My concern is that the floor has failed. It's not a sound job that's a simple re-tile. You've already addressed deflection issues, but adding a beam addresses deflection of the joists. The other concern is deflection of the plywood between the joists.

The standard there is 1-1/8" of ply under cement board, or Ditra with properly spec'd subfloor under it.

But I'd wonder about the installation of the cement board. Was it thinsetted down? If not, it could be vertical micro-movement of the cement board that has contributed to the failure of the floor. There's no sense in adding good over bad, so dig before you proceed is the best advice I can offer.

If you can't vouch for the competency of the original installation...which has failed...you can't, nor should you want to, depend on it to carry your new work.

Merry Holidays!
Mongo

Ditra might solve one of several problems. That is not enough help, to be sure you will have a good floor afterwards.

The cement board is thinsetted down (DH replaced a couple cracked tiles last summer, so got a good look). The joists are 16" OC--this is the first I've read that 1 1/8" plywood is required subfloor for ceramic tile. We've tiled multiple rooms in our house that have just the standard builder's 3/4" OSB that we put Ditra over. Adding that much height would definitely be a problem for the customer.

What would you recommend doing?

I wrote "The standard there is 1-1/8" of ply under cement board, or Ditra with properly spec'd subfloor under it. "

So that is 1-1/8" of plywood under cement board, OR if Ditra is used, then you just need Ditra with a properly spec'd subfloor.

As you know, a properly spec'd subfloor for Ditra can indeed be 3/4" OSB. So you're good there.

Sorry for the confusion!

The joists are 16" OC but there are still other numbers to know about the joists. Length of joists, unsupported. Cross section dimensions of joists. For example. E.g.

If you haven't been able to find out or even ask yet, then I add that factor to all the other factors influencing my vote too. The first factor mentioned above is the possibility "that the floor has failed" in terms of having too much spring in one way or another. Another factor is that you are learning as you go, too little too late. There are several variables and having a good grasp or understanding of most of them won't cut the mustard. Learning from an internet person is not the way to go. Risk is high. Something can be overlooked. Or only partially understood. My opinion. Hire mongoct for some ten-minute conference calls made when you are on site and have the run of the place.

Oops, mongo, didn't read your comment closely enough. Thanks for the clarification.

Davidro, joists are 2x10, span was 22' (what were they thinking?), now half that.

Learning from an internet person is the way to go when the internet person is mongo or one of his ilk. :-)

So given the above info (thinset cement board, 3/4" OSB, correct(ed) deflection), should it be ok to just Ditra over it?

If we pull out the tile and find they didn't screw it every 8" (the only thing not observable with previous tile repair), we'd probably go ahead and remove the board. But I'd like to avoid the painstaking hours of pulling out screws if it's advisable.

YOu can't tell if thinset was used under the backer board until that backer board is removed. We're not talking about between the board and the tile. That's a given for any tile installation. We're talking about between the subfloor and the cement board. You've got to thinset that also if the installation is to be correct.

There is absolutely ZERO way I'd ever take on this project without ripping out the entire bad installation and starting over from scratch. Especially since you are considering doing this for money. If you are acting as a pro, it's incumbent on you to do the best job you can correctly for that pay.

Also, you need to check with your local municipality about what type of contractor's license and insurance you're going to need. You really don't want to be in someone's home without those items protecting both you and them.

One of the tiles DH replaced had been replaced before (down to the OSB), so he cleared out down to the subfloor and could see the neighboring layers.

Believe me, we want to do this right. That's why I'm asking. Mr. Weedy works by the hour, so the "more work" will cost them more money. We want to limit their costs, but don't want to short them on quality as a result. Heck, I'd feel the same about my own floor: NO desire to shortcut and have to rip it out and do it again.

Mr. Weedy's a self-employed handyman & auto mechanic that works by word of mouth for friends and friends of friends. He just pulls me in for carpentry and tiling projects, as that's more my bailiwick than his. Our county does not license handymen or contractors, and he has insurance.

Pull the cement board. From what you've said, good chance that it was the joist span that killed it the first time around, But I'd hate to be in your husband's shoes, and find out that there was more to it...... the hard way. it's cheap insurance to redo the cement board, or, if the subfloor's 3/4", just go right over it with the Ditra. I'd only use the cement board if you need the height. It adds practically nothing for strength.

This is true. Joists are small here. In terms of the strength and stiffness that this particular floor needs, cement board adds nothing.

Have you posted somewhere what the room dimensions are? Is it a puny bathroom or a big open bathroom? If it's a big bathroom, people need to know more. If it's a small bathroom, it could be that one of its walls is in a place that will press down in the middle of the unsupported portion of joist span. Do tell.

