Ceramic Tile... Thinset on the backerboard... or a membrane?

Okay... after reading all the post on backerboard, the rights, the wrongs... I need some reassurance.
We have an 12'x16' room/add-on that was built off the back of our house (not part of the original construction), over a well ventilated dirt floor crawl space. The floor construction of this room is 1" plywood on 2x8 joist, 16" centers. My wife wants me to ceramic tile the area.
There is currently a threshold transition into the room (step down.) To make the new tile area level with the adjacent room I need to raise the project area about 2". My plan was to screw down exterior grade plywood, followed by cement backerboard (screwed and glued to the plywood 6" to 8" on center), then a waterproof membrane and finally a layer of thinset in which I will set the tile.
I've read enough posts that seem to verify... no matter how solid I make the subfloor, grout on an elevated subfloor will eventually crack. One piece of advice was to screw down the plywood but only "glue" the backerboard as a way to prevent grout cracking, however, the current plan was suggested by several tile guys that said by laying a membrane (tarpaper?) over the backerboard, the tile will float as an entire unit and be less subject to cracking.
NET: I guess I'm looking for a few more "thumbs up" before I jump on this approach. Is my plan sound or is there a better way to tackle this job? Thanks Don
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Don Task wrote:

You didn't say, 2x8 floor joists spanning 12'? I wouldn't tile over that. Now, if there's a girder support perpiedicular to and supporting those 2x8's joists at halfway (6') well then that's different. Anyway you need to determine L360 for deflection.
http://www.ttmac.com/deflection_limitations.htm

Not necessarily. But if you follow the advice you quote below it probably will.

Yes. Forget all the paper, membranes, and plywood. Make sure the floor is properly and -adequately- framed, and use a mortarbed substrate.
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On Sun, 17 Aug 2003 21:37:15 +0000, 3GCPO wrote:

Not a tile guy, but I'm with you. His way would seem to defeat the whole purpose of the backerboard.
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Okay... the other, less favorable approach (more trouble to me anyway) is to forget the whole plywood, backer board idea... and go with a 1 1/2 to 2" wire reinforced mudbed. This would certainly eliminate any flex concerns. Anyone know of any negative aspects with this approach (in this application?) Don
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Now you're talking! I knew you would figure it out.

only 1 concern, the transition to the adjoining room. I would install a marble or granite threshold.
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Don Task wrote:

Based upon your description, my advice to you yesterday was expert and sound. It hasn't changed.
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Anny is right as usual. A thicker bed will not reduce the deflection.
Bob
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