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Remodeling Flooring Bathrooms

Non-modified thin-set or Polymer-modified thin-set on OSB?

3 years ago

last modified: 3 years ago

What type of thin-set should I use between OSB subloor and 1/4" Durock cement board?

Manufacturers' data sheets for both types of thin-set mortar explicitly state that neither should be applied directly to an OSB subfloor.

1. So what type of thin-set mortar do I use?

2. If neither thin-set mortar will bond to OSB, what is the purpose of using it rather than solely screwing the cement board to the OSB?

3. Do I absolutely need thin-set mortar between OSB subfloor and 1/4" cement board?

Note:

1. I intend(ed) to apply thin-set mortar directly to OSB, then install 1/4" cement board directly atop with screws then tape and trowel joints (using thin-set mortar), then apply 1x coat of RedGard to cement board, then apply polymer-modified thin-set atop cement board, and last install porcelain floor tiles.

2. I would prefer not to eliminate cement board from the mix (i.e. 2x coats of RedGard applied to OSB, polymer-modified thin-set mortar applied atop RedGard, then Porcelain tiles applied atop polymer-modified thin-set mortar) provided this bathroom sits directly above my living room and dining room, both of which have hardwood floors. In other words, I want the extra layer of protection from a moisture/leak perspective.

Featured Answer

Aglitter

3 years ago

I would recommend looking into the Schulter systems for waterproofing. They have everything you would need within the whole system of products. We just used it on our master bathroom remodel and have been extremely pleased with the outcome, in use for over a year now.

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Comments (7)

PRO
Avanti Tile & Stone / Stonetech
3 years ago
I second that. For putting down Durock on OSB, you can use the cheapest morter available....try "Customblend" from Homer's. All that is there to do is fill voids and hold the Durock "up." The screws hold it down. If you are tiling over this, then a good modified morter is required. I guess you could use the redguard on top of this but, again, I have no idea of your project.....
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NoviceDIYer
Original Author
3 years ago
@Silverlined Did you all use Kerdi board on shower walls (to eliminate the need for cement board) or kerdi waterproofing membrane over drywall? I ask provided I’m a bit iffy at the thought of including drywall in the mix. The Kerdi board itself is pretty expensive provided I cannot find any business that sells single sheets at a reasonable price, and although both their shower kit and bulk pricing seem more reasonable, the sheet counts/square footage are over twice what I require.
mainenell
3 years ago
I used cement board and red guard.
Aglitter
3 years ago
last modified: 3 years ago
Yes, we used Kerdi board, the membrane, and the pre-molded forms and corner blocks. We hired a contractor who had taken Schluter training and was experienced in the application, so they provided the material. Do you have a nearby ProSource? They can get you everything you need to do it yourself if you want. Cement board and RedGuard like the previous poster mentioned is a viable alternative. This was our 2 time to gut that same bathroom to the studs with leakage issues from cement board and plastic liner installations, so we took no chances on doing it right this time.
NoviceDIYer
Original Author
3 years ago
@mainenell I’m thinking I’ll likely do the same given my project is a hall bath and pricing/sell of Schluter’s products seems geared towards bulk buys. Although, I may use Ditra for the floor.
NoviceDIYer
Original Author
3 years ago
@Silverlined I’ve not heard of ProSource but will check. Thanks for the tip.

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