What is a “green roof” and are they helpful to the environment?

Green roofs are vegetated rooftops, and they are gaining popularity in the United States. They are designed to alleviate some common problems in urban areas:

  • elevated air temperatures,
  • flooding, and
  • air and water pollution.

Out in the country, when rain falls, or snow melts, the water comes into direct contact with the ground. This precipitation is meant to move through the soil.

Flowers on a green roof in Chicago
Green roofs, like this one in Chicago, help capture water that might otherwise overwhelm the storm water system. Credit: B Dvorak

But, cities have rooftops, paved streets and sidewalks and other “impervious surfaces” that disturb this natural water cycle. Green roofs catch stormwater that would otherwise flow off roofs into rain gutters. This reduces urban flooding. The plants in green roofs capture carbon and clean the air. Green rooftops reduce energy use and help manage urban heat islands. They can also provide islands of habitat for urban wildlife, and even produce food in places where the land only long ago supported farming.

Green roof technology is still under development in the United States; however, research and innovation continues to expand the possibilities and benefits of urban greening with green roofs.

Green roofs are designated as extensive, semi-intensive, or intensive. The designation depends on their depth, type of vegetative cover, and function.

Variety of plants on a green roof
A diversity of plants on green roofs can help create an ecosystem that is good for the environment. Credit: B Dvorak

Extensive green roofs are low-maintenance. They have shallow soils or other growing media that are well-suited for low-growing plants. They primarily function as an ecological protection layer, such as a filter for stormwater. These green roofs are typically maintained at only 3 to 5 inches high and are not usually irrigated. This helps extensive green roofs to be well-suited for roofs with little load bearing capacity.

In contrast to extensive green roofs, intensive green roofs contain a greater variety of plant types and require greater maintenance and inputs of fertilizer and water. An intensive green roof is designed to look much like a city park. Green roofs used for rooftop farming are one example of an intensive system. Intensive green roofs require a roof with high load bearing capacity, as they may weigh 35 to 80+ pounds per square foot. They will also require a permanent irrigation system and are the most labor-intensive and expensive green roof type.

5 benefits of green roofs infographic
Besides looking nice and providing an outdoor space, green roofs can save energy and help capture and clean water.

In between extensive and intensive are semi-intensive green roofs, which are grown 5 to 7 inches high with perennials, small shrubs, and ornamental grasses. They do not contain tall-growing bushes or trees. Semi-intensive green roofs have moderate watering and maintenance needs and are suited for roofs that can support 25 to 40 pounds per square foot of load. They function as a stormwater filters and as habitat for urban wildlife.

The soil beneath the plants is typically composed of an engineered growing medium. The medium is high in minerals and low in organic matter (less than 8%). Research on the best growing media for green roofs was pioneered in Germany and Switzerland. During the 1980s, there were major advancements to development of lightweight growing mediums. This allowed green roofs to become part of standard building construction worldwide.

Read more about information about Green Roofs on Discover Soils here.

Answered by Mary Lusk, University of Florida

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