The green roof has evolved tremendously since mossy, thatch-roofed cottages dotted the countryside. Not only are homeowners taking a serious look at the environmental benefits of green roofs, but a growing number of business and industrial groups are realizing that a green roof can have a positive impact on their bottom line.

What Exactly Is a Green Roof?

If your idea of a green roof is a soggy layer of grass, think again.

Modern technology and building materials have greatly expanded the options for designing and building green roofs.

Most green roofs start as flat roofs, though it's possible to design one for a building with a sloped roofline. The simplest kind of green roof uses a shallow layer of soil 2 or 3 inches deep, planted with ordinary lawn grass or some other live ground cover plants.

More sophisticated green roofs have complex installations with a variety of grasses, trees and other plants; the most extravagant are roof gardens, which usually include some kind of hardscape like benches, tables, decks and walkways, as well as water features, barbeque pits and other amenities. The sky is, quite literally, the limit on what a green roof can be.

Some experts describe green roofs as either "extensive" or "intensive," a distinction that seems a little pointless. Simply put, an extensive green roof is a shallower layer of grass or ground cover that reaches from one end of the roof all the way to the other.

An intensive green roof, meanwhile, has a deeper and more varied layer of soil, which allows the planting of trees and other large plants.

Financial Benefits of Green Roofs

Now for the money shot: What good is a green roof?

After all, constructing a safe and effective green roof isn't always cheap, depending on the building and its existing roof structure. To keep the subroof dry, most green roofs require an extra layer or two of waterproofing membrane, as well as some kind of additional load-bearing support.

According to the EPA: "Estimated costs of installing a green roof start at $10 per square foot for simpler extensive roofing, and $25 per square foot for intensive roofs. Annual maintenance costs for either type of roof may range from $0.75 to $1.50 per square foot." That's much higher than an ordinary roof.

The payoff comes from the long-term energy savings -- especially in urban or suburban areas, where the "urban heat island" is strongest. The term refers to the increase in temperature that's found in virtually all urban areas. Because solar radiation warms up concrete, asphalt and other man-made materials much faster and hotter than it warms trees and greenery, a big zone of hot air -- a heat island -- surrounds urban environments year-round.

While this helps to keep cities warmer in winter, the urban heat island makes cities and towns unbearably hot in summer, so air-conditioners have to work harder and longer. The resulting spike in energy demand puts a real strain on electrical grids and sends summer energy bills through the roof.

A roof garden, however, can ease this financial burden: A study by the National Research Council of Canada found that an exposed roof can get as hot as 158 degrees F on a sunny day; a similar green roof stays relatively cool at just 77 degrees F. And while the average daily energy demand for air-conditioning with the bare roof was 6.0 to 7.5 kWh (20,500-25,600 BTU), on the green roof the demand to less than 1.5 kWh (5,100 BTU) -- a reduction of over 75 percent.

Other studies are highlighting the benefits of green roofs. The City of Chicago estimates that its City Hall green roof project (see photo, above) will provide cooling savings of approximately 9,270 kWh per year and heating savings of 740 million BTUs. This is roughly equal to energy savings of $3,600 each year.

How Buildings Benefit

Beyond energy savings, green roofs have a beneficial effect on buildings themselves. Most exposed roofs go through rather large variations in temperature and weather conditions. These extreme events -- heat waves, freezing winters, torrential rains and blistering sun -- cause the roof membrane to shrink and expand as the weather changes, thus shortening the lifespan of the roof.

For example, in the Canadian research noted above, a bare roof experienced daily temperature fluctuations of 83 degrees F, but the green roof reduced this variation to just 22 degrees F. And because Roanoke, Va., installed a green roof on its municipal building -- at a relatively low cost of $123,000 -- it now expects to add 20 to 60 years to the life of the roof.

Again, the EPA finds research that supports the economic value of green roofs: "A University of Michigan study compared the expected costs of conventional roofs with the cost of a 21,000-square-foot green roof and all its benefits, such as stormwater management and improved public health from the absorption of nitrogen oxides. The green roof would cost $464,000 to install versus $335,000 for a conventional roof in 2006 dollars. However, over its lifetime, the green roof would save about $200,000. Nearly two-thirds of these savings would come from reduced energy needs for the building with the green roof."

Green Roofs and the Environment

Another big advantage to roof gardens is their ability to manage rainfall, making it cleaner while also absorbing much of the rainfall, thereby reducing its quantity and easing the burden on local storm sewer systems.

When the Canadian researchers compared the runoff from a bare roof and a rooftop garden, the difference was astounding: The roof garden reduced runoff by 75 percent, filtered the water and made it cleaner, and delayed the run-off time by 45 minutes. For municipal wastewater systems that regularly discharge raw sewage after a rainstorm -- and in areas where runoff flows directly into area lakes and rivers -- this is big news.

An finally, there's a big quality-of-life advantage to a green roof. Who wouldn't like to head up to the roof at lunch to pick a few fresh tomatoes and see hummingbirds flit about a flower garden, or stop by after work for a drink and to watch the sun set? Given the proven benefits of green exercise and time spent in nature, the benefits of a green roof are impossible to deny.