There are a lot of factors involved in a flat roof. Most important are how long do you want it to last? How many penetrating items? Will it see any foot traffic? What are the repercussions if it would leak (a leak over your back porch is a nuisance, a leak over top of a bookstore is a disaster)? Finally, what is the budget going to allow? There are a lot of guys in pick-up trucks that will put down an .045 EPDM membrane and call it a roof. It will have crumby details around penetrations and edges and it will leak in a few years or less. By then they will have gone into business under a new name, so forget any warranty. The opposite is a professional roofer who has put down 1000s of squares of a variety of flat roof types and can evaluate what you need vs what you have to spend.