Try epdm flat roof membrane. I recon it is the easiest flat roofing material to install diy. Hot gas and tar not needed because it is an all cold process.
Have a look at this site; they have lots of example flat roof pictures using the rubber membrane: www.flat-roof-repair.co.uk
You could try epdm flat roofing membrane. Sizes are manufactured to fit most flat roof applications in a single sheet. It is just bonded to the substrate with special adhesive
Have a look at: www.flat-roof-repair.co.uk