Davidro, joists are 2x10, span was 22' (what were they thinking?), now half that.

That's more than sufficient, so long as they're set 16" OC.

Okaaaay, so back to my original question, which was this: given that the floor has cement board over a (now) sufficiently supported floor, can we just put Ditra and then tile over the cement board, or do we need to tear out the cement board?

i re read the thread. Sounds good to me. You already fixed the deflection problem totally. Or, I'll say "totally" but I haven't seen the beam and I don't know how you supported it... YOu keep asking about Ditra, which is for slab cracks not for springy floors. Actually, the word 'for' means "to work against" the problem named. Remind me why you want Ditra. Is it because you just want to feel good about buying it and installing it? It won't hurt but it's not the solution. That is what every post above said either explicitly or implicitly. So, I repeat, if you fixed the springiness which was the problem, why do you now want to add a layer of Ditra too? And please tell me why internet people all seem to want this product and not all the hundreds of other products out there. Why are you focusing on this product?

Davidro,

Remember, they addressed the springiness of the joists by adding a support beam. But they still need to address the springiness of the subfloor between the joists, since all there is right now is a single layer of 3/4" material covered with cement board. That's not sufficient to tile over.

To a certain extent Ditra can work for "springiness".

You can tile on Ditra that is installed over a single layer of 3/4" subfloor.

If using just cement board, you'd need a minimum of 1-1/8" of ply, then cement board, then tile.

The Ditra acts as a shock absorber of sorts to not transfer minor floor movement of the thinner 3/4" subfloor to the tile. Cement board translates all movement, thus the need for a stronger and less flexible, and thus thicker, subfloor/underlayment system beneath the tile over just cement board.

There are other products out there beside Ditra, but Ditra is readily available and easy to use DIY, there is good tech support for pros and DIYers from Schluter, they have a good distribution chain making it available in supply houses as well as box stores, and Schluter offers a full product line that can cross over from one application to the next. And the product performs as advertised. All that contributes to why I use it myself and why I mention it in my posts.

Davidro, if you have another product besides Ditra to recommend, then by all means recommend it. I'm always willing to pick up new ideas.

Weedy,

Going back a bit, if you can validate that the floor joist size, span, and on-center spacing, as well as the subfloor thickness all meet Ditra specs, that the subfloor is in good shape (not punky) and that the cement board is properly installed over the subfloor, then sure, you can Ditra right over the cement board.

Best, Mongo

If using just cement board, you'd need a minimum of 1-1/8" of ply, then cement board, then tile.

Ummmmm-- Davidro's right. Over 3/4" subfloor, you can use 1/4" cement board. Over 5/8" subfloor you need to boost it to 1/2" cement board, but you CAN use cement board over less than the 1 1/8". That spec is for going directly over double layer plywood. :-)

Hey Bill, Happy winter to you!

I'm not a fan of 1/4" cement board over 3/4" ply. I'll do Ditra over 3/4" ply, but not 1/4" CB.

I prefer the added stiffness of a second layer of plywood under 1/4" CBU.

Or just tile right on two layers of ply with something good and sticky like Kera/Kera, but that's heresy these days!

Thanks all for sticking with this thread. Here's my plan:
-Demo the tile
-Examine the cement board job. If it's all intact, screwed properly, and not destroyed from the tile chopping up, we'll leave it, Ditra over it, and tile.
-If it looks like the prior deflection damaged the bond with the floor (cracked? bent? loose?), if it's not firmly screwed down every 8", if it's "punky" (?), or if it's not otherwise firmly and obviously wedded eternally to the OSB subfloor, we'll yank it out.

Davidro: I hadn't considered just leaving the cement board and putting tile back on it. I guess I assumed that the movement of the original floor had compromised its integrity. So my thought was to either replace it or add a new underlayment over top.

I use Ditra because I tiled two bathrooms with cement board before I discovered Ditra and fell in love with its ease of installation and sexy waffles. ;-) Hundreds of square feet of tile later, I'll never go back. I'd also welcome enlightenment as to the hundreds of products out there that might be even better.

Mongo-- I agree-- I'd also prefer the stiffness of the second layer. But I'm so used to doing this on just about every new construction home I go into, so that the tile will come flush to the hardwood, it's now second nature, and I've never had a problem with it. If I had, I know the Durock rep. I know he would've backed me up, so long as everything else was right. :-)

Weedy-- unless there's no bond between the tile and thinset, I doubt very much you'll get the existing tile up without destroying the cement board.