Green roofs, well-established in Europe, are becoming a growing trend in North America.

Some benefits are tangible, like conserving energy and managing storm water runoff, while others are more speculative, such as improving air quality or having a positive psychological impact on communities. Green roofs are loosely defined as "landscapes over structure," and the methodology and vocabulary of green roofs are imported from Europe, especially from Germany, where green roofs have been required on most structures for over 20 years.  

Most German words translate easily into English - the two basic types of green roof are defined by their maintenance regimens: "Intensive" green roofs are deep, heavy, lushly planted, and they are often designed for 100 percent accessibility. They also require more intense work and maintenance.  

"Extensive" green roofs are shallow, which limits the plantings to those that can survive in shallow growing medium, and they are less likely to be designed for frequent human access. Their maintenance regimen is much less than an intensive green roof.

Virginia Russell, an associate professor in the University of Cincinnati College of Design, Architecture, Art, and Planning (DAAP), will say in her Oct. 24th presentation at the 11th annual CitiesAlive Annual Green Roof and Wall Conference in San Francisco that the terminology is confusing. She believes that intensive and extensive are used interchangeably in English and cause confusion.

She instead believes that green roof terminology should focus on sustainability rather than maintenance - but the proposal is likely to be more confusing than what is being replaced. Clearly if people are putting green stuff on the roof, that is good, let them do it. Saying a classification system needs to consider regional identity and should support regional methods and plant palettes rather than generic applications that are alike from coast to coast, will just come across as a top-down bureaucratic headache for the individuals who are most likely to spearhead plans for green roofs - they are usually not bureaucrats.

The only benefit seems to be 'uniquely North American green roofs', which would happen organically anyway. People inclined to want to create green roofs already know plants. But these newly classified green roofs will confusingly be identified by social and economic benefits as well as their environmental benefits. "Just as animal kingdoms have higher and lower orders, green roofs will have higher orders that are more 'intelligent' because of their advanced and highly evolved sustainability measures," says Russell. "The list of reasons to justify the development of a classification system for vegetated roofs is not complete without some mention of the difference between tough love and puppy love for vegetated roofs."