Green Roofs are Great! … But Not Cheap

Green Roofs are Great! … But Not Cheap

Green roofs are rightly in the technical news. A plant-covered layer of soil on a roof can provide insulation and cooling in summer ( Green roofs can provide insulation in winter ( The plants in a green roof can help decompose pollution. Green roofs even have their own web page at

Green roofs are defined as plant-covered roofs (not just a roof painted green). They function in two ways. First, the layer of soil and plant material is a significant addition to the typical roof insulation. Second, plants transpire moisture to the atmosphere and prevent heating of the roofing surface. (This is analogous to the way people sweat to prevent overheating.)

As additional benefits, green roofs can produce crops, they can be exotic small parks for office buildings or condominiums. At the very least, they add to the beauty of a city. I remember looking out from the Empire State Building in 1997. There were green lands in the distance, there was Central Park nearby, and there was the somber Hudson River. But, just below there was a wasteland of dead rooftops. Every green roof would have been wonderful to me.

Yet, in the phrase invented by environmentalists, “There’s no such thing as a free lunch!” A new building designed for holding the weight of a green roof at the top of its structure must be stronger. Furthermore, it must be more than proportionately stronger. Because that extra weight is at the top, there must be more support for transverse strains, sideways shocks. Earthquakes are rare in some areas, but they happen everywhere. The New Madrid quake in the early 1800s rang church bells as far away as New York City and Boston. There is a low, but real, possibility that either of those cities could have a major earthquake tomorrow. Other areas have higher risks.

Supporting the extra weights of green roofs for normal and potential earthquake strains requires additional support. For a wood-frame structure, that means heavier wood supports. For larger buildings, it means more steel rebar, more concrete, and/or more I-beams. Those additions cost money.

For existing buildings, there may not be sufficient design margin for adding a waterproof membrane and tons of soil. The choice regarding such structures may be adding extensive new support structures of finding other options. (Additional conventional insulation adds much less weight per unit area of building roof.)

A second issue is that the cooling transpiration costs water. Building owners in the American Southwest, where there are often water shortages, tend to use succulent plants, which need less water. That means they transpire less … but it also means they cool less. Using more water intensive plants requires greater water expense and maybe even political arguing to get that water.

Finally, a green roof requires regular gardening labor. A blacktop and gravel roof can be ignored for months, until the rains come. A green roof needs checking a couple times a week during dry weather.

In summary, green roofs are wonderful, but they are not free.


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