Concerns over climate change and water quality are affecting how municipalities, developers and brownfield professionals approach redevelopment projects. As LEED green building and stormwater management requirements are increasingly mandated by municipalities, some see the process becoming more involved.

However, meeting green building requirements, stimulating corporate interest and managing stormwater runoff on site doesn’t have to be extraordinarily troublesome or expensive. Many large cities throughout North America are making a strong case for incorporating green roofs, also known as rooftop gardens or eco-roofs, into a brownfield redevelopment.

Last year, installation of green roofs in North America was up 30 percent over 2006, according to the organization Green Roofs for Healthy Cities. Chicago holds the number-one position, having installed over half a million square feet of green roofs in 2007.

What Is a Green Roof?
A green roof is simply a vegetated roofing system designed to mimic conditions found in nature. They typically consist of the following components constructed from the roof deck upwards:

Waterproof membrane—like any rooftop waterproofing system, protects the building from water infiltration.

Root barrier—placed directly above the waterproof membrane, is usually imbedded with a copper-based compound that effectively prevents plant roots from penetrating the waterproof membrane and causing a leak.

Drainage layer—necessary to facilitate proper drainage from the green roof, but can also be utilized to retain a percentage of rainfall to support plant growth. The drainage layer may also incorporate a lightweight drainage media, such as roasted Arkansas clay, to store water and help provide a rooting medium for the overlying plants. Â

Growing media—a lightweight, engineered soil containing a proprietary mix of compost, vermiculite, and other inorganic material. Â

Vegetation—can be incorporated as seeds, plant cuttings, or seedlings. Depending upon the species selected, plants typically propagate quickly, and within two years, can completely cover the roof surface.  For a successful green roof application, the planting palette should focus on native or drought tolerant species (e.g. sedum varieties).

Additional materials—although not always required,  such as insulation, roofing felt, filter fabrics, and wind blankets are often used depending upon the application.

When speaking of green roofs, one can refer to either extensive or intensive green roof systems. Â Extensive systems are characterized by shallow growing media, generally 3 to 4 inches in depth and are limited by the types of plant species it can support. Intensive green roofs utilize growing media greater than 6 to 8 inches in depth and can support a larger variety of plants, including trees in deeper areas. A hybrid system, often called semi-intensive, uses growing media between 4 and 8 inches in depth. Â Regardless of the type, green roof systems provide similar environmental benefits.

Why a Green Roof?
Green roofs are an effective method for combating the Urban Heat Island Effect, cooling buildings and managing stormwater runoff.

In both urban development and suburban sprawl, the destruction of green space and the increased use of concrete, asphalt and other heat-absorbing building materials lead to an increase in surface and ambient air temperatures. As a result, cities retain heat long after the sun has set, increasing our reliance on air conditioning. Â

Green roofs can be used to cool a building. Through the process of evapotranspiration, heat energy that would normally accumulate in roofing materials is instead used to support a phase change for water stored in the growing/drainage media. Cooling a building with a green roof reduces the building’s energy use, subsequently reducing energy demand and energy production, leading to less pollution. In new construction, architects can take advantage of the cooling properties of green roofs to reduce the capacity and cost of the HVAC equipment needed to maintain building temperature and humidity.

Further, the use of impervious surfaces in construction reduces stormwater infiltration and increases the likelihood of combined sewer overflows and flooding. Green roofs can retain stormwater runoff, and since the green roof is constructed within the building footprint, additional space is not required for stormwater management. Recent studies in Chicago have shown that green roofs can retain 90 to 100 percent of runoff for small storm events (<0.25 inches) and 60 to 80 percent of runoff for large rain events (~1 inch). Other factors affecting storm-water retention include the composition and depth of growing media and the age of the green roof. In general, older green roofs with well established root systems tend to retain more stormwater than newly planted roofs.

Additionally, the vegetation acts as a carbon sink, filters pollutants from the atmosphere and rainfall, and provides a habitat for birds and insects. Green roofs can also provide noise attenuation, reducing ambient noise by up to 40 decibels for building occupants. Finally, green roofs increase the longevity of the roofing membrane. In most cases, green roofs last at least 50 percent longer than conventional roofs.

Planning, Construction and Cost
Although the components that make up a green roof appear simple enough, and construction is fairly straightforward, additional considerations must be evaluated during planning and construction.

Green roofs weigh significantly more than conventional roofs. Therefore, it is imperative that a licensed structural engineer or architect evaluate an existing rooftop before adding a green roof to a building. The saturated weight of the growing media and adjustments for snow load in colder climates should be included with the structural analysis. Retrofitting an existing structure is more challenging than adding a green roof during building construction. Often, the structural reinforcement necessary for adding the green roof will increase the project budget.

In most, if not all municipalities, building permits are required. A permitting department’s unfamiliarity with green roofs may increase the time needed to obtain the permit.

Other items to consider during construction include methods for hoisting materials to the rooftop, whether an irrigation system will be necessary, and access to the rooftop for building and landscaping maintenance. If the green roof will be occupied by people for any amount of time, plumbing vents may need to be extended, railings will need to meet OSHA requirements for safety, and at least two routes of evacuation should be present. Certain buildings may also be subject to access requirements necessary to comply with the Americans with Disabilities Act.

Although costs have become more competitive, green roofs are still expensive in the U.S. However, offsets from reduced energy use, lowered storm-water impact fees, and longer roof life will help pay for the installation over time.

The useable space provided by a green roof can add to property values and the well-being of building occupants while satisfying municipal rules and regulations for landscaping and stormwater retention.

Kevin M. Laberge, P.E., is an Environmental Engineer III with the City of Chicago’s Department of Environment.