It is important to understand the dangers in hiring an inexperienced installer when putting tile in an exterior application. Outside, your tile is constantly exposed to rain water which ultimately is the enemy that must be dealt with before it wreaks havoc. In areas susceptible to water, not only does the tile installation need to involve a drain matt, but everything has to be installed in the proper order as well.
First, you must make sure that the areas are pitched properly for the water to run away from the area that is tiled. Secondly, the drain matt must be installed on top of that first layer of setting material and not below it. Lastly, the tile is then adhered to the drain matt with a thin layer of thinset. When rain occcurs, the water goes through the tile or grout joints, penetrates the thin layer of thinset, hits the drain matt and runs off the area that is tiled, keeping water and moisture away.
A common mistake with an inexperienced tile setter is they either disregard the drain matt entirely or install the drain below all the setting materials. In either of these cases, the rain goes through the tile or grout joints and is unable to run off the tiled area and saturates the setting bed. At that point, when evaporation occurs, there is nowhere for the water to go but up and through the tile and grout joints which is called "spalling". This occurrence will literally destroy your tile and appear as if the tile is disintegrating or defective. This is a textbook case where there is nothing wrong with the tile and everything wrong with the installation. Beware of who you hire when it comes to tile installations. It is a complex process and if done wrong can be very costly to you.