May 5, 2004

That roof over your head may one day turn green

Photo by Mark Huppert, Catapult Community Developers In Europe, green roofs are often installed on top of ordinary roofs with no extra modifications.

In cities throughout the world, non-permeable surfaces have created stormwater management problems. Buildings and streets create an almost completely impervious surface, so rainwater causes flooding, which overtaxes stormwater collection systems and degrades stream habitat.

The conventional solution has been costly underground detention tanks that control the release of runoff. Rainwater races from the roof and pavement to the tanks, where it is held until the storm passes. The runoff goes through a cleansing device as it is slowly released from the site, entering the downstream systems, albeit in a controlled manner.

Low profile green roofs can provide these same functions while also reducing the amount of runoff that leaves the site. Green roofs clean airborne pollutants from rain, and reduce the rate and quantity of stormwater runoff. A building with a green roof requires significantly less conventional stormwater detention and cleansing facilities, and benefits downstream systems by reducing the volume.

Rainwater is absorbed by the soil, taken up by plants and returned to the atmosphere. It is common to decrease annual runoff -- and stormwater detention facilities -- by half.

Seattle-based Magnusson Klemencic Associates has developed a modeling tool that predicts the stormwater management benefits of green roofs, based on factors such as size and thickness.

Green roofs also provide insulation and reduce energy consumption, noise and the urban "heat island" effect. Furthermore, they prolong the life of roofs by protecting the roof membrane from UV rays and thermal shock.

After seeing green roofs in Sweden, Greg Smith, a Seattle real estate developer said, "Having viewed green roofs in person, I am convinced it is the roof of the future. It's attractive, long-lasting, cost-effective over the long term, and both socially and environmentally the correct application. I plan to install them in my properties." David Gold of Cathedral Park Place, a real estate development firm in Portland, vowed to have a green roof on one of his buildings by fall.

Duncan Chalmers, a project manager with Turner Construction, said he was impressed by how easy they are to install. "They're simple," he said. "They aren't rocket science."

One problem is that U.S. contractors don't have experience with green roofs, and often over-specify items such as the waterproofing layer, which unnecessarily increases the costs. This scares building owners away from installing green roofs. Green roofs in Europe are often installed on top of ordinary roofs with no extra waterproofing or structural modifications.

Germany uses

carrots and sticks

The use of green roofs began about 30 years ago in Germany. Germany offers financial incentives for green roofs, including direct subsidies ranging from 50 cents to more than $6 a square foot, based on the avoided costs of infrastructure maintenance and replacement.

Taxes and use fees are also levied on stormwater management facilities. In some cities, buildings with impervious roofs are required to pay a 100 percent utility surcharge. This can be reduced by up to 80 percent if the owner installs a green roof.

Another type of indirect subsidy lets developers count green roofs toward meeting open space requirements. Some land development ordinances in Germany allow green roofs to compensate for lost open space at a ratio of .50 to .70.

As a result of this carrot and stick approach, the green roof industry in Germany has grown about 15 percent to 20 percent per year. Other European countries with similar programs are also experiencing high growth rates.

The U.S. is benefiting from Europe's experience. Although green roof designs are generally regulated using existing standards for ballasted roofs by the International Code Council, the only accepted guidelines for green roof construction are those developed in Germany. These guidelines include industry standard tests for medium weight, moisture, nutrient content, grain-size distribution and other considerations.

Cost of green roofs

Conventional roofs cost approximately $6 to $10 per square foot, while the cost of green roofs in the U.S. is typically around $14 to $18, including plants, growing media and membranes. This is largely because green roofs are new here and installation is somewhat customized.

In European countries where green roofs are common, costs run between $8 and $15 per square foot, even though labor costs can be higher. Some of the increased upfront costs are recouped through the longer lifespan of the roof. In Germany, the oldest green roofs are 30 years old and going strong. The green roofs at Rockefeller Center, built in the mid 1930s, still have their original waterproofing membranes. Current estimates are that green roofs can last up to 70 years.

Higher upfront costs can also be recouped through energy savings. The insulating value of soil lowers the cost of heating and cooling.

In 1995, The Gap built a 195,000-square-foot office building in San Bruno, Calif., and expects by next year that lower energy costs will offset the cost of the green roof and all the other environmentally sustainable features of the award-winning building.