RBnDFW wrote:
I am finishing a bathroom counter.
It's approx 75"x25" and will have two sink bowls

Right now I have the top made from 3/4" plywood. It is not marine
grade plywood BTW. The tiles are cut and ready to lay.
My original plan was to use structural adhesive to attach the tiles
to the plywood and then grout.
But I bet I need a waterproof substrate - backer board, cement
board, Hardiboard? Problem is I don't want to make the top any
thicker than it already is, though 1/8" or so would be fine.
Any suggestions welcome on materials and method. I only want to do
this once.

1/4" backer board would form a more a stable layer to attach the tiles,
eliminating the risk of wet plywood expansion causing cracks. Neither Grout nor
most backer board are completely water proof.

I have been very happy with my tiled bathroom counter, with none of the problems
brought up by Pete. I do wipe up spills and water immediately with a towel to
significantly reduce maintainence, and used large tiles with flat surfaces.