After 10 years of evolution, the green roof of the American Society of Landscape Architects is producing a new and varied crop.

We recently came across this piece by Brittany Patterson at E&E Publishing on green roofs in the nation’s capital and their enormous (and necessary) benefits, which was originally published behind E&E’s paywall. E&E, which does excellent daily reporting on climate change and energy issues, has kindly allowed us to repost the article in full.

NATION’S CAPITAL BECOMES GREEN ROOF CAPITAL TO FIGHT EXTREME HEAT, HEAVY STORMS

BRITTANY PATTERSON, E&E PUBLISHING, LLC, JUNE 9, 2015

Nestled on Eye Street in downtown Washington, D.C., near the heart of the bustling city lies the headquarters of the American Society of Landscape Architects (ASLA).

From the front, the brick building looks like any other in the neighborhood, but take the elevator and a flight of stairs to the roof and you’ll find yourself surrounded by rows of green Sedum, blooming prickly pear cactus, and patches of lush butterfly milkweed and hare’s-foot clover. It’s almost possible to imagine you are sitting in the tranquil countryside, not just on the roof of a building covered in foliage.

As relaxing as they can be, green roofs are more than just easy on the eyes.

“Green roofs deliver multiple benefits for both combating heat and in the retention of stormwater,” said Kate Johnson, a program analyst with the District Department of the Environment (DDOE). “Both are issues we think are going to continue to be important in light of climate change. It’s projected to get hotter, and it’s projected we’ll have more extreme rain events.”

ASLA’s green roof captured 75 percent of the 29 inches of rain between July and May of 2007.

Now in its 10th growing season, ASLA’s green roof has weathered it all and has provided an immense amount of data on the environmental impacts of a leafy building canopy.

The data—the roof caught nearly 75 percent of 29 inches of rain that fell between July and May of 2007 and was as much as 32 degrees cooler than black roofs in the neighborhood—is consistent with benefits observed around the world.

As cities search for climate change mitigation strategies, green roofs have emerged as a way to chill an urban area’s core temperature, collect stormwater, clean up local air pollution, and reduce a building’s energy needs for a relatively small cost.

Stormwater regulations help spur boom

Last month, a survey released by the group Green Roofs for Healthy Cities found that the nation’s capital leads North America in green roof installations, with 1.2 million square feet installed in 2014.

In the early 2000s, through partnerships with a number of nonprofits and community groups, the district began offering subsidies for the cost of green roof installations through a program called RiverSmart Rooftops. The 2014–2015 program, which is being administered by the Maryland-based Anacostia Watershed Society, is offering $10 per square foot on properties of any kind, including residential, commercial, and institutional ones, and of all sizes. In some areas, up to $15 per square foot is an option.

The current program is set to run for three years. Based on past demand, DDOE allocated $100,000 in grant money per year, but program manager Stephen Reiling said the program isn’t even through the first year, and already money is being added to the budget.

“The demand’s been much higher,” he said.

The flurry of activity can partly be attributed to new stormwater regulations passed by the district in 2013, under which all new buildings constructed in the downtown area must retain the first 1.2 inches of rainwater.

“Especially downtown with dense development, it can be hard for developers to meet that,” he said. “Green roofs have become one of the best ways given the limited space.”

But aside from new regulations, Reiling said it appears there is an increasing interest in green roofs. DDOE doesn’t typically give rebate dollars to new developers in the downtown area that fall under these new restrictions. Instead, it’s targeting those who are retrofitting. The traditional roof has a life span between 20 and 30 years, and the idea is to “catch people in that sweet spot where they want to install a new roof,” he said. As an added bonus, studies have found green roofs have a life span closer to 40 years.

RiverSmart Rooftops is administered through DDOE’s Watershed Protection Division, which means the program’s mission is to help to reduce stormwater runoff, a problem that D.C. and many other older cities battle. As cities expand, more surface area is covered in impervious materials like roads, parking lots, or buildings that prevent water from being absorbed into the ground. In the parts of the district where the pipes are designed to carry both stormwater and sewage to the treatment plant, the overflow pours into Rock Creek and the Potomac and Anacostia rivers.

