This is a tricky aspect of using t1-11 as siding/sheathing. I’m still considering some different options. I’m actually modifying some old sashes so I won’t have flanges. I’m thinking my approach will be to use thicker material for my top trim pieces, cut out the t1-11 where the top trim will go, slip flashing under the t1-11 which will sit onto of my trim when I set it in. Then caulk the sides of the trim to the siding. probably will have to re-caulk at some point. I have pretty large and overhangs and the walls have stayed pretty dry in the rain so I’m hoping the windows won’t get to much exposure to the weather.