Green Roofs: Reducing the Heat Island Effect

Tue, 2009-09-22 10:16

I think garden roofs are cool. They’re just neat. Think about it - we have the ability to make our buildings blend in with nature and to create green space of the millions of acres of otherwise black tar roofs. Why are black tar roofs bad? Well, for starters a black tar roof raises the local temperature (just like a black asphalt parking lot) by as much as 60 degrees on the roof itself. The increased temperatures make the local environment much less habitable for many animal species, contribute to smog formation, and waste energy as the building now has to work harder to keep us at 70 degrees on the inside.

After temperature, a tar roof also speeds rain runoff, which, by the way, is about 10 degrees hotter after washing off the roof and then it starts killing the aquatic life in ponds and streams. The runoff is important because its like a dam breaking every time it rains. In developed areas, the streams become deep and narrow chasms because the water rushes through it so fast that the soil is eroded away. If you’ve ever seen a stream away from development, you know that it is shallow and wide meandering over the terrain. The fast runoff also means that the water has no chance to replenish the ground water, and because it runs off so fast, the pollutants (oils, etc.) have no chance of getting filtered out. Wow, so that’s a lot of reasons that black tar roofs are bad. Check out Google Maps sometimes to see just how many black roofs are out - even in Phoenix and Miami. It;s amazing.

The good news for green (a roof with a high Solar Reflectance Index) and garden roofs is that they are required by the LEED standards, an many municipalities are starting to encourage greener roofs. LEED Credit 7.2 requires that the roof must be 75% or greater high Solar Reflectance or 50% or greater garden roof. That’s pretty neat. Eventually, as our water resources become more constrained, I expect we’ll see many more green roofs. Even New York City has contemplated mandating them. Imagine flying over a city in an airplane, and all of the commercial roof top space is actually green. That would probably be pretty interesting to see. It's also probably something that won’t shock us by the time it happens because there will be such a slow transition to green roofs. Anyway, I have always thought green roofs were cool ever since I found out about them.

What triggered this blog post was a recent news story from the Charlotte Observer called "An Oasis in the Sky." Here are the highlights: A green roof would last 40 years, compared to 20 for a conventional roof. It would save about 10 percent in energy costs, or about $36,000 a year for that building. And its plants would absorb about 70 percent of the water that would otherwise go into uptown storm drains. Heated runoff from hot rooftops can endanger wildlife, experts say.