Most storms in the D.C. region unleash between 1 and 2 inches of rain, and 4-inch green roofs, the most commonly constructed thickness, are designed to catch just that amount.

“On our end, in the branch where I’m at, we’re looking at green roofs for stormwater control, volume reductions, and water quality,” Reiling said. “There are habitat and urban heat island effect benefits, as well.”

Offsetting the need for power plants

Cities are an appropriate target for green roofs partly because of the urban heat island effect, which can threaten lives during hot periods. The effect occurs when dark areas like black asphalt prevalent in cities absorb large amounts of heat.

According to a recent study commissioned by DDOE assessing the health impacts of different urban heat island reduction strategies, a 10 percent increase in vegetative cover can reduce mortality during heat events by 7 percent. Between 1948 and 2011, an average of 285 people died of heat-related causes in Washington, D.C. More vegetative cover could save approximately 20 lives per decade, the study found.

Green roofs also trap particulate matter, which can have a positive effect on regional air pollution levels. But the easiest way to sell green roofs to building owners and developers may be to cite their energy cost savings. Local climate dictates how big the cost savings will be, but in general, when green roofs are wet, they absorb and store large amounts of heat. When dry, green roof layers act as an insulator, decreasing the flow of heat through the roof, thereby reducing the amount of air-conditioning needed to keep the inside cool.

One study in central Florida measured year-round energy savings from a green roof on a 3,300-square-foot building. By the roof’s second summer, the average rate of heat absorbed through the green roof was more than 40 percent less than for an adjacent building’s light-colored roof. That reduction was estimated to lower summertime energy use by approximately 2 kilowatt-hours per day. Under winter heating conditions, the amount of heat transferred into the building was almost 50 percent less for the green roof than for the conventional roof.

“By saving energy, green roofs can help reduce emissions by offsetting the need for power plants, one of the biggest sources of particulates,” Johnson said.

Seeing greenbacks in green roofs

Green roofs also are having an economic ripple effect. Emory Knoll Farms owner Ed Snodgrass, for example, has farmed many different commodities over the course of his lifetime, from dairy cows to corn to soybeans. Nothing has done so well for him as what he cultivates now at his Maryland business—plants specifically designed for green roofs.

Today, Snodgrass has 10 employees on the payroll, and over 15 years, he has facilitated the installation of more than 1,400 green roofs covering more than 7.6 million square feet.

He said the general acceptance for green roofs has increased, in part due to increased stormwater regulations. Surprisingly, the very people who were once skeptical are one of the driving forces.

“In the beginning, I had roofers saying, ‘I spent my career getting water off of roofs; now you want me to keep it on,'” Snodgrass said. “Now, they’re the biggest advocates.”

The easiest candidates for the installation of green roofs are new buildings belonging to institutions that intend to occupy the space for an extended period of time, like schools, government buildings, and hotels.

The process for installing a green roof is fairly simple. Once a roof is structurally sound enough to carry the extra weight from the soil and plants of a green roof, it’s insulated and waterproofed, and a layer of water-absorbing material is added. That is followed by drainage and filtering mechanisms and soil and plants.

In addition to installing green roofs, the nursery stocks more than 100 varieties of green roof plants including Sedum, ground covers, herbaceous perennials, and grasses, specially designed to thrive in shallow and water-extreme conditions.

“These plants have to be real survivors,” he said.

The whole roof works together as a system, Snodgrass said, and as a result, its benefits come in aggregate, not from a single aspect of the roof.

“They catch stormwater, reduce energy costs of the building, extend the life of the roof’s waterproofing membrane because they protect it from sunlight, they’re nice to look at and an ecosystem for pollinators,” he said.

Not every roof can go green, however. The standard 4-inch green roof adds between 25 and 30 pounds of weight per square foot, and some roofs cannot be engineered to handle the stress.

They’re also expensive, costing anywhere between $10 and $30 a foot to install, as opposed to between $5 and $7 for the typical shingle roof.

Reiling at the DDOE said he thinks the price will fall once more people adopt green roofs.

“At some point, it’s going to become standard practice, and we’ll stop incentivizing it,” he said.

Copyright 2015, E&E Publishing, LLC. This article was reprinted from ClimateWire with permission of E&E Publishing, LLC, www.eenews.net.