New Tile Floor w/CBU underlayment

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Old 11-02-06, 08:19 AM
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New Tile Floor w/CBU underlayment

I need some clarification/advice for a new tile floor install. I"ve got a new sunroom and I want to do a tile floor. I"m planning on putting down 1/2 inch cement board (durock) as an underlayment. This will go on the wood subfloor. Questions: should I just lay the cement board on the subfloor and screw or nail it down, then thinset/tile on top of that? I've read that some prefer to use thinset under the cement board as well as under the tile. Could I get away with just laying the cement board down without fastening it since it's so heavy? Also, should I do anything with the seems of the cement board before laying the thinset and tile?

Thanks for the help
 
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Old 11-02-06, 12:10 PM
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Thinset is an absolute must under cbu in any tile installation. Anyone who says otherwise has no understanding of a ceramic floor assembly and their advice should be ignored. There are also minimum requirements for the structure under the cbu. Plywood must be exterior rated and t-g or blocked under all seams where the ply crosses the joists, not cdx, it must be exterior rated, properly glued and fastened to the joists, at least 5/8-3/4" in thickness, depending upon the minimums as set forth by the manufacturer, and so on. The supporting structure must also meet deflection requirements of l/360 for ceramic and l/720 for stone. What specificly do you have for a subfloor and what is your joist size and unsupported span and oc spacing?
 
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Old 11-03-06, 07:04 AM
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Thanks for the response. My existing subfloor is exterior grade T & G, glued and fastened properly. Joist spacing is 16 oc so I think the subfloor is proper. I'm ready to lay the CBU and it sounds like I definitly need to use thinset under the board which I will do. I will also stagger the seems so they do not meet and run along the seems of the subfloor. I also plan to tape and mud each cbu seem. Do I need to leave an expansion gap where the cbu meets the wall??
 
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