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MeasureTwiceCutOnce 09-15-2008 07:39 AM

Best way to ensure flat floor between two cement board slabs?
We decided to tile a small powder room ourselves, and hurried to mortar down the cement board without adequately filling some low spots; plus the cementboard buckled when we screwed it down (the cementboard screws strip 1 out of 3 times.) the real flatness problem was occurring where two boards met, and we've gotten advice that we can now make up the difference with the mortar. (but aren't sure if they mean a layer of mortar, dried, and then proceed, or just lots of extra mortar as we tile.)

The difference is as much as an 1/8" in spots. My boyfriend tried using levelquik to make up some of the difference, but had never used it before and it dried pretty quickly. it helped reduce the problem in the primary spot (where it was closer to 1/4" variation originally.) but at the one edge close to the radiator, he didn't feather it so there is about an 1/8" drop off. I sanded it down a bit with a mason's stone so as not to put too much dust in the house. (and then he went over it with a belt sander, and now the difference is maybe 1/16"). needless to say, neither of us wants to create a bigger mess and we disagree about how to proceed. we're laying 12x12 porcelain tiles over 1/4 CMB over 2" wood floor over subfloor. it's pretty solid, but we have another minor complication - a baseboard radiator around three walls. we keep getting more advice - just build up the difference with mortar, try dependable or ardex as they're easier to work with...(hire a professional ;-) Help!:(

JazMan 09-15-2008 10:34 AM

Sorry you're having all these problems.

Maybe you should hire a professional? If you can't even install the backer board correctly, or make the floor flat, you're doomed and shouldn't continue until you correct these issues.:yes:

Believe of not, it is not a crime if you're not handy around the house.:no:


Bud Cline 09-15-2008 10:36 AM

Sorry I didn't respond to your PM. I get so many of those things that to respond there and here both would consume way too much time. I like to keep it public for all to benefit.

The Level Quick is a self leveling compound not intended to be used with a trowel. You used the Level Quick RS I'll bet. "RS" means "rapid set".:)

You can now fill the low spots with thinset. Let it dry then proceed with your tile. No harm no foul.:)

When installing CBU (not sure what CMB is) you can perfect the plane after the board is installed. CBU MUST be installed in a fresh bed of thinset however.

Can't advise you on the baseboard heat. Sometimes those things are an issue and in some cases can keep one from installing ceramic tile in an area that has them.:(

MeasureTwiceCutOnce 09-15-2008 02:39 PM

Good advice
Thanks, Bud, appreciate the posting. :thumbsup:
We're not in such bad shape after all. The floor is actually pretty flat except in the one spot where the two boards met, and I think we'll be fine. My bigger concern with the radiators is that there is very little clearance, and if I had to feather towards that edge now that the levelquik was a bit high that I wouldn't have enough clearance to get the tile under it. As it is now, I think we'll have at least 1/4" of air space once the tile is set. I think it will be okay.

At least your posting was on the positive side. It wouldn't be much of a do-it-yourself network if I had to run and hire a professional for making one mistake with cementboard...I'd rather have to rip it out and start over than never have tried. We've rehabbed two houses - just never worked with cement board on floors before.

:thumbup: And for the record, the grout is cracking in the professional tile job in our other bathroom after only one year. Fixing that is my next project.


Bud Cline 09-15-2008 04:40 PM

In defense of JazMan.....many times people come to a DIY forum asking for assistance and quite frankly they shouldn't be doing anything beyond tieing their shoes each day. We see a lot of it. We have been at this (forums) for around ten years now and we can sometimes offer the best advice there is for some potential DIY'ers and that is for them to hire a pro.

Some cement boards have tapered edges while others do not so tapers have to be dealt with from time to time. The tapers cause dips otherwise. Cement boards when installed will follow any imperfections in a substrate, they are not floor levellers and they are not cure-alls. Once the cement board is installed THAT is the time to plane the floor, not before.

The cracking grout you talk about done by the pro is going to be movement in the substrate 9 times out of 10. So that's your next task...find out why that floor is moving. Re-grouting won't change a thing.:)

MeasureTwiceCutOnce 09-16-2008 07:48 AM

Sorry if I was touchy. I am a big fan of professionals and hire them regularly. And for the record, if I could have, we'd have hired someone to do this job...We just bought this house in March, and hired professionals to refinish our hardwood floors, rewire it completely, move a wall and prepare for our new kitchen cabinets, even though many of those things we have done very successfully ourselves in other houses...but money only goes so far, and we needed to get this done ourselves. Thanks again for your help.

MeasureTwiceCutOnce 09-16-2008 07:51 AM

and the other bathroom floor is laid over this orange waffle stuff (no cement board) the name of which i can't's supposed to be used to create a "floating floor" sort of once the tile is mortared to it. should have just gone with cement board. i wouldn't say the stuff is bad, but i'm not sure the contractor had much experience with it.

Bud Cline 09-16-2008 05:18 PM

It's DITRA from Schluter Systems and if installed properly it's about the best thing going today. It's not a floating flloor but instead it is more of an isolation product.

Earlier this year I inspected a failed ceramic tile installation over DITR and found that the DITRA had not bee installed properly. The necessary thinset was non-existant in some places and sparce in others. The waffles had not been completely filled either, and there was evidence that the waffles had bgeen skimmed the previous day.:)

MeasureTwiceCutOnce 09-16-2008 09:25 PM

Yes, DITRA. I read great things about it and suggested we try it, but since he hadn't used it before, that was probably a mistake. There are two spots where the grout has cracked, and they are hairline - almost invisible against the edge of the tile. But I can feel the tiles move a bit under foot. One of the tiles near the tub makes a clicking noise when I step on it, but no cracks - either the tile or the grout. Not really sure what to do about it except regrout. Suggestions? Do you use it? Do you prefer it to cement board?

JazMan 09-16-2008 10:49 PM

Hi Jaz again, :thumbsup:

Bud uses Ditra, I use Ditra, thousands of tile contractors use Ditra, It's a premium method. As he said it is the best thing going for most situations. Ditra has been on the market for over 20 years, with millions of sq. ft. installed. You can't beat it. :thumbsup:

There is no possibility that the failure you have is because you used Ditra. Everything else being equal, I bet you;d have more failures without it.

I would say the problem is improper or no subfloor prep, or it was not done by a 'real' tile setter that knew what he was doing, I guess you already said that.


MeasureTwiceCutOnce 09-16-2008 11:20 PM

Bud, what do you mean by waffles being completely the thinset would fill the waffles? I didn't watch the contractor put it down so it's hard to say what the problem is for sure. I had read a lot about it at the time, but don't remember much about it now.

If the substrate is in good shape, would you recommend DITRA for a DIY? We have to redo the floor in another bathroom - it is currently honeycomb tile over concrete. There are two settlement cracks that must be repaired first and we were planning to lay new tile (not sure what kind yet) right over that. Assuming that the floor is truly flat:whistling2: would you agree?

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