But, I would definately screw down the cement board with stainless screws. And I would for extra measures lay down some liquid nails in "S" shape all over the floor.
I would screw down the cement board every 8 inches.
Don't worry about the mastic comming up through the cracks in the tile, as the mastic is water based and easily to clean up. I would also use water proof mastic.
Remember, you want a good stable flooring to tile to and you do not want any buckles, humps, or areas that are not affixed properly.
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