You can use Schluter products to achieve this. However, your method is a bit off. You would put down cement BEFORE the membrane. To make it truly waterproof, you need to address where the deck meets the house. Sorry for the size but here's a pic of the Schluter method. Maybe try to save the picture locally and then blow it up a bit. I can't make it any bigger and still post it.

 

Cliff,

As Angus mentioned, you've got a few things backwards.

First off....Do NOT use OSB. Use 3/4" exp. 1 plywood subfloor. Then install a cement backer that is rated for ext. use. Over that is when you apply the waterproofing membrane. Now the proper tile for exterior applications.

All the above is assuming your deck framing meets specs, AND is slopped 1/4" per ft. away from the building for drainage. Start with the framing. What type of joists, size, spacing, species, grade and unsupported span? Is the deck elevated, what is its exposure on a compass?

Jaz

 

Thanks for the help, I really appreciate it.

It is an elevated deck, about 12' off the ground. It's a 2nd story deck.
The posts are 6"x6".
The beams are 6"x10".
The joists are 2"x10", spaced 16" oc.
The unsupported span is 12 1/2'.
The overall size of the deck is about 15' X 18'.
All the framing is pressure treated, injection.
The deck exposure is West facing.
The location is Western Washington, Olympia, WA. We get a fair amount of rain year round, most in the fall, winter.
I don't know the species or grade. I think it is douglas fir, it looks good, straight and true, no large cracks or splintering.

I was told it was sturdy enough to hold a 4 person hot tub.