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Old 04-21-2009, 09:46 AM   #1
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Granite Floor Tiles ...Best way to lay?


I am building my family room, home theatre. The current floor is 3/4" T&G ply, laid over 2X4 sleepers on the flat side 16"O/C, over a concrete floor. The floor is very solid, stable, and clean. Is there any problem with laying a granite tile floor over this base. I assume a standard thinset is useable. I have never done granite, so I am shooting in the dark here Done lots of ceramic and porcelain floors. Any advice would be appreciated.
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Old 04-21-2009, 10:54 AM   #2
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Granite Floor Tiles ...Best way to lay?


What was done to stop the capillary moisture transfer from the concrete floor? Where all these sleepers pressure treated lumber? Was a poly vapor barrier installed first over the concrete? Thinset is what to use, but you do not use it or lay tile over plywood. You need to cover with 1/4" cement backer board first or a isolation membrane like Ditra. Since it sounds like your floor may not have been installed correctly I would strongly recommend the Ditra to provide isolation membrane and a vapor barrier.

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Old 04-21-2009, 03:18 PM   #3
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Granite Floor Tiles ...Best way to lay?


Nice try but not that answer isn't what I would have said.

I agree with the parts about moisture barriers under the original slab and over it too. But the sleepers should not be treated lumber for indoor use, unless they are "kiln-dried" PT. The sleepers would also need to be fastened in such a way that they will not ever move. (Please let me know if anyone ever figures how to do that.)

If the above is done right, then you must install a second sheet of plywood over the 3/4" ply subfloor. Natural stone tiles require at least 2 sheets of ply totaling at least 1 1/8", but the more the better of course. ( I recommend 1/2" or thicker and not CDX). Then you thinset your 1/4" CBU or Ditra membrane, and then tiles.

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Old 04-21-2009, 03:29 PM   #4
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Granite Floor Tiles ...Best way to lay?


I kinda ommitted some details. sorry. Prior to placing the sleepers, the entire floor was coated with a heavy coat of rubberized mastic. The cracks were repaired with a prime coat of rubberized mastic prior to the main coat. A second coat of mastic was applied and a layer of 8 mil poly embedded in it. The premium grade PT 2X4s sleepers were coated with mastic on the floor side and ramset into place. A final coat of mastic was applied over the poly, and 1 1/2" durofoam insulation(one side aluminium foil/the other side plastic) was set between the sleepers. Any gaps between the foam sheets and the sleepers were filled with gap filler foam and topped with plastic sheathing tape. My floor prep was done 2 years ago. I had to wait for the money to put down the Black Galaxy granite. I finished my sons basement bedroom the same way, the floor received a finish of Asian Cherry hardwood, and I have had no problems with cupping or checking due to moisture. We do monitor and control the humidity in the basement, and keep it at 15 to 20%. The house is 3 years old and the basement is dry.
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Old 04-21-2009, 10:30 PM   #5
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Granite Floor Tiles ...Best way to lay?


15-20% wow, where are you? O<. we'll assume that part is fine? No way to know from here anyway. I hope you did not create a "moisture sandwich". Sounds like you investigated every step.

But, you still need another layer of underlayment over that subfloor and then backer or Ditra or something appropriate.

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Old 04-22-2009, 11:49 AM   #6
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Granite Floor Tiles ...Best way to lay?


Thanks guys, I am fairly confident the subfloor wont rot or shift, its been down for 2 years, walked on and seems solid. I will lay another 3/8 layer of ply over the 3/4", but not sure about "Ditra", its purpose, or if there is something else that can be used in its stead. I have always done tile over the plywood base with NO membrane. Question is, what is the objective of the ditra, where is it applied? In my last house, the builder used a cement screed to level and prepare the floor, then applied the granite with thinset( I believe thats what he used its 24 years ago, and I was not paying a lot of attention at the time). This is my first time DIY setting granite, so it seems like rocket science to a dumb sparky like me. lol

BTW we are in the city of Niagara Falls which is typically like a rain forest, located between 2 large fresh water lakes. My house is all hardwood and porcelain flooring. Consequently we need to be very mindful of the humidity levels.

Last edited by Blckbird; 04-22-2009 at 11:55 AM. Reason: Added comment
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Old 04-22-2009, 11:54 AM   #7
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Granite Floor Tiles ...Best way to lay?


Ditra 'uncouples' the solid tile+thinset assembly from the subfloor assembly, as they expand and contract at different rates - thereby creating stresses and eventually cracks.

Does other things too but was unheard of 15 years ago. Now it's a standard all unto itself...it's basically a plastic membrane. $2.50/sq ft approx.

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