Why Green Roofs Are Not A Fad

by Don Friedman on June 21, 2017

People have known about the heat island effect for some time, where the concrete, asphalt, stone, and brick of buildings and streets absorb more heat than a natural landscape would, where black roofs absorb heat, where human activity generates heat, and the relatively lesser amount of vegetation means that the natural cooling from plant respiration is missing. It’s not just nostalgia for the countryside that makes urban heat waves so unbearable, it’s actually higher temperatures. Worse yet, the heat island effect is not going away just because the climate is changing.

Some of these causes cannot be eliminated or even significantly changed: the forest that covered much of Manhattan before urbanization is not coming back. On the other hand, most of the land area of Manhattan, when view from above, is roofs, and that’s something we have control over. The city has been encouraging people for some time to paint their roofs reflective white or silver. That seems like a simple thing but it has been demonstrated to significantly decrease the amount of heat a roof will absorb on a sunny day. Now the program has expanded to include green roofs and other measures.

This is directly related to our work as structural engineers in some unexpected ways. First, the roofs on small buildings – rowhouses or tenements, for example – used to be uncluttered places, with nothing much other than vent pipes and tarpaper. Now we’ve got mechanical equipment for ventilation and air conditioning, solar panels, firefighters’ paths, and decks, and that’s before we even get to green roofs. In low-lying areas, commercial buildings are putting boilers and generators on their roofs in case another Hurricane Sandy happens. The structure of roofs is often not adequate to support those loads and wood structure is not allowed to support most mechanical equipment. So the new roof uses require roof-structure upgrades or dunnage above the roofs.

Why isn’t this a fad? Because roofs represent free space that building owners can use pretty freely. Because roofs are a large exposed surface with few or no openings (how many interior spaces have working skylights?) and therefore represent low-hanging fruit for insulation and reducing heat gain. Because modern mechanical systems are smaller, more efficient, and lighter than their counterparts even thirty years ago (when I started work) and therefore are easier to place where we want them. Because green roofs extend the garden area available to a building and are often potentially nicer garden areas since they have fewer shadows than rear yards. Because green roofs actually create positive cooling in addition to preventing heating. Because in the never-ending arms race of New York real estate, green roofs are a new amenity. Because greenery – even if it’s trays with ground-cover shrubs or grass – is more appealing than asphalt-impreganted paper.


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