Concerning green roofs, John Porterfield sent us this note:
Reflections on skinning a roof –
Green roofs have an albedo (reflectivity) of about 25%, while “cool” roofing will reflect around 90% of solar radiation. The immediate result: cool roofs return solar heat to the sky while vegetated roofs absorb heat that is released the following night. Cool roofing is designed to give off (“emit”) heat to sky at night. A cool roof, then, is a better way to avoid global warming than a vegetated roof.
When calculating structural load in a cold climate, design for rain-soaked soil, followed by freeze, followed by heavy snow (worst case load).
When counting benefit of stormwater management, compare cost to roof drains designed to retain and then release water.
Perhaps actively raising food on the roof would favor the (usually) costly vegetated roof, though consider raising food at ground level, perhaps starting or joining a community garden. Or an alternative roof gardening strategy: plants with broad leaves, in containers at existing structural support, that shade the roof in summer (think melons!), then expose a heat-absorbing roof to the sun in winter when urban heat can offset fuel use.
As regards having soil above my head . . . I am willing to wait!
Thanks for the letter, John.