A green roof doesn't refer to the color of the shingles or tiles. A green roof is actually a roof that is designed to grow plants. Like a rooftop garden, a green roof is covered with vegetation. These roofs are being studied in several urban areas by environmentalists, NASA and even companies that just want to save money.

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Standard roofs are dark and hold heat.

Pro: Reducing Heat Islands

Cities are sometimes called "Urban Heat Islands." Pavement, dark roofs and few green spaces absorb more heat and keep the entire area hotter. Since plants are lighter in color than the black or brown tars usually used to cover roofs, they reflect more sunlight than standard roofs. According to NASA, green roofs can be up to 50 degrees cooler than their standard counterparts.

Pro: Heat Reduction

Plants need several inches of dirt to grow. On a green roof, the dirt needed to grow the plants is another layer of insulation between the building and the sun or snow. According to Sustainable Industries, the City of Chicago saves 10% on its air-conditioning bill on buildings that have green roofs.

Pro: Air Quality

Green roofs offer another way to bring plants into a cityscape. More plants bring cleaner air to an area. During photosynthesis, plants use the carbon dioxide spewed by cars, trucks and even people and turn it into fresh, pure oxygen.

Pro: Storm Water Management

When it rains in cities, all of that water has to go somewhere. Water from rooftops is shunted down to the street or underground to prevent roadway flooding. Maintaining storm drain sewage and runoff is a full-time job. While it doesn't stop all rainwater, green roofs use the rain rather than feeding it into the sewer system.

Con: Expense

Designing and building a green roof is expensive. According to Sustainable Industries, a green roof will cost between $8 and $40 a square foot, including installation. Water barriers and retrofitting the roof to support an entire garden must be figured into the cost of installation. Modular units are less expensive than custom designed green roofs.

Con: Maintenance

A green roof doesn't do its job if all the plants die. Even with simple, hardy plants, landscaping costs must be considered.

Con: Repairs

With an ecosystem on the roof, finding and then repairing leaks or HVAC equipment becomes much more expensive. It means digging into the garden to expose the leak, repairing, then replanting.

Cons: Wildlife

Plants are part of an ecosystem. Insects, bugs, rodents and even small reptiles can and will set up house in green roofs. But they don't stop there. These creatures can end up burrowing or damaging the building.