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Old 09-08-2007, 09:59 PM   #5
Veteran DIYer -- Schluterville Graduate
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Join Date: Dec 2006
Location: SE Tennessee
Posts: 8,879
Originally Posted by Steve
I am pretty sure it is 3/4 inch plywood, so that only gives me 1" with the 1/4 inch backerboard.
I'm sorry, Steve, but cement board doesn't count towards the structural height requirement of the subfloor. Cement board merely serves as a suitable substrate for the stone - it adds no structural strength.

The purpose of the thinset under the cement board is to eliminate any voids that would allow deflection, which is bad for tile - especially slate.

You're in a situation where your joists and subfloor are not up to industry standards for natural stone. It doesn't mean that your installation is guaranteed to fail, but the odds are not in your favor.
Dan - a DIYer in SE Tennessee
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