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Ga Ga for Green Roofs

October 24, 2008

There are green roofs (like the moss-speckled roofs throughout the Pacific Northwest) and then there are GREEN roofs like the goat-groomed one atop Al Johnson’s Swedish Restaurant in Sister Bay, Wisconsin. The goats aren’t green-they’re normal goat colors-but the grass that grows on the roof is. The goats are just lawn mowers.

Why would anyone plant grass (or low-growing succulent plants) on their roof? To increase the building’s energy efficiency, of course-or just to feed their goats.

Transplanting a section of meadow onto an otherwise neat and tidy asphalt-shingled roof not only makes the roof look nifty (or tacky if the goats are slackers), it also reduces heating costs in the winter and cooling costs in the summer. On top of that, greening your roof can extend its life by reducing wear and tear from sunlight and big temperature changes. The plants even help reduce storm water runoff that can overwhelm water treatment plants during heavy rain.

Green roofs are becoming a tad more popular on single-family houses, but they are becoming a big hit in large urban settings like New York and Chicago. Businesses and developers are trading-in the standard flat-topped roofs covered in tar for flat-topped roofs covered in plants. Apart from giving surrounding buildings something more interesting to look at, the switch from tar to plants can lead to a big drop in temperature-for the roof and the entire city.

How big of a drop? We’re talking about the difference between frying an egg and what you wish you could set your thermostat to in the middle of winter. Black tar (and asphalt shingles) soak up the sun like…well, like black tar and asphalt. The surface of a traditional roof can reach over 160°F (an effective egg-frying temperature). Much of that heat is either passed straight into the building or is radiated into the surrounding air. (That’s part of the reason cities are usually 2-10 degrees warmer than surrounding areas.) Green roofs, on the other hand, stay much cooler (a comfy 80° F) thanks to the miracles of reflection and evaporation. AND they send less heat into the air, potentially cooling down cities. 

Goat comfort aside (you wouldn’t want them to burn their tootsies, would you?), what’s so great about a cool roof? Researchers are trying to answer that question by collecting data from rooftops in New York and California. So far, published studies have shown that green roofs reduce the daily energy usage of the buildings they top by 15% to 75%. (Big range, eh? Good thing they’re still collecting data!)

Anything cool is a welcome relief in summer, but most of us would prefer buildings to be a little warmer in winter. Thankfully, the vegetation (and extra layer of insulation) of a green roof acts like a big ol’ down comforter on the roof (even when the plants are covered in snow). But what happens to the goats in winter? Well that depends on how much they ate during the summer.

FYI: Building a green roof is a lot more technical than it sounds–it’s not just a question of a little grass seed and two bags of potting soil. A green roof is made up of A LOT of layers: Layers that are waterproof, layers that prevent roots from growing too deep, layers that let excess water runoff the roof so that all of the dirt doesn’t just float away, and of course, a layer of dirt and plants.

But not just any plants will do. Some folks use grasses (especially people looking for a place to graze their goats-or an excuse to rent some goats), but the majority of green roofs are planted with low growing, hardy, succulent plants from the Sedum family. 

Most green roofs have only a two to four inch layer of soil on top. There aren’t many types of plants that can survive in such a thin layer of soil-and that’s a good thing. It means that the plants that are supposed to be on the roof (the grasses and succulents) will thrive, but those that don’t belong (like oak trees) won’t have a chance.

Planting things on a roof is nothing new-the Hanging Gardens of Babylon are thought to be the earliest example of rooftop gardens-but keeping goats on a roof IS a relatively recent development. 

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