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.  To be recognized as porcelain, a tile must be impervious with < 0.5% water absorption.  This results in a frost resistant body capable of withstanding the rigors presented by freeze-thaw conditions. 

The second factor to note 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, for freeze-thaw areas, choose Porcelain tile with a Wet Coefficient of Friction>0.6and 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.