I'm in the process of building a walk in coop. I have the walls, buts the best roof. I'm thinking on putting a slant roof with plastic or metal roofing for a quick job.
I agree, if you're in NJ, your weather is much like ours. Lots of snow and wind! Make it strong, insulate and use metal vents. Check every crevice for drafts and especially for predator protection. And have fun...this is gonna take three times longer than you think! (And it's worth it).