Flat roofs has different types. Mostly used ones are buit up roofs. Following are the types –
Build Up Roofing (BUR): These roofs are build by alternating layers of bitumen and ply sheets that create a complete membrane.
Roofing Single Ply Membranes: Two types of membrane used – thermoset and thermoplastic. EDPM is a thermoset rubber-like membrane mostly used for low slop flat roofs. TPO and PVC are thermoplastic membrane used for commercial roofing mostly.
Polymer Modified Bitumen: Provide higher resistance against thermal cracking, rutting, aging, and stripping.
In my experience, though, built up roofs (BUR) are a dying breed due to needing a kettle of boiling tar on site.
Various membrane options are out there now that are easier to apply and have a number of benefits. TPO is remarkable easy to install and also comes in light colors which results in some energy efficiency.
Really, though, if you are asking this question, I’m guessing you should talk to a roofer or engineer (https://www.cityroofingorlando.com/) to determine the construction and needs of your roof. Depending on the location of your building, weather in your area, height of the roof, the materials used to build the structure of the roof and so many more things, you really will want an expert involved in this process to ensure your roof performs the way you want, lasts as long as it can, and comes in at your budget.