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 A green roof is a kind of building roof that is covered in vegetation. There are two kinds of green roof, extensive and intensive, categorized by their intended use and the nature of planting. The intensive green roof is named as such because it requires intensive maintenance and care. Intensive green roofs are more similar to garden plantings, involve soil depths suitable for a wide range of vegetation, and are often intended for occupation by roof users.
Extensive green roofs are usually accessible only for maintenance, and use planting strategies that require limited care and upkeep. Both kinds of roof reduce energy demand on buildings by providing insulation and absorbing heat from solar gain, and both can be used to augment evaporative cooling strategies. Green roofs fit into the cool roof concept.
 comment on this definition on the Index of Re-inventing Construction blog
Source: WhatWow
 Benefits for the individual buildings include improved energy efficiency and optimized thermal insulation; benefits at the city level included the mitigation of urban flooding; and also in terms of global warming through minimizing the consumption of fossil energy.
Roof policy
A conceptual project for a public policy to promote roof gardens (green roofs) on existing buildings in the city of Buenos Aires on a large scale won the Holcim Awards Gold 2005 in Latin America. The conceptual project aimed at improving urban policy is refreshingly innovative in proposing roof gardens throughout the city. Highly commended is the ambition to improve the conditions of individual buildings as well as more extensive environmental problems at a metropolitan scale.
Benefits for the buildings include a substantial improvement of the energy efficiency of housing, as well as increased comfort due to optimized thermal insulation. Benefits for the city include mitigation of urban flooding as a result of water retention as well as the reduction of heat islands due to vegetal transpiration. At a world scale, such measures have a positive influence in that they help to reduce global warming both by minimizing the consumption of fossil energy for heating and by controlling CO2 during plant growth.
Important is that the proposal can be applied equally to existing and new buildings. A beneficial social component of the work is that is promises the potential of creating new jobs in the region for construction and maintenance, but is not necessarily limited to this context. Not only demonstrating how the overall material quality of the city can be ameliorated, the project presents an ecologically responsible vision for large urban conglomerations.
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