In your case, a roof poses a few issues. First off, the roof greatly increases the probability of structural failure in a windstorm, specifically the roof coming off, or taking the framing with it if it comes off. You probably don't have snow load issues unless you live in the mountains, but wind definitely needs to be accounted for.
I realize it does not rain too often, but no one ever builds a perfectly flat roof, it would not drain. There is a small, carefully controlled pitch even on a "flat" roof. I have no idea if there are building codes in your area, or if you are subject to any, but you certainly should evaluate if the framing is adequate to support a roof before you go about installing one.