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

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

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