There are plenty of obstacles to overcome when convincing developers — and the public — that green roofs are better than a rooftop swimming pool, garage or plain old shingles.

But two municipalities in the Chesapeake Bay watershed are throwing everything they’ve got at the challenge, with the hope that green roofs — as well as rain gardens, bioswales and a host of other water-absorbing practices — will help to reduce the amount of polluted stormwater flowing off their concrete landscapes to the nearest river.

Now that the District of Columbia and Maryland’s Prince George’s County have rolled out programs meant to encourage more such installations, are they seeing the predicted deluge of new projects?

These were among the questions circulating at the 14th annual CitiesAlive Conference in November in Washington, DC, where the nation’s capital was highlighted to an international audience of 500 as a leader in green roof implementation. The city has installed more square feet of green roofs than any other in North America for each of the last five years.

Green roofs cover a waterproofing membrane with layers of soil and vegetation, ranging from low-lying sedum to small trees and shrubs. Originally recognized for their ability to reduce the heat island effect in urban areas, green roofs are emerging as a timely tool for cities looking to absorb their stormwater runoff, still the fastest growing source of water pollution in the Bay watershed.

“Stormwater is what the industry wanted to talk about,” said Sara Loveland, co-chair of the conference and partner at the District-based consulting firm Annette Environmental Inc. “This is what the industry leaders told us is driving green roofs.”

That’s particularly true in the nation’s capital, where since 2014, stormwater regulations have required new developments to retain the first 1.2 inches of rain that falls in a storm. In a city already crowded with concrete — and prevented from growing skyward by building height restrictions — experts say green roofs are often the best option to absorb water with limited space.

“If you’re a downtown developer, you basically have to do a green roof on your lot to capture rainfall there,” said Jeffrey Seltzer, associate director of the District Department of Energy & Environment’s stormwater management division.

While green roofs might be the developer’s best onsite option, they also can take credit for green infrastructure installed elsewhere in the city through the District’s innovative stormwater credit trading program. At an average of $15 to $20 per square foot, green roofs are one of the most expensive ways to absorb stormwater, and, after their soils are saturated, they can’t absorb as much water during a heavy rainfall as a large water retention pond in a less developed area.

If, for this and other reasons, a developer would rather have a swimming pool on the roof of a new apartment complex than a sea of sedum, the company can purchase credits from a property that is absorbing more than its share of stormwater through the city’s burgeoning market to meet its regulatory requirement.

In this way, “the District has assigned a financial value to stormwater retention,” said Kahlil Kettering, urban conservation director with The Nature Conservancy. That, he said, makes the city “ground zero for this work.”

It’s why his national nonprofit will be breaking ground on a first-of-its-kind project in the coming months focused on absorbing stormwater with a suite of best practices so it can generate credits for other properties to purchase. The conservancy and its partner in the project, Encourage Capital, received a $1.7 million private investment from Prudential Financial, Inc., earlier this year that will help fund the project upfront, though they haven’t announced additional details about its location or scale.

A secondary benefit of the trading program is that it could help place green infrastructure — along with its societal benefits of reducing heat and increasing green space — in low-income areas of the city that might not see such investment otherwise. But that altruistic aspect of the program is also one of its obstacles.

Jane Silfen, an associate at Encourage Capital and partner with the conservancy in their green infrastructure-focused District Stormwater LLC project, said one of the challenges is one has to build the green infrastructure before one has the credits in hand that will pay for it.

“That creates a near-term investment problem,” said Silfen. “You’ve got to build the green infrastructure before you can generate credits.”

Matthew Espie, environmental protection specialist at the DOEE said that supply of credits is beginning to slowly stack up. In addition to the District Stormwater’s work, another organization is pursuing a project of similar scale intended to generate credits specifically for the trading market, though details have yet to be announced.

Both projects picked up speed after the District announced in September that it was working with a nonprofit, the Center for Watershed Protection, based in Ellicott City, MD, to spend $13 million on a credit-purchasing program in case the market was not ready to absorb them when they came online.

But projects don’t have to be built just to generate stormwater retention credits to participate in the program. About 85 percent of the projects that must meet stormwater requirements have exceeded their regulations so far, making them eligible to generate credits that could be sold to the other 15 percent of projects that are choosing to purchase credits.

As of late November, the program had 178,500 credits — one credit equals to one gallon of water retention per year — available for purchase by the 35 projects that are going to need to acquire credits soon.

“We have enough supply to meet the demand for quite a while,” Espie said.

Experts at the green roofs conference praised the program for its ability to engage private industry to help meet the city’s stormwater goals, which call for retrofitting 30 percent of its concrete landscape with green infrastructure by 2040.

“The system uses current economic development to help pay for the sins of past economic activity,” said Daniel Nees, director of the environmental management center of the University of Maryland.

Some aspects of the program, though, have progressed more slowly than advocates initially hoped. Doug Siglin, executive director of the Anacostia Waterfront Trust, said his organization has tried to get into the business of generating credits for the program, but the startup obstacles — and costs — are formidable.

“You have to acknowledge the upfront management costs” to building projects that generate credits, Siglin said. “Investors, so far, don’t want to pay those costs; they want to finance the projects.”

Siglin said the business opportunities are still promising, especially in a quickly redeveloping city like the District, but that more nonprofits, investors and agencies need to help lay the groundwork.

DOEE director Tommy Wells said the credit trading has been successful so far and that the approach works best in a city that’s rebuilding as quickly as the District. But neighboring Prince George’s County in Maryland has found that working with private industry is just as key when trying to absorb stormwater on 500 urban and suburban acres.

Adam Ortiz, director of the county’s Department of the Environment, said clean water regulations mean his jurisdiction has to retrofit about 15,000 acres in the next 10 years with tens of thousands of rain gardens, green roofs and water-retaining projects.

“One of the challenges is government does not always do things quickly,” Ortiz said. “Every dollar is precious in government, and we can spend that dollar to leverage other social benefits if we do it thoughtfully.”

Prince George’s has about $1.2 billion in the bank to spend on retrofits, generated from a stormwater management fee initially required, but now merely permitted, by state law, to see that the state’s largest jurisdictions have adequate resources to reduce polluted runoff to the Bay.

By outsourcing these projects to private construction companies, Ortiz said the county was able to cut in half the price-per-acre of new stormwater controls. The county expects to have 950 acres of new projects constructed this year alone.

University of Marylands’s Nees and other experts say the entire country, and especially the Chesapeake Bay region, is watching these localities to see how their public-private approach pans out.

Other communities, such as the 2,700 municipalities in Pennsylvania, might have to lean even harder on the private sector to meet their obligations without the fees that have generated funds in other areas, Nees suggested.  The best path forward, he suggested, comes by “turning these issues not into an environmental cost but an economic investment. When we’re able to do that, we’ll solve the big problems.”