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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.

A_J 06-05-2014 03:29 PM

My local HD doesn't carry anything suitable in 5/8", but I think Lowes does, so you might try there if you have one. I ended up getting the plywood for my bathroom at a lumberyard - better quality and cheaper than the box stores.

awdpsi 06-05-2014 03:44 PM

AJ, I can go to a local lumber yard, the only issue I have is the truck issue. HD has those trucks you can rent to take your crap home...

A_J 06-05-2014 03:56 PM

What some friends did in that situation is to rent the truck at HD (just buy something else like thinset if they require a purchase), and use it to pick up the wood at the lumberyard. Last time I checked Uhaul was pretty reasonable for a half day pickup rental as well.

jadnashua 06-05-2014 04:04 PM

The 3/4" nominal (23/32") sheet should work fine. To install the Ditra, you'll need a modified (A118.11) mortar, and a premium unmodified (A118.1) to install the tile. You can buy just a premium unmodified and the modifier as a liquid in a jug, and then, depending on whether you mix it with the modifier (in the proportions they say on the instruction sheet) or just water alone, you'll get the two different types of mortar required. SOmetimes, that's easier, rather than having a half bag of each type to throw away at the end. Note, at least around here, HD does NOT sell a premium A118.1 mortar, only a really low-level, entry budget version...Lowes generally does have one, and any good tile shop should have one as well along with the modifier if you want to just buy a single type of mortar. SOme people think the liquid modifiers are better than the dry stuff already mixed in the bag, but I have no direct experience in comparing the two.

A premium dryset (unmodified) mortar has more cement, smaller sand particles, less extraneous crud, and generally, just works better than the entry level stuff (characterized by the low price - often in the $5-6/bag range). If a local shop carries DitraSet, that works quite well, but Lowes generally has a good one, too.

cx 06-05-2014 04:12 PM

Welcome, Scott. :)

The large size of your tile has nothing at all to do with the requirement for the rigidity of your subflooring.

If you elect to use plywood thicker than nominal half-inch you'll need to pre-drill all the screw holes in the plywood to make your attachment effective.

My opinion; worth price charged.

awdpsi 06-05-2014 04:36 PM


Jim great info, I am aware of the modified to the back side of the Ditra and the unmodified to mount the tile. I can run to lowes to get the thinset from there if you think its better.

CX, do you think that 1/2" think ply is good enough? Is it worth looking for 5/8? It would be better for me height wise to go to 1/2" I just figured that going a bit thicker might help additionally reduce deflection.

Anyhow, thanks again to everyone. I am going to attempt to try and get at least one piece of the CBU up tonight. Fingers crossed it comes up without an overwhelming amount of difficulty.

jadnashua 06-05-2014 05:55 PM

Having your planks on the diagonal means that they are spanning further than if they were perpendicular, so while 1/2" is generally the minimum for when they are perpendicular, a 5/8" subfloor is strong enough to support tile, and is the minimum I'd want to consider underneath my tile over them. 3/4" is also fine, but the difference may not be warranted, but it would reduce the between joist deflection, but with the planks and the 5/8", you're already above the minimum. Your call, your money. People have indicated that finding a decent sheet of 1/2" is harder than when it is thicker (the thinner stuff tends to be warped, and both cutting it to size and then screwing it down can be a pain).

awdpsi 06-08-2014 03:50 PM

We'll I just wanted to stop back and thank everyone for their advice. You guys caught me at the right time and stopped me from making a big mistake. It took a while but I pulled all the cement board up.

Now I am going to try and call a local building supply to get the correct plywood this week. Hd and lowes both don't have anything besides sheathing.

I guess Ill start tiling the shower in the mean time. Thanks again!!

awdpsi 06-10-2014 09:43 AM

So I found a place local and I'm getting BC sanded 5/8 plywood. Being this is a bathroom there is no reason to coat the plywood in Redgard before thinsetting down the DITRA correct?

I have some left from the cement board in the shower, just wanted to make sure its not worth it before I throw it out.

Thanks again!

jadnashua 06-10-2014 09:50 AM

If you want your bathroom floor to actually be waterproof, you install a band of Kerdi over the seams. The mat itself is waterproof if you seam it. So, there's no advantage of any additional waterproofing underneath it. Some people put some Kerdiband along the floor/wall transition to make sort of a bowl, then install their tile or baseboard over it. If you do something like that, you may want to use one of their expansion joints there. Not sure how long RedGard will last in the bucket once opened then resealed...you could use some around the toilet flange, but it's kind of overkill.

