The Dirt on Green Roofs

Encouraging home and business owners to turn their rooftops into gardens may have seemed absurd just a decade or so ago, but today’s emphasis on environmental design is changing that mindset. “Green” roofers say these plans can reduce both energy consumption and stormwater discharge. For landscapers interested in creating building-topping designs, business could eventually grow through the roof.

Cultivating a successful design
Creating a successful green roof is more complicated than simply throwing down some topsoil and seeds and waiting for Mother Nature to work her magic. The first step is determining whether the installation will be “extensive” or “intensive.” Extensive green roofs are more common and easier to maintain, featuring sedums and other hardy plants that can thrive in hostile rooftop environments without regular irrigation. Extensive roofs tend to be at less than six inches deep.

Intensive designs are more like rooftop botanical gardens, and may require irrigation systems and regular landscape maintenance. As a result, these projects offer opportunities for experienced landscapers to excel. Thoughtful plant selection is critical to these plans, according to Angie Durhman, green roof manager for Skokie, Ill.-based Tecta America, a leading green-roof manufacturer and installer. This is because greater sun and wind exposure can make a rooftop a significantly harsher environment than surrounding ground-level locations.

“I think one of the biggest reasons green roofs fail is that the right plant material isn’t chosen,” she says, noting that the use of the wrong soil composition can also lead to failure. “A rooftop isn’t the same environment as the ground level.”

In either approach, standard asphalt or modified bitumen roof surfaces are covered by a waterproofing system to protect the underlying surface, plus a drainage layer to prevent pooling and often a filter fabric. This is then topped with lightweight, engineered growing medium, which replaces heavier topsoil to minimize structural loads. Some manufacturers offer extensive-type systems that combine various components into modules for a paver-like installation.

Because installation requires an understanding of how roofs are designed and built, roofers are often involved in green roof projects, in collaboration with landscapers, who have experience in sourcing plants and other materials. Landscapers will need training if they’re interested in moving their businesses above the ground level.

“The ramifications of working on a built structure are much higher than working on the ground,” says Steven Peck, founder and President of Green Roofs for Healthy Cities, a Toronto-based association serving the green-roofing industry throughout North America. “One slip of a shovel can cause hundreds of thousands of dollars in damage.”

Green Roofs for Healthy Cities has developed a Green Roof Professional training and certification program that could be a good starting point for interested landscapers. Training includes four full-day courses conducted in a number of North America locations. Classes cover design basics, bidding and quality-assurance essentials, waterproofing and drainage, and plant selection. The organization will begin offering a certification test at its 2009 annual conference, to be held in Atlanta, Ga.

Additionally, landscapers would need to become familiar with Occupational Safety and Health Administration (OSHA) guidelines for working on roofs. Added insurance also may be required by the building owner or by the roofing installer, especially if the landscaper is acting as a subcontractor, Durhman says.

Growing environmental appeal
The aesthetic benefits green roofs offer are pretty obvious – after all, who wouldn’t rather look out onto a plant-filled expanse than a layer of asphalt topping? But it’s the roofs’ environmental benefits that are driving adoption, especially in larger cities, such as Chicago, New York City and Washington, D.C.

First off, by covering black asphalt with engineered growing medium and plants, designers can help reduce the “heat-island” effect that can turn downtown areas into steam baths in the summer. Plants also are natural air cleaners, according to Peck.

Reducing stormwater runoff is another big green-roof benefit. The systems can capture 60 percent or more of rainfall that might otherwise flow from the roof through city sewer systems, according Durhman. In many cities, stormwater flows through sanitary sewer systems and can overload those systems during heavy rainfall. In these cases, cities may be forced to release untreated sewage into area waterways.

Finally, Durhman says green roofs actually can extend the life of standard roofing membranes by eliminating exposure to damaging ultraviolet rays – perhaps even doubling standard 15- to 20-year lifespans.