What tile should I use outdoors?

Both Frost Resistance and Coefficient of Friction are criteria that must be considered when selecting tile that will be installed horizontally outdoors in freeze-thaw areas or in areas with similar environments such as walk-in freezers.    

Frost resistance is determined by the water absorption rating of a tile.  Porcelain tile is impervious and must have < 0.5% water absorption.  This is the most frost resistant body and, thus, resistant to the rigors presented by freeze thaw conditions. 

The second factor to include when selecting a tile for outdoor usage is Coefficient of Friction, i.e. how slip-resistant is the tile?  The ADA (Americans with Disabilities Act) recommendation for slip-resistant accessible routes is a WET COF > 0.6 (> 0.8 for ramps). 

So, choose Porcelain Tile with a Wet Coefficient of Friction > 0.6 and verify that any trims selected meet the same criteria.

These combined guidelines are critical when selecting tile for exterior horizontal installations.  For vertical exterior installations, Frost Resistance is the primary factor.