HomeDepotAssociate 06-10-2014 05:15 PM

Great stuff here. . Kind of piggybacking on the original posters Q's here, this would be good information for me as typically I dont engage customers on subfloor issues but a common question I get is what kind of wood subfloor to install and some of you say HD doesnt have anything that works? We are on opposite sides of the US (me and the original poster) but have any of you found acceptable underlayment for tile at HD (or lowes) for that matter? And if so what? So I can go peek around in our lumber dept. Happy to see this guys situation fixed!

jadnashua 06-10-2014 05:33 PM

To be honest, the product details shown on the HD website leave out a few critical things; they only list the ply's sides - as in say BC, and leave out the type of glue used. Now, most panels are made with either exposure I or exterior rated glue, but it is critical for use as a tile underlayment. And, technically, to be used as tile underlayment, it should also have in the grade stamp 'underlayment', meaning that all of the internal plies have either no knots or have been plugged to provide a solid) no voids, panel in all layers. And, since most people do not know how to read the grade stamp, a little more help would be useful. As you'll note, the stocked material is not the same all around the country, so what's available at one store may not be available at another - to minimize costs, they source heavier things like this as local as they can. So, in one place, you might have pine, or fir, or whatever type of wood, and, while that can matter if you're talking ultimate strength, it all comes down to the grade stamp to determine actual suitability.

HomeDepotAssociate 06-10-2014 07:01 PM

So anything grade B or C?

jadnashua 06-10-2014 08:30 PM

Sheathing grade ply can have voids in any one or more of the plies. The BC, AB, AC, CD, etc. grade stamps are only an indication of the two external surfaces. You certainly do not want any D faces on the outside, but you also don't want any of them INside, either! Ever look at the edge of a cut sheet and see a void? It doesn't always end up solid unless it is classified as underlayment grade, which is different than the external surfaces grades, and the glue used, is a separate, but important thing.

There's an article in the 'Liberry' that talks about the grade stamps and what underlayment quality ply is L335 from the American Plywood Association.

HomeDepotAssociate 06-12-2014 01:03 PM

awesome! thanks. that's really good info much appreciated

awdpsi 06-14-2014 07:27 PM

I just had another quick question. In my reading of the preparing underlayment article which I found in the liberry I read something that I should be using a screw that penetrates both subfloors every 6" but I should try my best to avoid screwing into joists.

Should I definitely try to avoid joists when screwing the 5/8 ply to my old subfloor?

I am using a 2" exterior screws, laid the ply perpendicular to joists leaving 1/8" gap on all seems and intend to use one screw every 6" avoiding joists directly. And I will be predrilling all holes as per cx's advise. Sound good?

PC7060 06-14-2014 07:36 PM

The 2" screws will be a bit long, I'd go with the 1 5/8" so you maximize the threaded portion of the shank.

The spacing of the screws is based on 25 / 75 percent of the joist spacing. So if the spacing is 16"OC, the rows screws will be at 4" and 12".

awdpsi 06-14-2014 07:45 PM

So essentially you're saying all the screw rows will be between the joists.

PC7060 06-14-2014 08:40 PM

You got it! Seems counter intuitive but works very well.

A_J 06-14-2014 09:03 PM

I believe another advantage of the 1-5/8" screws is that if you do hit a joist the screw isn't long enough to really grab it.
With diagonal planks, you won't be able to place the screws in a perfect grid. You'll miss the planks every so often. Fortunately with deck screws it's quite obvious when you do because they won't countersink.

awdpsi 06-18-2014 03:21 PM

Ok hopefully last stupid question related to this subject. The plywood sheets ( I have three of them).

When the butt up against each other should I leave 1/16" or 1/8" gap for expansion or they can butt up against each other because they are screwed?

I have left small gaps around the perimeter edge under the drywall.

Thanks again for the help!

Houston Remodeler 06-18-2014 04:00 PM

Small gaps are required by the plywood manufacturer.

Not a stupid question at all !

awdpsi 06-18-2014 08:52 PM

Well I'm glad you didn't think that question was stupid because here comes another one. I bought deckmate brand #8 1-5/8 in. Star Flat-Head Wood Deck Screws (Home Depot).

Are these good to use? I'm having so much trouble in certain areas getting them to grab effectively to the dimensional lumber subfloor. I'm sure it's because the wood is old and it's on diagonal with gaps in between most boards.

Anyhow two more questions as they relate.
1. Is it ok to leave any holes in the plywood from the screws that don't attach we'll and I remove
2. Since ditra is going over plywood is it ok if some screws are flush with the plywood or do they all have to be recessed.

Houston Remodeler 06-18-2014 08:56 PM

0- might wanna switch to ring shanked nails and a nail gun

1- the holes will be fine left alone

2- flush is fine.

awdpsi 06-18-2014 09:23 PM


I will look into the nails. Could I use those #9 durock/ wonder board corrosion protected screws? I have a bunch of those in 1-5/8. They seem to really grab better on one I tested.

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