I am getting ready to start an exterior tiling project and could use some advice. I have a 8'x20' exterior covered patio that I am planning on installing porcelain tile over. The patio is covered on 3 sides and unless it rains and wind blows hard which it did last night, not much of any water gets on patio. I was planning on simply installing 1/4" concrete tile backer board to 3/4" osb subfloor after putting down thinset between osb and tile backer board, then installing tile over backer board then installing epoxy grout. Friend of mine thought i should install a coat of redguard to protect subfloor just in case. Any thoughts or suggestions would be much appreciated. We are in north central washington state so we do get freezing and snow.