Twin Cities residents walk past Angie Durhman’s work every day, but few of them are likely to notice it.

Durhman’s one-woman business designs and maintains greenroofs — roofs that are topped with a layer of vegetation — in the metro area and nationally.

Greenroofs have become increasingly common, as developers pursue sustainable design incentives and environmental certification. Durhman is part of an industry that is growing up around this trend.

“It’s still nichey,” Durhman said. “But the general public usually knows what a greenroof is now, even if they haven’t installed one before.”

Durhman has been designing and maintaining greenroofs in the Twin Cities for a year, under the name AD Greenroof, based in Minneapolis.

After spending seven years managing greenroof construction and maintenance for Tecta America, an Illinois-based contractor that entered the industry when it was in its infancy, she decided to move to the Twin Cities in 2012.

She now maintains one residential and two commercial greenroofs in the metro, including 2 1/2 acres on top of the Target Center arena in Minneapolis — one of the largest greenroofs in the country. But she’s hoping to contract for several more.

“They don’t need a whole lot of maintenance, but they do need some so the weeds don’t take over,” said Nathalie Shanstrom, a landscape architect with Kestrel Design Group, which designed the Target Center’s greenroof.

During the months it isn’t blanketed by snow, Durhman visits it and the other greenroofs she maintains periodically to ensure the vegetation cover is holding up.

During the rest of the year, Durhman has kept busy with the design side of her business — greenroofs can take up to three years to implement. Presently, she’s designing two greenroofs, one in Florida and one in Nebraska.

From a distance, a greenroof might not look much different than an average backyard, but it’s very different — right down to the soil.

Rather than the loamy topsoil found at ground level in much of Minnesota, a greenroof is rooted in a layer of pea-sized, porous rocks. These absorb and retain water, and are less likely to blow away than soil.

And the vegetation isn’t standard Minnesota fare, either.

“Greenroof systems … have to grow in more of an extreme climate,” Durhman said. “You have to think in terms of alpine conditions.”

She says the plants that thrive on a greenroof are usually heartier succulents called sedums, or stone crops like chives.

The average cost for such a roof can vary greatly. Durhman says they average about $25 per square foot for a new roof, and $12-18 per square foot if built on an existing roof.

Durhman estimates there are roughly 200 such roofs in Minnesota. But that’s growing.

In the decade since she graduated from the University of Michigan with a master’s degree in horticulture, the greenroof industry has taken off. It grew 24 percent in 2012, the most recent report by Green Roofs for Healthy Cities, a nonprofit focussing on encouraging the trend.

“Many cities across North America have recognized the public benefits of greenroofs and have taken various policy measures to encourage their widespread installation,” the organization reported in its Annual Greenroof Industry Survey. “However, there is still enormous potential for growth of new green roofs on tens of billions of square feet of buildings across North America.”

In Minnesota, anyone building an urban structure that will occupy an acre or more must obtain a construction stormwater permit.

To be granted the permit, the Minnesota Pollution Control Agency requires such structures to be designed to retain at least one inch of stormwater per rain event. This standard is aimed at minimizing the pollutants that leech into lakes and rivers when it rains, and a greenroof is designed to do exactly that.

And there are other incentives. Greenroofs earn builders points toward LEED — Leadership in Energy and Environmental Design — certification.

Durhman says greenroofs make buildings more energy-efficient by absorbing sunlight during the summer and acting as an extra layer of insulation in winter.

Durhman says buildings can earn about four LEED points for installing a greenroof.

The Penfield, a recently completed mixed-use development in St. Paul’s downtown, is topped with a greenroof that doubles as a communal “backyard” for its apartment tenants.

The Penfield has applied for LEED Silver certification.

Although she doesn’t maintain the Penfield greenroof, Durhman was hired by the city of St. Paul, which subsidized the Penfield construction, to conduct a study of the roof. She takes regular measurements of rainwater runoff, soil temperature and relative humidity.

At the end of the year, Durhman will compile a report for city officials, which they’ll use to inform decisions on whether to fund similar projects.

As greenroofs become increasingly common, Durhman is hoping to increase the maintenance component of her business.

Although the Target Center roof is a significant source of income, she estimates she would need at least 10 Target-sized roofs to give up her design work and make maintenance her full-time job. But there few maintenance jobs to be had.

But, fortunately for Durhman, she doesn’t have much competition. Yet.

“There are a lot of people who are trying to get into the market now,” Durhman said. “They might not understand that it’s still a lot of hype, and not that many actual projects.”

Durhman depends on her decade of experience in the industry to compete for these limited opportunities. Since the industry is relatively young, she says, there aren’t many contractors with her level of experience.

“There’s a small handful of us across the country,” she said. “But that’s increasing.”

Nick Woltman can be reached at 651-228-5189.

Follow him on Twitter at @nickwoltman.