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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?


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.

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