Flashing between wall and covered porch

Hopefully I can explain this well enough, so here it goes. My house has a covered concrete entryway that is recessed into the house. The foundation however is not recessed, i.e. I can stand in my basement and look up at the bottom of the concrete porch. So the concrete porch is in direct contact with the sill on all of the walls and imagine this (I know hard to believe wood in contact with concrete rots) but the sill is rotten on one side. I am trying to figure out how to fix this, and make sure it doesn't happen again. In a perfect world I assume that I would put some sort of flashing between the two and replace the sill with PT lumber? I don't think this is the right design in general! Thanks Jeremy
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