The temperature atop Chicago's City Hall building on an average summer day is usually 14 degrees to 44 degrees cooler than the county office building across the street. The reason: the county building has a typical black-tar roof, while City Hall has a green roof planted with grass, plants, and flowers.
The idea of planting grass or other vegetation on the roof of a building or house may seem like a holdover from the '60s counterculture movement, but proponents say such roofing systems are the solution to a wealth of environmental problems plaguing cities and jurisdictions across the country.
“Green roofs reduce resource consumption and are restorative in nature,” says Steven Peck, founder and president of Green Roofs for Healthy Cities, a Toronto-based nonprofit industry association and research network. “They clean the water, clean the air, and contribute to the health of a city.”
grow your own way According to GRHC, a green (or sod) roof is actually a multilayer system consisting of waterproofing and root-repellent membranes, a drainage system, filter cloth, a lightweight growing medium (soil), and vegetation. Green roofs may be installed as modular systems—with all of the aforementioned elements already prepared in movable, interlocking grids—or as individual components that are installed separately.
There are three types of sod roofs: intensive, semi-intensive, and extensive. The green roof research program at Michigan State University reports that intensive green roofs “use a wide variety of plant species that may include trees and shrubs, require deeper substrate layers, are generally limited to flat roofs, require intense maintenance, and are often parklike areas accessible to the general public.” Peck says such roofs also have a higher load capacity and, thus, are often used as rooftop decks.
Extensive roofs, on the other hand, are limited to herbs, grasses, mosses, and drought-tolerant sedum. “They are low-cost, low-maintenance, low-weight, and usually [measure] less than six inches” deep, he continues. What's more, extensive roofs are not designed for public occupation. Semi-intensive roofs, meanwhile, combine the two systems and measure about six inches deep.
What are the benefits of planting grass on your houses? Apparently, there are several. Among other things, a green roof “can add great insulative value,” says Lori E. Ryker, a principal of Ryker/Nave Design in Livingston, Mont. “There's a practical side to it.” Laura A. Blau, AIA, LEED AP, a principal of Philadelphia-based BluPath Design, explains the benefits in this way: “The roof is not getting direct sunlight, so it's a good insulator for the house.” At the same time, “it retains water, which helps keep the structure cool.”
A green roof also helps cool urban areas by reducing the so-called “heat island effect”—an occurrence in which dark surfaces, such as asphalt roofs in a city, absorb sunlight and radiate it back into the atmosphere as heat. It's this effect that makes the Chicago county building so much hotter in the summer.
In areas where storm water runoff is a problem—nearly everywhere, as it happens—green roofs help reduce stress on sewer systems in periods of heavy rain. “The roofs hold the water until it goes back into the atmosphere through evaporation,” Blau explains, or they delay the time when runoff occurs. In fact, a green roof may retain up to 90 percent of the rain that falls on it, depending on the depth of the growing medium, GRHC says, and it acts as a natural filter for the water that does run off.
Sod roofs also recover lost green space, filter particulates from the air that move across the roof surface, reduce energy consumption, and improve sound insulation. Some architects even use sod roofs to add aesthetic appeal to a house. “The way we approach design work is to first look at site appointments,” says Eric J. Cobb, AIA, principal of Seattle-based E. Cobb Architects. “The overlapping of landscape and architecture is very rich,” he adds, “and the green roof is a good way to do that. It's a good way to lift the ground plane up.”