Roof with pink bldg_Flickr_jcetnikEcoroofs with their fuzzy fescues and plump sedums might seem like a quaint, perhaps even self-indulgent display of one’s environmental bonafides, the instantly recognizable, Prius equivalent of the roofing realm.

But it turns out that living roofs offer more than green window dressing. According to Northwest-specific research, ecoroofs significantly reduce stormwater runoff and their insulating properties cut energy use. Additionally, though more difficult to quantify, the roofs provide plants and soil that are hospitable to bugs and birds. The vegetation helps clean the air and can reduce the “heat island” effect in which cities and their pavement and black rooftops are hotter than undeveloped landscapes.

The amount of stormwater benefit depends on various factors including the thickness of the soil (thinner can actually be better than thicker because it dries out faster between storms) and whether the soil and plantings cover all or just part of a roof.

In a 2008 report published by the city of Portland’s Bureau of Environmental Services, green roofs reduced stormwater runoff by approximately 56 percent (the range was between 26 and 86 percent, see Table 4 of the report). Runoff during downpours—also called “peak flow”—was cut 96 percent (see Table 5).

In a 2007 report for the city of Seattle, ecoroofs on commercial buildings around the city curbed runoff between 65 and 94 percent over an 18 month period.

So what, you might ask? The volume of rainwater streaming off traditional rooftops is staggering. For a typical Northwest house, one heavy rainfall will send 10 bathtubs worth of water cascading off the roof. That water picks up pollution that’s delivered straight into streams, lakes, and the sea. It causes flooding, erosion, and landslides. Over the course of a typical year in central Puget Sound, the roof from that single home is generating about 26,600 gallons of stormwater.

So installing a green roof that sponges up even half the rainfall that would normally drain from it can help.

  • Runoff is a big, expensive deal for cities and counties that are required by state and federal laws to try to control stormwater. They’re spending millions of dollars every year trying to reduce flows and clean up the polluted runoff. Controlling stormwater where it falls has become the preferred approach for treatment because it’s cheaper and more effective in many cases than building pipes and gutters, retention ponds, and treatment facilities.

    Horizontal roof_Flickr_Rob HarrisonWashington’s Department of Ecology, for example, is working on an update of stormwater regulations that should increase the use of low-impact development—the name for the strategies that treat stormwater naturally. In addition to green roofs, LID techniques include rain gardens, rain barrels, and porous or permeable pavement.

    But some builders and developers are pushing Ecology to make the use of LID optional—despite the fact that stormwater experts say it’s the most environmentally sound way to handle the dirty runoff. In a great stormwater article by the Tacoma News Tribune’s Rob Carson, those opposed to LID requirements argue that rain gardens don’t work in certain soils and on sloped properties, which can be true. But they can’t make that excuse for ecoroofs, which can be built on roofs that are flat or sloped up to 40 degrees. Yes, they’re more expensive upfront, but affordable over the long term (I’ll delve into the economics of ecoroofs in my next post).

    Thanks to a growing track record of success, green roofs are catching on in the Northwest. Some noteworthy highlights:

    It’s clear that the green-roofs movement is gaining traction, in part because carefully documented studies are demonstrating their sizable benefits. Stormwater is a problem with countless sources, and a diverse range of tools that can be implemented in all sorts of conditions are needed to solve it. Science is showing that green roofs needs to be one of those solutions.

    Plus, they can be positively Hobbit-y cute (see photo above).

    Downtown Portland ecoroof photo jcestnik courtesy of Flickr user jcestnik under a Creative Commons license.

    Hobbit green roof photo Rob Harrison courtesy of Flickr user Rob Harrison under Creative Commons license.