Most roofs come with a life expectancy - people say "this is a 30 year roof or a 20 year roof". That means you can expect the roof to start leaking by the time it hits that timeframe.
There are things that can cut the life of your roof, like having branches from trees lying on the roof (I don't think you should cut down the tree because the branches are touching it, just keep them trimmed back). Fungus can attack the roof (this is what makes the roof look stained). This staining can be cleaned off (it's not that expensive) and it extends the life of your roof. Storms can also cause the roof to fail before it comes to the end of its useful life. I see lots of roofs with missing shingles that have blown off in a storm and hail is hard on a roof.
When it is time to replace the roof, there are many options. Asphalt shingles are the most common here in Hoover, Alabama. Roofs can also be made of cedar shake, slate (which was common in Louisiana years ago) and metal.
If you want to "go green", consider a metal roof. They are long lasting, come in multiple colors and styles, help to deflect heat in the summer, are often made of recycled materials, and are recyclable. Asphalt shingles are short-lived, made of oil (here we go with petroleum again), are rarely recycled and give off toxic VOCs. If you have to go with asphalt shingles, make sure you choose a 50 year asphalt shingle that contains recycled materials.
People are wondering what they can do to help with global warming. We all have heard about the light bulbs, but every decision we make when we go to the store should be filtered through the question, is the best choice for the environment. A metal roof may be more expensive initially, but in the long run, it will save you money and help the environment.
Here in Hoover, Alabama there are not many Homes for Sale with metal roofs. Perhaps in the years to come, this will change.