So first off studying Laticrete's PDF files is that tiling over a single layer of plywood is a No-No.
The total plywood thickness must be a minimum of 1 1/4" (32mm) thick. To achieve this a double layer of 5/8" or some 3/4" topped with 1/2" is a start.
We never like using plywood thinner than 1/2" and in order for these floors to be acceptable the bottom layer needs to be tounge and grove. The TTMAC requires that all plywood be at least 5/8" thick so 1/2" plywood is out of the question.
Here is a link to Laticrete's recommendations for tiling directly over EGP - http://www.laticrete.com/distributor...face_prep.aspx
They have four drawings at the above link showing some great cross sections;
Bonding Ceramic Tile, Stone or Brick Over Wood Floors TDS 152
LATICRETE Drawing ES-F143
LATICRETE Drawing ES-F150
LATICRETE 254 Platinum DS 677.0
Never use a non-modified thin-set to bond to plywood. The stickier the better and Laticrete 254 is one of our all time favourite thin-sets.