Going Green on Top

Green roofs, already popular in Europe, are starting to gain momentum in the United States as an environmentally sustainable building practice. Some cities, including Chicago, now offer property tax and other incentives to buildings with green roofs.

According to Peter Belisle, president, Project and Development Services (PDS), Jones Lang LaSalle, green roofs can cost much more per square foot than a standard commercial roof, but they provide numerous advantages, both tangible and intangible.

“Among their many eco-friendly benefits are increased insulation to help reduce energy requirements, lessening of the urban ‘heat island’ effect, and reduction of carbon dioxide and pollutants in the surrounding atmosphere,” Belisle said. Green roofs also can contribute to credit toward the U.S. Green Building Council’s LEED (Leadership in Energy and Environmental Design) certification.”

Additionally, green roofs can extend the serviceability of a roof by two to three times its normal length, and help with rainwater retention.

There are a wide variety of choices available, from park-like roof gardens to low-maintenance self-sustaining systems. In fact, about the only thing all green roofs have in common are their basic components, which typically include, from bottom to top:

Awaterproof membrane that also serves as a barrier to root penetration through the roof;

Alayer of rigid insulation;

Amat to aid drainage, and, if desired, water retention for re-use in the building;

Aspecially engineered soil mix; and

Some type of vegetation.

While there are many variations, there are two basic types of green-roof systems: extensive and intensive (some roofs are a hybrid, containing elements of both).

Extensive roofs, or roof gardens, typically feature a wide variety of aesthetically pleasing plants, similar to a park. They generally require more regular maintenance, including irrigation, feeding and trimming.

Study Details Storm-Water Benefits

A green roof has the potential to lower storm-water runoff from 65% to 94%, translating to a significant reduction in the demand placed on wastewater-treatment plants during storms, according to a two-year study of green roofs in Seattle. The Seattle Green Roof Evaluation Project was conducted by Magnusson Klemencic Associates (MKA), a Seattle-based structural and civil engineering firm.

In addition, the study found that green roofs allow building owners to reduce the size of storm-water-detention tanks needed for peak rainfall. This can offset the cost of a green roof by 30% to 60%, according to MKA.

Extensive systems utilize hardy, drought-resistant perennial plants. From the soil composition to the choice of vegetation, they are designed to survive the local climate with minimal care.

Extensive roofs require less additional structure support, and are often more practical for sloping roofs or tall buildings.

“While extensive roofs can also be attractive,” Belisle said, “they are especially valued for their contributions to environmental sustainability.”

From his work in helping various Jones Lang LaSalle clients incorporate green roofs into their properties, Belisle has identified three important pitfalls for building owners to avoid with regard to green roofs:

Unrealistic assumptions about cost. Cost data on completed green roofs ranges from $8 to $28 per square foot, with the average in the $10 to $12 range.

“This is in addition to the standard roof assembly underneath the green roof,” Belisle said. “The annual cost of maintaining the green roof also needs to be considered.”

Hiring inexperienced consultants. Find a horticultural/landscape consultant with specific green-roof experience who can advise on selecting the best plants, irrigation methods and soil mix for a particular environment, taking into account a range of factors, from climate to roof height.

Also, hire an experienced project manager who can coordinate the activities of all involved parties, including landscape architects, structural engineers and building operators, who must understand how to properly care for the green roof once it is installed.

Inflexible design and construction. Green roofs should be designed in a way that minimizes the load on a roof’s weakest points, such as flashings and perimeter, and that is flexible enough to accommodate changes of plan, even repairs, over time.

“One smart choice for an extensive system is prefabricated trays that can be moved not just to change the design pattern of roof vegetation, but to provide easy access for inspection or repairs in any particular area,” Belisle said.