Hello, I have an underground masonry garage with a concrete-cast roof. Now I'm draining and we found a method to seal from the outside on the "roof". The walls are cement stone that we are sealing according to Pordrän. The plan is to backfill on top with soil and then grass.
If you want to be really secure for many years, then it should be an EPDM sheet that comes in one piece or glued together (vulcanized).