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What is a Green Roof?

Green roofs involve growing plants on rooftops, thus replacing the vegetated footprint that was destroyed when the building was constructed.

A large green roof above a light grey building - spans an entire city block
The convention center of the Church of Jesus Christ of Latter-day Saints in Salt Lake City, Utah; includes both intensive and extensive green roofs.

Germany is widely considered the leader in green roof research, technology and usage. It is estimated that 12% of all flat roofs in that country are green and the German green roof industry is growing 10% to 15% per year. Modern green roofs can be categorized as ‘intensive’ or ‘extensive’ systems depending on the plant material and planned usage for the roof area. Intensive green roofs utilize a wide variety of plant species that may include trees and shrubs, require deeper substrate layers (usually > 15 cm (6 in)), are generally limited to flat roofs, require ‘intense’ maintenance, and are often park-like areas accessible to the general public. In contrast, extensive roofs are limited to herbs, grasses, mosses, and drought tolerant succulents such as Sedum, can be sustained in a shallow substrate layer (< 10 cm (4 in)), require minimal maintenance, and are generally not accessible to the public.  What can be grown depends on such factors as climate, microclimate, substrate depth and composition, and whether supplemental irrigation is available.

A green roof covering a large garage.  Overlooked by a 3-story building.
An extensive green roof covers the garage providing an aesthetically pleasing view for the building occupants (Photo courtesy of Behrens Systementwicklung GmbH, Germany).
A green roof with large trees and other flowering plants.
An intensive green roof atop the Coast Plaza Hotel in Vancouver, British Columbia; has the appearance of a wooded forest.
A large grassy green roof.  White roof vents and 3 people can be seen in the picture.
An extensive green roof in Kassel, Germany