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04snr 01-22-2011 08:36 PM

OSB - Cement Board & Leveling
I have ripped up the tile and removed the Schluter Ditra 1/8" underlayment, and it left the fabric adhesive that attached to the OSB plywood subfloor. Now, I am ready to prepare the subfloor for some travertine tile, but I need some guidance regarding this step as I need to level one side of the bathroom.

I had planned to use some cement self-leveler, mortar and some 1/4” hardiebacker, but I am stumped because of the OSB plywood. I would like to use the self-leveler, but it says that it is not recommended to be used on OSB. So my next plan was to install the hardiebacker board. The manufacturer recommends using mortar or thinset, then screwing down the hardiebacker board. The thin set also says that it shouldn’t be applied directly to the OSB plywood.

As you can see I am a bit puzzled. :no: I am curious what materials and sequence I should follow in preparing the floor for leveling and installation of the hardibacker. Thanks so much!

JazMan 01-23-2011 12:14 PM

Hi 04,

OK, so your subfloor (the sheet on the joists) is OSB, right? Is that the only sheet? Is the OSB the original or was it replaced? Did you have a problem with the old installation or are you just changing color?

Let's start with your selection of travertine in a bathroom. Are you sure you wanna do that? I mean it's pretty but you'll have more up keep and you'll have to do additional prep to get the floor within specs for natural stone tile.

I'll bet you didn't know that. Natural stone tiles requires a much stiffer subfloor system than regular ceramic or porcelain do. Chances are your joists will not meet specs. Plus even if your joists were improved to specs, you will need a double layer subfloor plus a concrete backer or Ditra under the tiles.

When to level the floor depends on which method you select. OSB is suitable as the sub for under Ditra or concrete backers, you just can't tile on it directly. Your second sheet of underlayment should be plywood though, leave the OSB at the store.


04snr 01-23-2011 07:47 PM


Thanks for the input.

The reason I pulled the tile was because the floor wasn't level, then I found that all the builder had done was put down the OSB subfloor and Ditra on top, then tile. The house is a couple years old and has a single original piece of ~3/4 inch OSB plywood. I wasn't very excited to see the OSB subfloor...

I understand that travertine requires more preparation (subfloor, sealing, etc.) and that deflection in the joists has to be reviewed. What is the best way to gather the joist spec's? Is there a good website to find the spec's and the thickness of a second layer to strengthen the subfloor.

So the best steps would be: 1) add a new layer of plywood (thickness?), 2) Add 1/4 CBU or Ditra 3) apply self leveler and finally 4) thinset the tile?

Thanks again for your help.

bluebird5 01-23-2011 08:38 PM

close but no cigar!! add the plywood, then self leveler, then ditra or cbu, then tile. if you use ditra use a modified thinset under the ditra and non modified to lay your tile and when you add the 2dn layer of plywood do not use long screws if you have 3/4 subfloor and are adding 1/2 inch you only want to use 1 and 1/4 inch screws bc you want and "uncoupling" factor

good luck

JazMan 01-23-2011 09:26 PM


Is there a good website to find the spec's and the thickness of a second layer to strengthen the subfloor.
Yes there is..........right here, you've already found it.:thumbsup:

Tell us the type and size of joists, species & grade if at all possible, the o.c. joist spacing and the span from face-to-face of the supports.


close but no cigar!! add the plywood, then self leveler, then ditra or cbu, then tile.
Ah......well no, as you said close but no cigar. This is the right way; Add ply, self leveler then Ditra then tiles. OR ply, concrete backer, then self leveler and then the tiles.

I agree about 1/2" underlayment grade ply, (or... the thicker the better). Fastened with 1 3/8" or 1 1/2" flooring or deck screws. I like em longer than 1 1/4" cuz the first 1/8" is a point which does not hold much.

Let us know the numbers on the joists.


bluebird5 01-23-2011 10:21 PM

yaz why couldn't the self leveler go under the cbu but its ok to go under the ditra? wouldn't it be better under to get a 100% smooth surface on top?

Thurman 01-24-2011 07:44 AM

This sounds like the flooring being used in the last 6-8 years or so in my area. It is 3/4" OSB tongue & groove for flooring. Funny thing is, it is only sold at Lowe's here, not Home Depot or the local builder supply store. This is also a darker color than "normal" 7/16" OSB, a sort of "dull army green" type color.

JazMan 01-24-2011 11:06 AM

When using Ditra all floor leveling and repairs are done first. With CBU's you should do it after the board is nailed/screwed to the floor. Fasteners into cement products doesn't work so good. Another reason to go premium with Ditra.


bluebird5 01-24-2011 05:32 PM

your right!! you must have done this before i didn't think about having to nail through the self leveler thanks yaz

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