Simple question about underlayment for tile counter


[Hopefully a simple question with a simple reply; on Usenet, you never know]
Helping a friend tile her kitchen counter, I'm making my bill of materials. Just want to double-check one thing.
Question is: what thickness plywood should I use?
The plan is to put plywood over cabinets, then "wonderboard" (1/2" cement-fiberglass board) over that, using thinset to bond it, then tile over that. I was planning on using 1/2" plywood (counters are 24" deep), but now I'm wondering if I should use 3/4". Or would that be overkill?
And what grade? Seems to me that since it's there for support only (the cement board provides the smooth surface for tiling) that I could get away with something like CDX.
Thanks to those who can provide helpful advice.
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David Nebenzahl wrote:

When I did my tile counter, I read books and asked around. I used: 1. 3/4" exterior plywood over counter. 2. 1/2" wonder board over plywood 3. 1/4" wonderboard over 1/2 wonderborad 4. 12" granite tiles.
So the total thickness is 2", which can be covered by 2" edge tiles.
Comparing the other costs of the kitchen, the few sheets of cement boards are really nothing.
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Bald Eagle spake thus:

Just curious why two layers of wonderboard? Planning on driving a truck over the counter when not used for kitchen stuff?

Cost isn't the issue, but 2" seems excessively thick for this job.
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