Green roofs keep houses — and cities — cooler

Green roofs keep houses -- and cities -- cooler

Admission: $15,

"The basic idea is that it’s a really low-maintenance roof garden," said ecologist Colleen Butler, who will lead a discussion on green roof design Wednesday at City Park While a traditional roof garden would be a space for people to enjoy, "The primary function of a green roof would be for environmental purposes," she added.One of the main purposes is flood control. "The rain falls on the roof and gets absorbed by the soil of the green roof," Butler said. "It prevents the water from becoming storm water, and then having the municipality have to deal with that water with pipes and drains and pumps."

Another benefit is cooling. Cities are hotter than the surrounding natural areas. "In a natural region like a forest or a wetland, you have rain hitting the ground, and then it goes into the soil and is taken in by the plants." Plants, Butler said, "consume crazy amounts of water, and they evaporate that water out."

That cooling effect is called evapotranspiration; it’s akin to the way sweating cools our skin, but on a landscape scale, she said.

"So if we have water falling on buildings, the soil absorbing that, plants using that, then that can have a major cooling effect," Butler said. "And not only for the buildings, but also if we have enough green roofs in an area, it would have a cooling effect for that whole area."

And there are softer benefits. "Just the aesthetics. People are just happier when they have access to nature," she said.

From the bottom up, a typical green roof begins with some sort of waterproofing membrane. On top of that is a drainage layer, which could be something as simple as pebbles or stones.

Next should come a filter fabric, and on top of that comes the soil. Well. "I wouldn’t quite call it soil," Butler said. "It’s not really like a garden soil you would think of. It tends to be called ‘growing media’ or ‘substrate,’ and is very fast draining. It has a lot of larger particles of things like expanded shale and expanded clay, so it’s not going to get compacted like a topsoil would, and it also helps drainage."

And into that go the plants. "And that’s basically the whole thing," she said. "It doesn’t really have to be that complicated."

For green roofs in New Orleans, Butler in general suggests slow-growing, succulent, fleshy plants.

"We grow something called Side Oats Gramma, which is drought resistant but also can thrive in long periods of rainfall, which is kind of what New Orleans has," said Michele Pyne, Green Building program associate for Global Green USA. Global Green operates a demonstration house — part of its Holy Cross Project — at 409 Andry St. in the 9th Ward, which has one of the very few green roofs in New Orleans.

It’s also important to remember that a rooftop is a much harsher environment than the ground, Butler said, "so even if you’re a master gardener on the ground, you have to be extra conservative when you’re translating that onto a roof."

Green roofs have been around since the 1950s in Germany, but are now catching on in America in cities like Chicago, Portland and Seattle, Butler said. The delay has a lot to do with incentive, as building owners in Germany are charged for the storm water they produce. Chicago’s green-roof boom was driven largely by a former mayor, and in Portland, a similar push was made by a single city planner. Absent that type of individual will, Butler said, it will likely take some sort of policy changes to get the ball rolling here.

But she remains optimistic. Green roofs have become "a real symbol of sustainability," she said, "and there’s been a big explosion in green-roof designers and researchers and companies, so it’s kind of an exciting time right now."


To see the Global Green green roof in person, the Holy Cross Visitor Center offers tours Mondays and Fridays from 11 a.m. to 4 p.m. and Saturdays from 10 a.m. to 1 p.m. Call 504.525.2121 or visit or for more information.

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