Tin is good, or plastic corrugated roofing. Plastic is probably cheapest but if you live in colder regions there's a chance of cracking. There is also another type of corrugated roofing its thicker than tin and plastic and won't rust or crack, its Corrugated Cellulose Fiber/Asphalt Roof Panel...
Seems like you know what you want. I would recommend taking a little more time to make blueprints if you are going to make you'r own coop. When we got our chickens we ordered ours online (not recommended) They are good coops but don't seem to last much longer than a year. We then converted our...