It amazes me that people in the US would even consider installing a roof that would only last 20 years.
In the UK, you wouldn't consider reroofing anything that your grandparents remember being installed. Ie. Stuff doesn't get reroofed till it's 100 years old. Even then, you'll normally inspect and only replace the damaged bits.
My 350 year old house still has part of its original roof and slate tiles etc.