Green Roofs

Did You Know?

A green roof can actually be installed at ground level. These types of green roofs are often installed as the roof of a parking garage or another underground building.

Green Roof Benefits

Economic Benefits

Although installation of a green roof usually involves higher upfront costs than a traditional roof, there are many economic benefits that can make up for this. Increased R–value (a measure of the resistance of a material to heat flow) of the roofing system, along with reduced temperatures on the roof lessen HVAC loads, resulting in energy cost savings. A green roof may improve property values and marketability, especially in urban areas with little green space. The University of Michigan performed a valuation study comparing a 2,000 square meter conventional roof and a green roof. The study looked at a range of benefits of green roofs including stormwater management, improved health benefits due to reduced pollution, and energy savings. Over its estimated lifespan of 40 years a green roof would save about $200,000, of which, nearly two–thirds would come from reduced energy costs. The economic benefits of any individual green roof will, however, depend on its design, geographic location, surroundings, and the building itself.

Improves Stormwater Management

A green roof also helps control storm water runoff and retention. The increased urbanization of towns and cities has resulted in less green space and more impervious surfaces. Precipitation generally runs off the roof of a building into gutters and flows into a storm sewer. From the storm sewer it either enters a municipal water treatment facility, or it is directly deposited into the ecosystem via lakes, streams, and rivers. Any dirt or contaminants on the roof are picked up by the water and are flushed into the storm sewer as well. In cities with a combined sewage overflow system in which rain water runoff, domestic sewage, and industrial wastewater are collected in the same pipe for treatment, too much storm water can cause a city’s sewage system to overflow, discharging sewage into streams and rivers. A green roof can help prevent this by retaining water in the plants and growing medium, thus slowing and reducing the amount of storm water entering the ecosystem and, consequently, reducing flooding and erosion. Data collected from a study by researchers at the Pennsylvania State University, Center for Green Roof Research show that green roofs captured up to 80% of rainfall during rainstorms, compared to 24% typical for standard roofs. As green roof plants mature and root systems grow, storm water retention may increase. In some cities, buildings with green roofs may be eligible for lower storm water management fees due to the fact that green roofs reduce the amount of storm water that leaves the site. The growing medium and plant material of the roof also act as a filter and help to neutralize acid rain, and trap dust and airborne particles.

water retention chart.

Chart showing average water retention for a tranditional roof vs. a green roof. Source: "Green Roofs in the New York Metropolitan Region, Research Report," Rosenzweig, et. al.

Reduces Urban Heat Island Effect and Improves Air Quality

The temperature in cities is often higher than surrounding rural areas, a phenomenon known as the urban heat island effect. Large amounts of paved surfaces in cities absorb solar radiation and re–radiate it as heat, which increases the local air temperature. For example, during the summer months in New York City, the daily minimum temperature is on average 7.2ºF warmer than the surrounding suburban and rural areas and even more during heat waves. Green roofs not only help reduce the urban heat island effect by covering conventional dark roofing surfaces with vegetation which absorbs less heat, but they also use solar radiation to evaporate water from the growing media and transpire (the absorption of water through a plants roots and release of it through its leaves as a vapor) moisture from the plants. This process of evapotranspiration lowers the temperature on the roof by using heat from the air to evaporate water.

graph of average roof temperatures

Graph showing average roof surface temperatures for both "greened" and "non-greened" roofs. Source: Penn State Center for Green Roof Research.

Insulates the Building

A dark, heat–absorbing roof surface increases demands on mechanical systems, making it more difficult to adequately cool a building, whereas a green roof reduces the temperature of the roof and, therefore, the building itself. The extra layers of a green roof also serve as insulation. This decreases the amount of heat passing into the building, reduces cooling loads, and offers some insulation during the heating season, although, it is important to note, it does not replace the need for additional thermal insulation. The benefit is greatest for buildings with a high roof–to–wall ratio (i.e., buildings with a greater amount of roof area vs. exterior wall area). The additional layers of roofing and growing medium also help minimize the noise on the interior from mechanical equipment on the roof.

Improves Efficiency of Mechanical Equipment

Cooler roof temperatures produced by a green roof help boost the efficiency of rooftop mechanical equipment by making the air on the roof cooler. When in cooling mode, HVAC equipment must pre–cool outside air to get it to the required temperature. If the air on the roof is made cooler by a green roof, this process is easier and uses less energy. Therefore, lower air temperatures on the roof improve the efficiency of heat–rejecting rooftop HVAC equipment because it is operating at a lower ambient temperature.

Reduces Greenhouse Gas Emissions

The reduction in cooling loads also helps reduce greenhouse gas emissions from fossil fuel combustion associated with the use of HVAC equipment. Adding plants and trees to the urban landscape in turn increases photosynthesis, reducing carbon dioxide levels produced by vehicles, industrial facilities, and mechanical systems. It also increases oxygen production.

Extends Roof Life

A green roof can increase the life expectancy of a roofing system by protecting the roofing materials from direct ultra–violet radiation and extreme temperatures. As a result, the roof structure can require less maintenance, saving the owner money in replacement costs over the long–term life of the roofing system. A well–maintained green roof can more than double the number of years before a roof needs to be replaced compared to a standard roof, making up for some of the added costs of installation.

green roof with beehives

Beehives on the roof of the Chicago City Hall help polinate the green roof plants and provide homes for the urban bee population. Courtesy of the City of Chicago


Provides Urban Amenities

An accessible green roof increases urban green space and improves the comfort and enjoyment of the building’s occupants by providing an aesthetically–pleasing view or environment for meetings or recreation. Some green roofs incorporate urban agriculture and include herbs and vegetables that can be harvested for use by the building’s occupants or the community. A green roof can also provide a refuge for insects and birds that have lost their natural habitat due to urban development and the loss of green space.

interpretive sign for green roof

Informational signs can accompany green roofs to educate building occupants and visitors to the benefits of the green roof and how to protect the resource.