You may want to conside using this or a similar product.

http://www.schluter.com/6_1_ditra.aspx
"MiamiCuse" wrote in message
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wrote in message
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On Sun, 17 Feb 2008 11:08:30 -0500, "MiamiCuse"
wrote:


wrote in message
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What kind of surface are you installing
over this?

Ceramic tiles, a preslope for shower pan, more mortar on top of it and
more
tiles. Thanks.


Typically they will lay tile over a membrane down here. Concrete will
crack and transfer the crack through the tile otherwise.

I have not seen membrane under tiles in the places I lived and visited.
Seems they just apply thinset and tile on. Wood floors or laminated floor
a moisture barrier is common. There is a layer of moisture barrier on the
underside of the slab which I torn up when digging up the trench. I will
need to fix that back in. Thanks.