Re: low pitch roof covering?
Before you evaluate the different flat roofing materials, you should ask yourself which one is the most common in your area. Also, consult with LOCAL roofers. You want to get a roof that is widely used, reasonably priced and with relative long life.
Modified is inexpensive, easy to roll and install, with about 10 years life expectancy. If you get this one, get the type you torch down, not the stick on kind. I have a roof like this on a section of a house, it's older than 10 and still in fair condition. I check it once a year and apply tar where needed.
Rubber is inexpensive, a bit more difficult to install, with about the same life. However, I've seen some rubber roofs with terrible flaws, holes, bubbles, etc.
I will choose modified, if it's popular in AL.