City roof gardens create a nice natural habitat for pollinators and migrating species, and additionally “they provide open green spaces for property owners and the public to enjoy.” Green roofs are known to improve air and water quality, lessen storm-water runoff, lower building energy consumption, and reduce urban heat island affect.

Green roofs are constructed as a series of layers including:

• a waterproof membrane
• a root barrier
• drainage mat
• an erosion control fabric
• lightweight engineered soil, and vegetation.

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Contemporary vegetated roofs have little in common with old "earth sheltered" buildings of the 70s. A vegetated roof is an integrated system, with everything engineered for its performance in the system from the roof membranes which keep water from entering the building to the "growth media" engineered soil that sustains the plants.

But, while roofs are the easiest surface to consider greening, they aren't the only surfaces that are being greened. A number of buildings are now sporting vegetated walls, as well. And these new green walls are something more than just ivy covered buildings of yore.

The current darling of green walls is the new Museum du Quai Branly in Paris, France, a project by the noted architect Jean Nouvel which incorporates 8600 square feet of living wall with more than 170 different species. Nouvel's collaborator, Patrick Blanc has been installing his Plant Walls for more than 10 years, but the prominence of this recent project has brought a flood of new attention to the concept.

These green walls use the plants as the exterior surface of the building, over construction materials that keep moisture out of the building and retain heat in a more typical fashion. The plant material protects the building from sun and rain in a manner similar to a rainscreen.

Green walls are not only for building exteriors, either. Beamish-Munro Hall at Queens University in Kingston, Ontario, Canada has an indoor biowall to serve as a living air filter for the building to reduce CO2 and VOCs in the building's air. An indoor biowall can use a wider range of plants, since the hardiness of the plants is less of a concern inside a controlled environment.

Photo Credit: Takenaka CorporationGreen concrete is another surface greening method being developed in Japan. This system is currently being tested for steep slope embankments, where erosion would remove ordinary soils and plants. It would also be possible to build buildings with precast concrete walls which were covered with green, growing plants.

Green building surfaces allow for more plants to exist in the urban environment. They add the air cleaning properties of plants to a loaction where plants have been removed in order to build a building. Green building surfaces can provide a more amenable habitat for local wildlife. A green roof will provide more habitat for insects and birds than a conventionally constructed roof. Green roofs help to slow water runoff from buildings which would otherwise go into storm sewers. They also reduce heat island effect and aid in making cities a little bit cooler in the summertime. Green surfaces aren't a requirement for a building to be green. But they can certainly provide a number of benefits, from the aesthetic to the practical.