What should I use as a waterproof membrane for an outdoor tile project?
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?
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