Hey, I'm in Massachusetts, too! Definitely build it with a substantial pitch especially if you're planning on the polycarb panels. Remember the back to back to back blizzards a few years ago? Snowload can put substantial stress on a roof and we often get wet snow that sticks. Having a decent pitch to the roof helps prevent the snow from accumulating.
Personally, I used metal panels over a plywood roof with 2x4 rafters for the roof of my coop and run.
Personally, I used metal panels over a plywood roof with 2x4 rafters for the roof of my coop and run.