When I was travelling in North America I noticed that a large proportion of buildings had flat roofs. ie thay did not have a pitch. Looking down at them from above, they had what appeared to be a bitumous membrane mixed with gravel.
Why not have sloped roofs, so that the water drains off?
It seems that this is just asking for a leaking roof or buildup of snow in winter. Does it have anything to do with ‘roof avalanches’?