Green Roofs

“Choosing an environmentally responsible roofing material can be one of the greatest challenges of green building. Roofing materials tend to be expensive, damage- and failure-prone, and often contribute to demolition waste. An option which has long been popular in Europe, and is gaining increasing favor in the United States, is the vegetative-cover, or green, roof. Designing a roof with plant cover has several environmental benefits. It can reduce rooftop temperature, in turn reducing building cooling costs and even preventing urban heat islands. Additionally, green roofs are an important tool in stormwater management, because they prevent runoff. Finally, plant-covered roofs can play a role in providing urban habitat for songbirds and butterflies, and in improving air quality.”

From the U.S. Department of Energy.

So what exactly is a green roof?

Linda Velazquez writes by email that “a traditional roof garden is a roof deck (concrete, wood, etc.) with potted plants in various types of free standing containers,” while green roof is an extension of the existing roof which involves a special root repelling membrane, a drainage system, a lightweight growing medium, and plants.

“All the green roof component layers cover the entire roof deck surface. Specifically, the roof deck, insulation, waterproofing membrane(s), root resistant layer, drainage, non-woven filter fabric, engineered soil mix, plants (and sometimes a biodegradable erosion control blanket). So, the entire deck has unimpeded drainage over the entire roof, and the weight of soil and plants is more evenly distributed over the entire roof. And the plants are planted directly into the soil, which in effect, looks like the roof deck.

And there will be a few additional layers (protection, vapor barrier board, etc.) that are applied over an intensive green roof system, as the soil depths are deeper, weights are greater, etc. You can also add architectural accents, like paths, arbors, fountains, etc., but these are built onto the roof deck before the layers are put on the roof deck.”

In an article on green roofs, Katrin Scholz-Barth writes:

There are two distinctly different types of green roofs: intensive and extensive. Intensive green roofs require a minimum of one foot of soil depth to create a more traditional roof garden, with large trees, shrubs and other manicured landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. Intensive green roofs add considerable load (from 80 to 150 pounds per square foot) to a structure and require intensive maintenance. These roof gardens are, however, designed to be accessible and are used as parks or building amenities.

In contrast, extensive green roofs range from as little as 1 to 5 inches in soil depth. Depending on the soil depth and type of substrate, loads can vary from 15 lbs/sf to 50 lbs/sf. For instance, historic green roofs in Berlin, Germany, built around 1900, weigh about 42 lbs/sf. Extensive green roofs are not designed for public use but can be accessed for routine maintenance walks, generally performed once per year. Extensive green roofs are primarily built for their environmental benefits.

Although everything from earth-bermed [partially underground] houses to balconies with potted plants has conveniently been termed a ‘green roof’ at one point or another, this article strictly defines extensive green roofs as elevated roof surfaces that are entirely covered with a thin soil and vegetation layer. They are not necessarily sod or grass roofs... Additionally, the term eco-roofs is not used in this context because products such as wood shingles are also made of natural, renewable materials, and would thus qualify as an eco-roof.

Green roofs can create recreational space while simultaneously address urban environmental issues like smog, climate change, stormwater management, and energy conservation, though there is the additional upfront expense for builders. Europe is far ahead of the U.S. Green Roofs for Healthy Cities:

“In North America, the benefits of green roof technologies are poorly understood and the market remains immature, despite the efforts of several industry leaders. In Europe however, these technologies have become very well established. This has been the direct result of government legislative and financial support, at both the state and municipal level. Such support recognizes the many tangible and intangible public benefits of green roofs. This support has led to the creation of a vibrant, multi-million dollar market for green roof products and services in Germany, France, Austria and Switzerland among others.”

From City Farmer, Canada’s Office of Urban Agriculture:

“In some parts of Germany, new industrial buildings must have green roofs by law; in Swiss cities, regulations now require new construction to relocate the area of greenspace covered up by the building’s footprint to the rooftop - and even existing buildings, some hundreds of years old, must convert 20% of their roofspace to pasture.”

Green roofs also cool down urban areas in summer, saving energy. The combination of dark surfaces and less vegetation in cities creates what’s called the heat island effect:

“On warm summer days, the air in urban areas can be 6-8°F hotter than its surrounding areas. Scientists call these cities ‘urban heat islands.’ The higher temperatures in urban heat islands increases air conditioning and raises pollution levels.”