[I am on vacation this week in Phoenix and then Kansas City so I bring you GUEST POSTS!  I met each of the guest authors somewhere along the way and asked them to contribute.  Hope you enjoy.]

By Linda McIntyre

Does the notion of plants on a roof seem bizarre, even dangerous, to you? If so, that’s understandable–there aren’t a lot of green roofs in the US yet. But their numbers are growing, because green roofs–relatively simple installations usually featuring low-growing herbaceous plants, not park-like roof gardens with furniture and trees–can be useful functional tools when properly designed, installed and maintained.

A green roof has a waterproofing membrane like any relatively flat roof. Above the membrane, plants grow in a layer, usually four to six inches deep, of mostly mineral-based medium that looks like gravel, not soil. A drainage layer comprising coarse aggregate or synthetic sheets ensures the system will drain properly after it reaches field capacity, and fabrics are available to protect the membrane from roots if the material isn’t naturally repellent. These layers make up the green roof assembly. Think of a ballasted roof with plants growing among the stones.

Why put a green roof on a building? It can help control stormwater runoff (a big and growing problem in cities), reduce energy costs (especially for air conditioning), and provide amenity space for employees or residents (if a building’s loading capacity allows for such access, a green roof assembly can provide a cost-effective garden). The green roof assembly, when properly designed and installed, protects the membrane, extending the lifespan of the roof. It can earn LEED points and make a building more marketable. If built on a mass scale, green roofs can mitigate the urban heat island effect that makes densely-built cities so uncomfortable in hot weather. A green roof aggregates benefits in a way that other approaches, such as reflective white roofs, which also save energy, can’t. And it will almost certainly look better than a black tar roof.

Maybe you’ve seen a couple of green roofs and been unimpressed. Not every green roof is flamboyantly beautiful, and not every project is successful. There’s no “Consumer Reports” for green roof components, and trial and error is unavoidable with any new technology. Green roofs are common in Europe, but still relatively new here, and we have a more variable climate and a different building culture. Research, project data, and the accumulation of experience are helping to fill knowledge gaps and making it easier to design and install successful green roofs.

A green roof isn’t the right choice for every building. It’s always going to cost more, and installation will always be more complicated. Especially for the first year or two of its life, it will require more maintenance than a conventional roof. But, more and more, it’s a choice worth considering.

Linda McIntyre is a writer specializing in environmental and urban issues. She is working with nurseryman and consultant Ed Snodgrass on a book about designing, installing, and maintaining green roofs, to be published by Timber Press in 2010.