Pollution Prevention Fact Sheet: Green Rooftops
What is a green rooftop?
A green rooftop, also called a “Living Rooftop@, is a thin layer of vegetation installed on top of a flat or sloped roof (Figure 1). The vegetation can range from turfgrass to shrubs or even trees, depending on the climate and the load-bearing capacity of the roof.
Why use a green rooftop?
Rooftop area as a percent of total impervious area ranges from 30 to 35% in suburban development to as much as 70 to 75% in downtown business districts, and maybe as high as 80% in some warehouse/semi-industrial districts. Even partial control of these areas can reduce overall runoff volume by up to 50%. In highly urban areas, rooftops can be an unsightly element. Green rooftops and rooftop gardens provide an aesthetic appeal in the community.
In the summer, green rooftops retain 70 to 100% of the precipitation that falls on them; in the winter they retain 40 to 50%. Green rooftops can reduce the total annual runoff volume by 50 to 60%.
What are green rooftops made of?
A green rooftop typically consists of several layers, including a waterproofing membrane, insulation, protection layer, drainage layer, filter mat, soil layer, and vegetation (Figure 2). Green rooftops may have an internal drainage network that directs an overflow away from the roof to inhibit ponding. Turf based green rooftops range in weight from 5 to 30 lbs per square foot, while rooftop gardens typically range from 40 to 100 lbs per square foot. The thickness of a green rooftop can range from 1 inch of soil to over 75 inches of soil, depending on the intended use and load-bearing capacity of the roof. Green rooftops can be built on roofs with slopes up to 30 degrees.
How do I get a green rooftop?
If the projected live load of a green rooftop is greater than 17 lbs per square foot, consultation with a structural engineer is required. Rooftop gardens with large trees and seating areas (Figure 3) will require more structural support so it is recommended to consult a professional before installing a green rooftop.
How much does a green rooftop cost?
The initial cost of a green roof can be 30% greater than a conventional roof, but long term maintenance and energy cost savings can offset this cost increase by a lifespan increase of 50%. Two green roof projects in the city of Toronto ranged in cost from $33 per square foot to $55 per square foot for re-roofing and green roof installation. One source states the green rooftops prolong the life of a conventional roof by 20 years (www.greenroofs.com). Green rooftops can be warranted up to 15 years, and the plant material is typically warranted for one growing season.
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