Hi Erich
What are you setting the tile on? How much is it sloped?
There are many products available for exterior installations, and we can discuss those if need be.
Versa bond is a thinset that has powdered polymers already in the mix. There has been much debate on this forum and in other places about whether or not the thinsets mixed with latex liquids outperform the polymer modified thinsets.
I just looked at my Custom Building Products notebook, and their polymer modified thinset that pre-dated Versabond said it can be used for exterior applications.
Before getting started, it may be best to call Custom's tech people for their advice. I have used lots of Versabond, but always for interior work.
http://www.custombuildingproducts.com
Please let us know what they say.