT should say exposure I or exterior rated glue. Other than that, the BC sides should be fine, but you probably don't need that nominal 3/4" sheet, but it will work if you want the extra strength. Since it's a small area, the cost is not that much. When a panel is designed for underlayment (that's sometimes included in the grade stamp), every ply will be solid - either with no knots, or if there were, they were plugged. IOW, all internal plies should be C grade or higher as well as the exterior ones.

awdpsi 06-05-2014 02:42 PM

Thanks for replying...I know to you guys this seems simple, but I just want to do this the best I can, so I appreciate all the help!

OK so this 23/32 in. x 4 ft. x 8 ft. BC Sanded Pine Plywood will work ok for the subfloor and with DITRA on top without using cement board? You were just saying that the 3/4" thickness might not be necessary right?

I couldn't find 5/8 at Home depot and I want to make sure I do better then 1/2" ply because I want this thing to be solid with the large tile.

thanks again for all your expertise, I REALLY appreciate it.

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