Alternatives to bituminous membranes for concrete roof?
There are several "rubber" roof materials for flat roofs; neoprene, EDPM, butyl, TPO, etc. Search "rubber roof" on the net for your location to find out what is available , what is needed for your UV, temperature, etc , and what is required for installation. I have EDPM pond liner; small parts of it have been exposed to weather for 20+ years and seem to be "good as new".