20-Year Prospect
The fact that roofing is considered a green job means prospects for the future are really good. Over the next twenty years, it's likely there'll be some changes in roofing that'll require you to up your skills a bit. New materials and the installation of solar panels are growing in popularity now. With all that solar tech, we predict a bright future for this career...hehehe.
Some things about being a roofer probably won't change, though. People won't stop needing them, for one thing. Unless robots take over the Earth ahead of schedule, human roofers are going to be in high demand for a long time. There's a reason why a roof over one's head is considered the bare minimum for comfortable living.
Another thing that won't change is the nature of the job. It's hard work and you'll be outdoors in the heat or the cold doing your job. The upside of this is roofers can rest assured that their jobs won't be outsourced or become obsolete. What will become obsolete is your gym membership. Unlike your desk jockey friends, you won't need to work out after working up on a roof all day.