Putting vegetation on the roofs of city buildings may reduce global warming, and save money long term. This will also provide energy savings, storm water runoff reductions and possibly lower city temperatures in summer.

It is estimated that the higher cost of building a green roof compared with a conventional roof may be recouped in about five years. Many cities are offering tax incentives to adopt green roofs.

Green roofs provide a natural layer of insulation, keep buildings cooler in summer and warmer in winter. Energy savings vary and daily energy demands can be reduced by about 15%.

Such insulation prevents some of the expansion and contraction that takes place when a roof is completely exposed to temperature extremes.

Vegetation also captures rainfall and can slow the rate at which runoff goes into storm-water systems, reducing the load on sewers.

On a larger scale, green roofs may help ease global warming. Heat is created when dark-colored, impermeable surfaces radiate heat back into the air. The effect makes cities 2 to 10 degrees hotter than suburbs.

Currently most green roofs are being installed in new buildings. To retrofit a green roof requires hiring an architect, landscape designer and green roof consultant, among others. Insurance policies and existing current weight and fire hazards may require compliance.

In 2002 Battery Park City started requiring new buildings to plant greenery covering 75% of the non-mechanical area of their roofs. Since then, about two acres of green roofs have been installed or planned in the neighborhood. In June 2008, New York City offered a one-year property-tax credit for the installation of green roofs on at least 50% of available roof space.

The amount of green roofing in New York more than doubled from 2004 to 2007.

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