i have a corrugated roof and it works well its on a slant enough that nothing can sit on it. to keep the hawks from getting your chickens try planting like 5' by 5' patches of shrubs or prarie grass or anything that will give them cover and do this every like 15 or 20 feet in your yard so they can run into it for cover, it also gives them bugs to eat. i have found my chickens in a patch of prarie grass my mom has when a hawk swooped at them(i was able to punch it as it flew by me). another thing you may be interested in trying is fishing wire if you have a tall enough fence or trees. on my neighbors pool they run fishing wire across the top(above head height by about 2 feet.) to keep birds from landing in it and flying low across it. it seems to work and if you run enough line to keep a hawk from getting through or even enough that it had good odds of it running into the line as it swoops. its a long shot idea but wont take much work if you have a fence and a staple gun.