Surely the idea that a chopper pilot would choose to land on the roof of a building when he could land on the ground, even a busy junction, is somewhat strange? Cars and pedestrians would be likely to have at least some chance of getting out of the way, whereas those in a building would not, not to mention the added danger to the crew if the roof gave way.
Also surely the C of G of this craft would mean it stays upright when the blades are seized?