I am finishing up a “shed remodel” . Yes you heard right. The pitch on the existing shed was less than 1-in-12. Consequently it leaked and rotted. I pulled the existing and added a 3-in-12. My question is this. when I finish my last course of shingles, (by the way, it is a shed roof) how do you keep water from running down under the shingles. Do you flash the last course so wter will stay ON TOP of the shingles.

Thanks