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