By replacing the standard black and white urban shingles with plant material such as green grass, goldengrass, wildflowers and even small trees city dwellers can, and in many places are, altering local climate. Since plants absorb rather than reflect heat municipalities, businesses and homeowners are slowly beginning to tap into an concept that the Vikings had when they built shelters with tovtaks (turf roofs).Living roofs produce food, clean the air and conserve energy
I imagine with existing advanced farming trickery and future development the roof might deliver quite a lot of food. I do think a glass house on top of each building would make more sense then farming out in the open.
Then the greenhouse would be a source of heat rather then just a layer of insulation.
Each person would still need a lot of space, if we build 2 floor houses and use downstairs for living we could match supply with demand 1:1
Bury the house into the ground and we can slope the rest of the garden up to the roof.
Those who master their Pythagoras know the surface area goes up but it would also allow us to some what aim the garden towards the sun.
Thus, more sun, more surface, great insulation and a most amazing landscape.