Roofing supply companies make tapered insulation sheets. If the parapet is high enough, I should think you'd be able to go over the existing roof with a layer of foam sheathing that's draining in the right direction and cover it. ONE TEAM - ONE FIGHT!!! Mike They do sell tapered insulation, but a tapered insulation system is very difficult to engineer properly. Many things to take into account, but the 2 biggest factors are the slope of the existing roof and the installers ability to follow the installation instructions and diagram. That being said, they do work when properly done. I would never recommend installing a tapered system over an exiting roof. If there is no slope or negative slope, the better solution is to replace the roof with a roof covering that is designed for that. CTP (Coal Tar Pitch) and EPDM both can be installed over a structure that has no/negative slope. Mod Bit, as identified in you sample photos, will not remain watertight. It needs positive slope towards a drainage point. The specifications are a little different for either of those systems when they are installed over a no/negative sloped substrate, and as home inspectors we can never see if those have been followed. The bigger problem is finding a good roofer to install either of these systems in a residential application.