Owen Rose of Montreal wrote about extensively gardened roofs in On Site 17: Water. These Montreal rooftops are more than container gardens, and not as heavy as green roofs with their .5-2m of earth (generally known as green roofs or intensive gardens) which required quite massive structural support. New, highly efficient substrate of 3-15cm allows garden plots on almost all kinds of construction. Rose calls them blue roofs, perhaps because of their water retention: they collect rainwater up to a point and release the rest into city storm drains, lessening the load on infrastructure during intense storms.
We had an article by Helmuth Sonntag in On Site 2: Houses on a rolling rooftop garden in Weisbaden, planted with rows of lavender and rosemary. It is a bylaw requirement there and in much of Germany that the roof collect water and that the water be stored.
The sense that the rooftop is the displaced ground plane is part of Le Corbusier's 1923 Vers une Architecture: that development should not take away access to land. He saw land as a source of leisure, but land is land, and if you want to grow kale on it, so should you be able to.
When I got my small Inglewood house in Calgary the neighbourhood was mostly old Saskatchewan farmers who had come off the land in the 1930s and 40s and gone to work for the CPR. Inglewood is next to CPR's Alyth Yards in southeast Calgary. Flat land, Bow River, sandy soil substrate with good drainage, a warm micro-climate, and, in the 1980s hardly a tree to block the sun on all the huge gardens in the back yards. Leafy neighbourhoods were a sign of wealth, poorer neighbourhoods were quite bleak. Well, with gentrification, trees now flourish and my yard is completely shaded and I can't grow a thing. The roof gets more sun than the ground. I would like a flat platform over it that I could put a garden on. I think I'd rather this than photovoltaic cells even - lower technology, a parasol for the roof, lots of food.