In areas of high-density development, where pervious surface and open ground make up 10% or less of total surface area capable of absorbing or diverting storm water runoff, greenroofs provide significant social, environmental and financial benefits:

Thermal heat island map of D.C. Red and orange areas show heat concentration over high-density portions of the city. Photo courtesy of G.E.M.

• Reduce cooling and heating energy consumption. Greenroofing improves building insulation for reduced energy consumption year-round. This means fewer brown-outs and less pollution created by air conditioners, heating units, and power plants. An energy study for the city of Chicago estimated peak demand would be cut by the equivalent of one small nuclear power plant if all the city's roofs were greened.
• Reduce energy costs. The study said Chicago could save $100 million in saved energy annually with greenroofs. A one story greenroof structure can cut cooling costs by 20 –30%.
• Protect the roof's structural elements from harmful UV rays, wind and extreme temperature fluctuations.
• Extend the life of the roof by providing enhanced roof membrane durability—up to 50 years or more. The greenroofs at the Rockefeller Center were built in the mid 1930's, and still have their original waterproofing membranes.
• In some cities, offer builders increased floor-to-area ratios for contracts that include greenroofs in their building design and construction.
• Increase property values by utilizing previously unused space for roof decks and attractive roof landscaping.
• Lower stormwater utility fees, and earn energy tax credits, grants and subsidies. The Gap expects to have saved enough money by 2005 from lowered energy costs to have paid for the greenroofs and all the other environmentally sustainable features of their award-winning building.
• Provide urban jobs and business opportunities for every sector within greenroofing: design, installation, structural components, horticulture, soils, roofing materials, machinery, and contractors.
• Reduce ambient air temperatures in high density urban areas, therefore reducing the Urban Heat Island effect. In the summer, city rooftops can reach 175° or more! See the orange and red areas of the D.C. thermal map (above) to see how paving and density create heat islands.
• Absorb CO2, filter air, bind dust particles and reduce glare. 1,000 square feet of greenroof can supply 110 people with oxygen and remove 41 pounds of airborne particles per year.
• Absorb and filter heavy metals, acid rain and pollutants from storm water, including cadmium, copper, lead, nitrate, zinc, diesel soot, VOCs, hydrocarbons and pesticides.
• Reduce runoff flow rates and retain 50–95% of rain water. This eases city sewer infrastructure and reduces combined sewer overflows into the Potomac, Anacostia and Rock Creek.
• Provide needed green habitat and nesting areas for urban birds, butterflies and other insects.
• Provide acoustic insulation and reduce noise pollution by 50dB with a barrier of plants and soil.
• Find respite from the "concrete jungle." Creative designs can include flowing water features, architectural details, meadows and landscaped seating areas that can help urban residents and employees unwind and de-stress.
• Foster a sense of community among apartment and condo dwellers, as well as among office or industry workers. Intensive greenroofs can provide amenity space for leisure activities including community gardening. High-end greenroof installations can become local hot-spots for community gathering.
• Build community capacity through volunteer installation projects in public spaces such as libraries, recreation centers and schools or park kiosks and restrooms. School greenroofs also make a cutting edge addition to high-school environmental science, biology or horticulture curricula.
• Integrate and blend our buildings into the natural environment. This can have the added benefit of assisting with national security by hiding buildings from thermal imaging and overhead view.
• Transform commercial and industrial structures from "big box" concrete eyesores into architectural assets that offer the community multiple benefits.