Traditional roofs, like the one shown below (322 Dundas), use a bitumen layer to keep rain out:
Bitumen is very sensitive to light, and is damaged by light. That is why bitumen roofs need to be covered in stones (to keep the light off the roof). The roof shown above is a raw bitumen surface (not covered in stones) and will break down quickly. Moreover, bitumen is essentially coal tar, and, as we all know, coal is a source of fuel and is thus very combustible.
It does not make sense to make roofs out of fuel (coal tar), especially, since many buildings are now "smoke free" and thus people often climb up on roofs to have a cigarette break.
The combination of foot traffic, smoking, and sunlight can take their toll on the traditional rooftop membrane.
The SolarSave roof is a major step forward in flat roofing technology:
This roof is part of the "Urbeach" (urban beach) project.
The goal is to outfit as many of the flat rooftops as possible, with solar energy.