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Do you have to put thinset between subfloor and cement board?

Do you have to put thinset between subfloor and cement board?


Old 11-03-08, 05:21 PM
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Join Date: Aug 2005
Location: United States
Posts: 94
Do you have to put thinset between subfloor and cement board?

I have been Googling and about half of the sites I find say you can attach the CB directly to the subfloor, and the other half say to use a layer of thinset between the subfloor and the CB. WHich is it? And why will a level of thinset help? Lastly, is there a certain trowel size I should use for this layer of thinset? Thanks!
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Old 11-03-08, 05:28 PM
Gunguy45's Avatar
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Join Date: Dec 2007
Location: USA
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Well..as they say, you CAN do anything. What the manufacturer of the CBU recommends/requires is what you need to go by.

I've never seen any of the Pro's on here say you can skip the thinset underneath.

Wall's of course don't get thinset, but they don't have 250 lbs walking on them.
Old 11-04-08, 03:37 AM
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Join Date: Mar 2005
Location: USA
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The thinset helps to insure that the CB sets firmly on the floor with no "air" pockets that can cause movement and potential issues with the finished tile floor.
Old 11-04-08, 03:01 PM
Join Date: Oct 2008
Posts: 82
Sounds like quite a messy pain in the arse. Ive never had a floor with 3/4 ply and steel mesh come apart untill i destroyed it. Why do you want your floor to last forever? Dont you think in 10-30 years you may want to change it? Just my 2 cents.
Old 11-05-08, 01:42 PM
Join Date: Jan 2006
Location: USA
Posts: 3,489
Every manufacturer of cement board requires that their cbu be set in a fresh bed of thinset and mechanically fastened to the subfloor with screws or nails. Thinset is not optional,it is a requirement, and to leave out this step is to be asking for failure. What you should be doing, is reading the installation instructions for the cement board you will be using. You can find it on the manufacturers website.

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