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Another alternative to traditional roofing materials is a rooftop garden, or "green roof." On hot summer days, the surface temperature of a vegetated rooftop can be cooler than the air temperature, whereas the surface of a traditional rooftop can be up to 90°F (50°C) warmer.
What Are Green Roofs?
A green roof consists of vegetation and soil, or a growing medium, planted over a waterproofing membrane. Additional layers, such as a root barrier and drainage and irrigation systems may also be included.
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What Are the Benefits of Green Roofs?
Green roofs are an attractive roofing option that can reduce urban heat islands by providing shade and through evapotranspiration, the release of water from plants to the surrounding air. They also:
- Reduce sewage system loads by assimilating large amounts of rainwater.
- Absorb air pollution, collect airborne particulates, and store carbon.
- Protect underlying roof material by eliminating exposure to the sun's ultraviolet (UV) radiation and extreme daily temperature fluctuations.
- Serve as living environments that provide habitats for birds and other small animals.
- Offer an attractive alternative to traditional roofs, addressing growing concerns about urban quality of life.
- Reduce noise transfer from the outdoors.
- Insulate a building from extreme temperatures, mainly by keeping the building interior cool in the summer.
How Do Green Roofs Mitigate Storm Water Runoff?
As impermeable surfaces like buildings and pavement replace open space and vegetation, green roofs can play an increasingly important role in storm water management. During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off. Researchers estimate that three to five inches of soil or growing medium absorbs 75% of rain events that are one-half inch or less.
Green roofs also filter pollution from rainwater. This is achieved by the root systems' bacteria and fungi, which utilize the natural filtering processes of bioremediation and phytoremediation. As a result, the non-point source pollutants, nitrogen and phosphorus, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature.
What Kinds of Green Roofs Are Available?
There are two types of green roofs: intensive and extensive. Here are some characteristics of each:
Intensive
- Requires minimum of one foot of soil depth
- Accommodates large trees, shrubs, and well-maintained gardens
- Adds 80-150 pounds per square foot of load to building structure
- Regular access accommodated and encouraged
- Significant maintenance required
- Includes complex irrigation and drainage systems
Extensive
- Requires only 1 to 5 inches of soil depth
- Capable of including many kinds of vegetative ground cover and grasses
- Adds only 12-50 pounds per square foot depending on soil characteristics and the type of substrate
- Usually not designed for public accessibility
- Annual maintenance walks should be performed until plants fill in
- Irrigation and drainage systems are simple
Are Green Roofs Cost-Competitive with Traditional Roofing Options?
Currently, the up-front cost of an extensive green roof in the U.S. starts at about $8 per square foot, which includes materials, preparation work, and installation. In comparison, the cost of a traditional built-up roof starts at about $1.25 while cool roof membranes start at approximately $1.50 per square foot.
Extensive green roofs cost more than traditional roofs because they require more material and labor for installation. Another factor affecting price is that green roof contractors are limited in number. As the demand for rooftop gardens increases in the U.S., and as additional contractors come into business, up-front costs will likely decrease.
However, it is widely known that up-front costs do not tell the whole story. Taking into account future summertime energy savings at the time of purchase brings the price of a green roof closer to that of a traditional roof. Depending on local construction codes, it also may be possible to do without storm water infrastructure investments.
Another factor reducing the cost of a green roof is that vegetation can extend the life of a roof. This is because less solar energy reaches the roof substrate, limiting damage from UV radiation as well as daily temperature fluctuations, which cause repeated contraction and expansion.
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