Tiled Kitchen Countertop

I'm making a ceramic tile countertop for the kitchen. I'm using 3/4" pressure treated plywood with 1/2" cement board applied overtop, with a skim coat of thinset between the plywood and cement board. I'm wondering about spacing of the cement board screws...I've read that 8" spacing is recommended, but at the price of the cement board screws, this seems like alot.
What about backsplash tiles...do I need to apply cement board first, or can I apply the tile directly to the existing painted dry drywall. If cement board is recommended, do I use the same spacing?
Why are special cement board screws required? Why can't you just use drywall screws?
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Water/moisture is the reason for the special screws. Regular drywall screws rust way almost immediately when exposed to water.
Yes you can apply the mastic directly to the drywall and place your back splash.
You should not be worried about a few dollar to do it right. Compared with what could happen if you do it wrong. This is probably the last counter top you will have in this home.
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