Is slate tile suitable for exterior flooring in a colder climate?
Is this a pressure treated deck on post and beams?  Or on top of a foundation? 

What ever you do, forget the waferboard and plywood.  Use only pressure treated plywood.

Over time water will find its way under the tile and freeze and crack or separate the tiles. How long it lasts has to do with how well the tile and grout is sealed and how many voids you have under the tiles.  

I did find a new solution.  I haven't used it but it would be the only way i would install tile on a deck.  It is a high strength fiberglass grate that installs on top of the joists. The tile is then glued on the grate. The grout lines are left open for water drainage.

http://www.gratedex.com/pg_moredetails/pg_moredetails.htm