To some extent, green roofs have been around forever. The fabled hanging gardens of Babylon and the still-existent sod roofs of Scandinavia predate recorded history and show that, at least in some places on Earth, humans have always found reasons to grow plants over their heads. The advent of the high-rise took this idea of skygardens to new levels. In the 1930s, elaborate roof gardens were put in place on top of the plaza buildings in New York’s Rockefeller Center, and other examples of tower-topping putting greens, herb patches, and sculpted hedges abound. Other than their altitude, these green roofs, known as “intensive” in the industry, differ little from gardens at grade. Almost always installed over waterproofed, reinforced-concrete slabs with drains, they feature soil depths of up to 10 feet, deep enough to grow sizable trees.

Intensive green roofs require a good deal of time and money, both in their design and construction as well as in their upkeep. Their motivating impulse tends to be aesthetic and recreational. However, a newer type of green roof has become more and more prevalent in this country over the past 10 years. Known as an “extensive” green roof, it is a system that has been engineered and developed in Germany since the 1970s, specifically for building performance and environmental sustainability. These roofs manage stormwater runoff, mitigate the heat-island effect, and create habitats for birds, insects, and other critters. They are also known to double the lifetime of a roof by acting as a barrier between the roof’s waterproofing layer and the elements.

There are many types of extensive systems, each suitable for a different set of project parameters. What separates them from intensive green roofs is primarily that they tend to feature soil depths of 3 to 6 inches and are lightweight enough to be installed on top of existing structures, opening up the possibility of “greening” vast swaths of the built environment. There are now several companies in North America offering off-the-shelf extensive green-roof systems for building owners looking to meet increasingly stringent stormwater retention codes with a minimum of effort and maintenance. But for those seeking a more holistic approach, there is much to consider.

“You have to decide for your client whether you want a design solution or product solution,” explains Ed Snodgrass, a leader in horticultural consulting for green-roof projects and co-owner of Street, Md.–based Emory Knoll Farms, a perennial nursery specializing in green-roof plants. “For a product solution, there are a number of companies that would say, if you want a green roof we’ve got you covered so you don’t have to think about any design questions at all. They will come with a shiny brochure and you pick your option. If you want a design solution, you have to look at all of the layers that make up a green-roof system and decide what is best for your client’s objectives.”

Analyzing the Layers

The first consideration for any green-roof designer is the loading capacity of the roof itself. This is especially true when the project is to be applied to an existing building, in which case the designer must team with a structural engineer to determine how much weight can be added and where. At Chicago City Hall, which was completed a decade ago as a pilot project to determine the benefits and feasibility of green roofs on city municipal buildings, the Elmhurst, Ill., office of ecological-design firm Conservation Design Forum (CDF) custom-tailored a system to meet the varying capacities of different parts of the historic structure’s roof. “Part of our approach on any green roof that we do is to maximize the habitat and therefore the depth of soil,” explains David J. Yocca, principal landscape architect at CDF. “In most retrofit projects, such as city hall, we adapted the green-roof system to the characteristics of the structure, varying the thickness of the growing medium based on available loading.”