Proponents of living roofs argue that they have met most, if not all, of the technical challenges involved in grafting a biological layer onto the top of buildings of almost any scale: everything from a vegetable stand or bus stop to the ten-acre roof of Ford's truck plant in Dearborn, Michigan. While the average cost of installing a green roof can run two or three times more than a conventional roof, it's likely to be cheaper in the long run, thanks largely to energy savings. Vegetation also shields the roof from ultraviolet radiation, extending its life. And it requires a different kind of care, akin to low-maintenance gardening.

There are still philosophical challenges to be met, many of them having to do with the very idea of what a roof should be and how it should perform. Clients tend to want roofs that are easy to maintain and are uniformly green year-round, perpetual lawns in the sky, not seasonal grasslands. Builders and architects tend to want interchangeable, standardized, universal solutions, the kind of green-roof systems now being offered by some of the big corporate players in the living-roofs industry.

A living roof, though, is not just a biological alternative to a dead roof. It requires a different way of thinking altogether. A standardized green roof such as a carpet of sedums is better than a conventional roof, but it's possible to build living roofs that are even more environmentally beneficial—locally grown, so to speak. The goal for some researchers now is to find ways to build living roofs that are ecologically and socially sound in every respect: low in environmental costs and available to as many people as possible.

Stephan Brenneisen, a Swiss scientist and a strong advocate for the biodiversity potential of living roofs, says simply, "I have to find easy, cheap solutions using materials that come from the region." That means less reliance on plastics and other energy-intensive materials between the roof structure and the plants themselves. What matters isn't only whether living roofs work. It's how to make them work in the most sustainable way, using the least energy while creating the greatest benefit for the human and nonhuman habitat.