One or two layers of Hardibacker 500 under tile?

I'm redoing the shower enclosure around my tub. I'll be using new tile. One layer of Hardibacker 500 over the studs (and over thick plastic) will give me 1/2 inch of buildup and will be flush with the surrounding drywall. The quarter round that I'm going to use on the edges seems to indicate that my base should be 1/2 inch beyond the drywall. Should I use two layers, or one layer over something else, or am I just thinking completely wrong?
The tub is surrounded by three walls. The walls where the tile/concrete board will end are currently standard drywall. Is this OK or should i replace this drywall.
I have already seen the debate on floating a new base vs. concrete board and have chosen concrete board. That is unless someoone wants to come in and float a new base for about the cost and time investment I would spend on installing concrete board myself.
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I spaced my hardibacker off of the studs by using some cedar strips on top of the studs. I guess cedar is good because it's easy to get and it's rot resistant (the old drywall in the shower was spaced off the studs with cardboard). I certainly wouldn't bother with 2 layers of hardibacker.
I completely replaced all three walls with hardibacker. Obviously the back wall will all be hardi. On the other two walls I would extend the hardi at least 12" past the edge of the tub. My side walls only extended 4" past the tub, so that was easy.

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Yen wrote:

Drywall over the studs, then use the 1/2" Hardi over that. Makes a very solid surface and gives the 1/2" you need for the radius trim.
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Thanks for the replies. The drywall under does sound stronger. Should I use the green drywall or is there something better?
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An even solider (is there such a word) surface would be to use 1/2" plywood under the backerboard. This allows installation of surface mount hardware such as handrails anywhere. This is what I did in my showers, now I don't have to remember where I installed blocking. The entire wall can be used to mount things.

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Eric Tonks wrote:

Yep. That would solider. In most cases when grab bars are installed in an application where backing wasn't installed, 2-3 toggles per flange are most sufficient installed over tile board and drywall or mortar.
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Randy Calhoun wrote

I think it would be prudent to follow what the building code stipulates. now's the time to do it right.
http://home.att.net/~alt.hvac/faq.htm
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