What kind of roof? how big? made of what? What part of the country are you in (frost line / depth of footer)? Are you in an earthquake area or are there provisions in your local building codes for this stuff? One of the things that proper footing/foundation should also provide is tie-down strenghth for hurricanes or high winds so everything doesn't just lift off. Roof design and construction also might need to take into consideration snow loads if you are in the north or midwest. Even if a permit is not required there are certainly many design considerations that you need to take into account. Also, if you don't go with building permit and proper design what does your homeowners insurance policy say about it if it breaks and hurts your house, property or people? Will they pay if it is an unapproved structure? I don't mean to sound like a wet blanket but I would want to know all of these things.