Expert interviews and analysis of the latest studies indicate that plant-covered green roofs can save both individuals and society money over the long run, but seeing a net return on investment is a long-term proposition.
Green roofs can lower energy bills significantly, especially in the summer. Brad Rowe, a professor of horticulture at Michigan State University, said that savings can range from 7 to 50 percent, depending on the type of vegetation, the depth of soil, the local environment and other factors. “Every roof is different,” said Rowe.
There are other advantages as well. “A green roof is the last roof you need to put on your building,” said Professor Stuart Gaffin of Columbia University, who recently completed a study (PDF) of green and white, or high-albedo, roofs in New York City. The vegetation shields the top of the building from the sun’s UV rays, which degrade building material over time.
But installing a roof with its own mini ecosphere can cost more than twice as much as a traditional one, which makes narrowing the cost benefit gap with traditional roofs tough. Not only do green roofs cost more to install, they also cost money to maintain.
A study (PDF) released in 2008 by Portland State University concluded that, based on the assumption that green roofs last 50-100 percent longer than traditional roofs, “it is likely that the amortized expenditures approach parity,” the study said.
Translation: a green roof can break even over the long term — 50 years or so. For shorter time spans, the ROI may not be quite there.
Another cheaper, environmentally-friendly roofing fix is a white, or high-albedo one. White roofs reflect back more of the sun’s energy, lowering summer cooling costs (they actually have a slightly detrimental effect on heating in the winter).
Gaffin said that white roofs cost a dollar or less per square foot to paint on, and achieve savings of about 13 cents per square foot a year, for an ROI of seven years. They also extend the life of a roof, though not as much as a green one.
White and green roofs also have externalized benefits, savings that are not realized by the owner, but by society at large.
The plant life on green roofs absorbs rain water, reducing the amount of waste water flowing into municipal sewer systems during storms. In New York City it has been estimated that a 50 percent green roof infrastructure would save the city $18 million per year in storm-water treatment.
In Germany, home owners are assessed a storm-water drainage fee, spurring the installation of green roofs there, according to Rowe of Michigan State. “It’s like a toll road,” he said. “If you use it, you pay for it, if you don’t, you don’t.” He said some municipalities in the US are toying with the idea.
Both green and white roofs also reduce the urban heat island effect, and, because they reduce building energy demands, reduce carbon emissions as well.
Of course there are other benefits to a roof covered in lush green vegetation that cannot be so easily quantified. From the Portland State study:
Other potential benefits of green roofs, such as air/water quality improvements, commercial crops, wildlife habitat, recreational areas, improved esthetics and an overall improved sense of wellbeing clearly exist but credible monetary values have not yet been established.
Portland State offers a Green Roof Energy Calculator on its website, which can estimate energy savings.