Rooftop gardens cool the buildings they're built on, reduce absorption of heat that makes cities "heat islands," and provide other benefits, according to an excellent City of Chicago website.
Says the Chicago web site, green rooftops
- reduce heating and cooling bills
- extend roof life by protecting it from weather conditions
- absorb carbon dioxide and reduce air pollution
- lower temperatures in and around the building in the summer
- insulate a building in the winter
- reduce storm water runoff, flooding, and water pollution
- provide a relaxing retreat
- enhance the aesthetics and increase property value
- provide space for urban food production
The simple science is this. Black tarred roofs absorb heat. "When increased temperatures meet air pollutants, it results in the accumulation of smog, damages the natural environment, and jeopardizes human health," according to Chicago. Lighter colored roofs, including those covered with greenery, reflect heat. Green roofs designed for visitation also are enjoyable.
Covering a roof with vegetation sounds easy but it isn't. The major problems are the building's support structure, types of plants, availability of water, exposure, and maintenance. Small-scale gardeners generally can overcome all these problems because they garden in pots and planters, can set up wind screens, and can carefully control their water regimen.
For a commercial, institutional or other large building, the problems are magnified. Plantings are generally larger than that of a rooftop "backyard" gardener, watering is automated, screening larger and more complex and plant containers large. Structural issues are important -- can the roof hold all the extra weight. Planting mix is important -- soil isn't necessarily the answer. Watering a large area adds more weight to the rooftop garden and automated watering systems aren't as careful as an individual who provides tender loving care. Planters are by design much larger and heavier than an individual's little rooftop plot.
Despite the complex problems of planting a large roof area, new buildings around the world are beginning to sport green fringe. This growth of green rootops is far from the scale of reforestation going on all the time around the world, so green rooftops won't have a measurable impact any time soon on Global Climate Change. Nor is the growth spurt likely to have scientifically measurable impact on city temperatures, but the effort to make such an impact must begin somewhere. A green rooftop is as good a place to start as any. And for the little guy, "green on."
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