# Accidentally used unmodified thinset between plywood and cement board



## tb34 (Aug 16, 2021)

I am working on a project and hope I didn't screw things up. This looked like a pretty helpful group with some knowledgeable guys. See attached pic. Putting down cement board on top of 1/2" plywood that was on top of the 3/4" subfloor. I know I should have staggered the boards when installing (first mistake), but just now realized I also didn't use the latex polymer additive for the thinset between the cement boards and plywood (second BIG mistake). Joists are 2x10 and 16". Each board has about 50 screws in them and all seams are tapped and mudded. I used Mapei Floor Tile Mortar thinset. I'll be putting porcelain chevron tile on top with heated floor wire elements. Do I need to rip out and redo (uggh). Thanks in advance for the replies.


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## Pounder (Nov 28, 2020)

You know it's best to ask for advice before you start, not after you finish.

My WAG is that it will work. Though the place to actually get your question answered would be the manufacturer.


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## tb34 (Aug 16, 2021)

Pounder said:


> You know it's best to ask for advice before you start, not after you finish.
> 
> My WAG is that it will work. Though the place to actually get your question answered would be the manufacturer.


Good point. I will give them a call.


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## CarpenterRN (Dec 3, 2009)

The purpose of thinset under cbu is not to bond the boards to the floor, but to fill any gaps/depressions in the subfloor. Some guys don't even use thinset. You'll be good.


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## We Fix Houses (Aug 15, 2007)

Call Mapei tech support and get their input. Hopefully like Custom or Laticrete the folks that answer the phone have a very good understanding of what occurs in the field.

Call Custom and present the same scenario like you used their product. see what they say.

Get back to us if you get this kind of input.


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## tb34 (Aug 16, 2021)

National Gypsum said the same thing as CarpenterRN. The thinset is not necessary under the cement board and is mainly used to fill in voids of the subfloor. They said as long as I use modified mortar to install the tiles everything should be just fine. Thanks gents!


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## Snobnd (Jul 1, 2008)

I no longer use cement board for tile are use Detra!
When bonding plywood to a subfloor I always use at least an 8 x 8 box pattern but typically I will go 6 x 6, having a floor screw gun really helps the job out, good luck !


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## avenge (Sep 25, 2008)

Don't know why this wasn't posted 
This site is for professional contractors visit DIY Home Improvement Forum


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