Complex sentence: A hard but flexible layer of rigid foam seals all tiles to each other and to the steel structure thus ensuring a water tight roof for tens of years, Polyurethane foam truly is an amazing invention of man.
Split into simpler sentences: A hard but flexible layer of rigid foam is then created and seals all the tiles to each other and to the steel structure. Thus, by ensuring a water tight roof for tens of years, it's clear that Polyurethane foam truly is an amazing invention of man.