If you have the chance to visit the observation deck of Chicago’s John Hancock Building, you will have an unobstructed 1,030-foot-high view of the city and the lakefront. After you take in the spectacular view, you will begin to notice certain details in the panorama below you. One of them is the large amount of rooftop gardens.
Chicago, which has more green roofs than any other American city, boasts more than 7 million square feet of rooftop garden space within its city limits. The city’s Green Roof Project has its roots – pun intended — in a concerted effort to keep the city cooler after a 1995 heat wave claimed the lives of some 700 residents.
What is a green roof and how can it benefit your family? Like its name implies, a green roof, sometimes called a rooftop garden, is a vegetative layer that is grown on a rooftop. According to the Environmental Protection Agency, (EPA), a green roof can provide shade and remove heat from the air through a process called “evapotranspiration,” basically reducing the temperatures of the roof surface and the air surrounding air. On a hot summer day, the surface temperature of a green roof can be cooler than the actual air temperature, whereas the surface of a conventional rooftop can be as much as 90°F warmer.
Green roofs can be installed on many locations, from large industrial complexes, to public buildings to private homes to homesteads. They can run the gamut from a park-like setting that is fully accessible to the public to a simple two-inch groundcover. In addition to the cooling advantage in the summer, here are some other benefits of a green roof:
- Reduced energy: Green roofs absorb heat and insulate buildings, thereby reducing energy — and the energy costs — needed for cooling and heating.
- Better quality air: Vegetation on a green roof works to remove air pollutants and to reduce greenhouse gas emissions.
- Reduced storm water runoff: Green roofs can slow storm water runoff, filter rainfall pollutants and help prevent flooding from heavy rains. Green roofs can absorb up to 90 percent of rainfall.
- Less noise: Homeowners report that green roofs absorb noise, helping to keep outside noise from coming in and helping to prevent inside noise from being heard outside.
- Longer wear: Greening your roof may help it to last longer. The plants help protect the roof from the sun’s ultraviolet rays and from other damaging elements, as well.
- Aesthetic value: Green roofs add beauty to the environment and provide food and/or a habitat for birds and other species.
According to the EPA, the cost of installing a green roof ranges from about $10 to $25 per square foot. Yearly maintenance costs range from $0.75 to $1.50 per square foot. Although these costs are higher than for a conventional roof, energy savings will offset those costs.
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There are two types of green roof designs. An “extensive” green roof has two to six inches of growing media and low-maintenance small plants, while an “intensive” green roof has several feet of growing media and much larger plants. Intensive green roofs, depending on soil depths and the substrate and other materials used, can increase the weight load on a roof by as much as 200 pounds per square foot. Plans for intensive green roofs must be approved by a structural engineer for safety.
An extensive green roof begins with a bottom layer of hot-applied rubberized asphalt or a cold-applied layer synthetic rubber, which is designed to protect the roof from damage from moisture and roots. A layer of pebbles or a geo-composite drain mat goes on top of this membrane layer to provide drainage. Then a filter cloth is placed on top of the drainage layer.
The uppermost and visible layer of an extensive green roof is the two to six inches of a growing medium, which hosts hardy small plants, such as sedum, talinum, chives and delosperma. Other options are a variety of herbs, grasses and wildflowers. Choices depend upon your climate, your growing medium and the soil depth you have for planting.
An extensive green roof is designed to require very little maintenance. Drought-resistant plants are used that require little to no watering after they are established and weeding is usually only necessary once or twice a year.
How can you tell if a green roof will work on your existing home? Prime characteristics are a sunny location mostly unshaded by trees and a flat roof with no more than 30 degrees slope. An extensive green roof will add more weight than a conventional roof – perhaps as much as 20 pounds per square foot – so you will need to consult a structural engineer and a professional that is familiar with green roofing.
Since many homeowners who are considering a green roof also are interested in solar panels, you may be able to do both. One way is to install a green roof on the flat sections of your roof and solar panels on the angled sections.
Here are some helpful sites for more information on green roofs:
Do you have a grass roof or would like one? Tell us why you want a green roof in the comments section below.