Green is one good color. In the pocket, it denotes wealth. In the yard, it portends fertility. But on the roof, it causes delay.
Oh, but delay is a good thing.
A University of Pittsburgh study, just completed, provides reams of evidence that green roofs successfully delay water runoff from roofs. Such delays can help prevent rainwater and sewage overflow into combined municipal and the Allegheny County Sanitary Authority sewer lines.
When overflows occur, municipalities and Alcosan must divert polluted water into streams and rivers. Correcting that problem in the next five years could cost $3 billion to $10 billion for Alcosan and its 83 member communities, according to Alcosan and Allegheny County projections.
But Pitt's study of green roofs atop Giant Eagle grocery stores in Shadyside and Homestead offers one natural method to help reduce overflow during the average rainfall. The study, with Dr. Ronald Neufeld, a Pitt environmental engineer, as principal investigator, revealed that green roofs not only reduce runoff but push back the peak water flow after a rainfall by hours, reducing flow to sewer pipes.
Three Rivers Wet Weather, a nonprofit organization affiliated with the Allegheny County Health Department, funded the $355,000 study. The organization assists municipalities in dealing with water-quality impacts on public health.
Withholding water atop a roof after a downpour spreads out water flow over a longer period of time, giving municipal systems and Alcosan additional time to manage water flow and get waste water to Alcosan successfully for treatment. The net result is that less sewage overflow ends up in local rivers.
"Nothing I've seen in the data suggests that green roofs are not a positive way to realize energy savings, control storm-water runoff and reduce the frequency of wet-water overflows, especially in average rainfall," said John Schombert, Three Rivers executive director. "It really can help solve the small-rain events."
Pitt used sensors and other equipment to monitor temperatures and water flow on the roofs. Sedum, planted on the roofs, can withstand extreme temperatures and weather conditions on a rooftop.
City pavement and concrete, along with traditional roofing, do not absorb water nor delay its flow during rainfall. But soil and plants do.
From April 2006 to April 2007, sensors and data loggers at the two stores recorded 24 storms ranging from 0.07 inches to 2.2 inches of rain. From April 2008 to April 2009, Pitt recorded 95 storms ranging from 0.02 inches to 2.42 inches of precipitation.
The study determined that soil and greenery on the roofs retained water for up to three hours or longer, compared with traditional roofing, which sends water straight to the downspout.
The Giant Eagle on Centre Avenue, Shadyside, has a 41/2-inch layer of soil on 12,000 square feet of its roof. Another area of roof was left with gravel covering a traditional roofing membrane for comparison.
The older structure housing the Giant Eagle in Homestead used a different technology. There a series of square modules with egg-carton-shaped containers inside were covered with a 11/2-inch layer of soil used to grow plants. The modules were lifted onto the slanted roof with a crane and positioned in a fashion similar to laying sod.
The thick and thin roofs over the Giant Eagle stores improved water quality and offered some insulation benefit for the building, although the thicker roof showed better results in water retention. Still, the thin roof showed that green roofs atop older buildings offer welcome results.
"The results demonstrated that in comparison to the conventional roofs, green roofs retained significantly more water, moderated temperature increases and decreases of the roof, and had marginal effect on the chemistry of the discharged runoff," the study states.
In particular, the green roofs showed an ability to neutralize acidity.
The study also showed that soil and plants not only delay the flow of rainwater, but also retain a large portion of the water indefinitely. Green roofs, however, cannot prevent runoff when rainfall is torrential.
The green-roof study, exceeding 300 pages, adds hard science to the discussion of green-roof technology, Dr. Neufeld said.
"This might be one technology suitable for future application," he said. "It retains rainwater until a point of saturation. The peak of rain is diminished. The peak is pushed back and overflow is diminished."
The Pitt study also showed that a green roof can moderate roof temperatures by absorbing solar energy. Resulting evaporation of moisture in soil and plants cools the roof.
"Anytime you can reduce runoff and the pollutants they carry into waterways, that's a plus," said Nancy Barylak, Alcosan spokeswoman, noting the high cost the region faces to correct the overflow problem. "This is going to be a big issue, and it's going to impact everyone."
David Templeton can be reached at firstname.lastname@example.org or 412-263-1578.