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When we designed our new Interstate Maintenance and Operations Center building, among the sustainable features included in the plans was a 22,500-square-foot vegetative roof, called a green roof or ecoroof.
The vegetative roof offers two key benefits over a traditional roof:
It’s been two years since our North Interstate facilities buildings were complete, so let’s check in and see if the roof is delivering on these expected benefits.
Based on precipitation data from the United States Geological Survey (USGS), for the two rainy seasons Portland has experienced since the ecoroof was installed, approximately 1,430,634 gallons of rain water have been “treated” by the vegetative roof.
The Portland Bureau of Environmental Services includes green roofs as approved stormwater management facilities because much of the rain that falls on them is captured by the soil and plants growing on the vegetative roof.
The water that is eventually shed from a vegetative roof does so in a much more gradual fashion as the water must first filter through the ecoroof system. Reducing the speed that rainwater moves is a major factor in relieving strain on the city’s wastewater treatment facilities.
In addition to slowing down how fast water sheds from our Interstate facility's roof, the vegetative roof also filters organic debris and some chemical pollutants from stormwater runoff. This filtering action improves the overall quality of the stormwater before it enters the sewer system and is treated as wastewater.
Based on USGS precipitation data, the vegetative roof has already “treated” an amount of water equal to 35,764 baths at 40 gallons per bath. That’s a lot of water!
Did you know that a newly installed commercial roof has a functional life expectancy of 15 to 20 years?
The two reasons that cause a roof to fail over time are exposure to ultraviolet light and thermal cycling, which causes the expansion and contraction of the roof materials over the seasons.
An “extensive” vegetative roof, like the type installed at our North Interstate Facilitates building, covers more than 90 percent of the roof surface with three inches of lightweight soil and plants. Because of this, ultraviolet light exposure for the vast majority of the roof is reduced to zero.
Also, the soil and plants act as a blanket limiting the energy gain in the roof and minimizing the impacts of thermal cycling. Because the roof is protected by a cover of plants, the roof life expectancy may now be extended two times or more over the life of a traditional, uncovered roof.
“If the lifespan of the bureau’s roof is extended to 30 to 40 years from 15 to 20 years, that represents a significant savings to the Water Bureau over the life of the building,” notes Jon Crumrine of A-Tech/Northwest, Inc., the company that helped design and install our ecoroof.
And, after two years of service, we now have the data to begin examining and quantifying the benefits of our ecoroof.
With almost one-and-a-half million gallons of treated stormwater later, and a roof that is aging at half the usual rate, it’s safe to say that the vegetative roof is paying dividends or ratepayers and easing the burden of our City’s stormwater infrastructure.
The ecoroof is one of the many reasons our North Interstate Facilities were awarded with the LEED® Green Certification.
Read more about how this designation saves money, reduces costs, and lowers carbon emissions.