After alot of research I think I figured out what Bob was using. I am posting as reference. What Bob was using was EPDM rubber. EPDM was mainly designed for industrial built-up flat roofs. The manufacturer of EPDM will not sell to do-it-yourselfers, only to authorized EPDM roof installers(I guess the manufacturer doesn't want to get a bad reputation due to bad DIY installation.) So you must find a roofer which is authorized, which means that it will be an expensive roof solution. As luck would have it the very same EPDM rubber is also the most popular DIY pond liner. TetraPond being the biggest name. I got it from the local TetraPond supplier for 99 cents a sq ft.
I used mineral spirits to clean the EPDM and basic rubber contact cement to glue it down.