Cities want to make your rooftop gardens profitable

A new EPA study predicts that as cities grow hotter, replacing flat black rooftops with plants could cool those cities back down.

On a recent Saturday morning in the Lincoln Heights neighborhood of Los Angeles, the city's River Revitalization Corporation is showing off a plan to add green space to an area that's now dominated by heavy industry. And Lee Christie of architecture firm Perkins+Will is explaining some options.

"You'd be looking at more raised beds or more greenhouses," she says, "which really opens up the possibility for rooftops."

Rooftops have a lot of hidden potential. A new EPA study predicts that as cities grow hotter, replacing flat black rooftops with plants could cool the cities back down.

According to Phil Morefield, one of the co-authors of that study, "any sort of well-designed, well-maintained green roof will give some benefit for the building that it's installed on."

Those benefits include reversing urban warming, absorbing rain before it overburdens sewers, and providing habitat for butterflies.

There's just one problem: green roofs come with a big up-front cost. So now, some cities are experimenting with financial encouragement.

For example, Austin lets developers build more floor space if they include green roofs. And Seattle gives out credits and discounts for rooftop gardens.

"Properties that take advantage of that credit range from single family homes to a regional airport," says Seattle Urban Designer Dave LaClergue, "so they're very different in size and scale."

The gardens on top of the Chloe Apartments in Seattle. That's the Space Needle on the right.

But it's been a learning process. Many of Seattle's first rooftop gardens died, since they were designed with assumptions based on what worked in east-coast cities. And a garden that cools one city might have less of an effect in another.

"There really isn't a 'one-size fits all' strategy," says Britta Bierwagen, another co-author on that EPA study. "And there are a lot of things to consider."

Financial incentives are similarly fickle. In Nashville, a credit of $10 per square foot of green roof hasn't attracted a single taker. But Portland, Oregon, got an overwhelming response to a credit of just half that much.

That's because green roofs need to be customized to what the market in each city wants, as much as to the weather. One market might respond best to grants, while another prefers tax credits.

For example, in Portland, the incentive amount was determined in part by the region's damp climate and sewers that combine wastewater with stormwater. "It was the amount that we could apply that essentially would cost less to manage a gallon with a green roof than it would with a pipe," explains Portland Environmental Program Coordinator Matt Burlin.

And green roof incentives aren't just for major cities. In tiny Saluda, North Carolina, the Polk County Community Foundation provided a $6,000 grant for a green roof on the new restrooms at Pearson's Falls.

On a recent afternoon, foundation President and CEO Elizabeth Nager stopped by to see how the plants are filling in. It's looking good: "The roof resembles the forest floor in the glenn below the waterfall," she observes. "There are smooth rocks that fill the space where you expect to see traditional gutters."

It's more than just a few rocks and sage bushes, of course. It's part of a national experiment that's happening right over our heads.

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