This article was published originally on 10/27/2010
A recent trend in going "green" is to partially or completely cover the top of your building with growing plants. These "living roofs" can provide substantial benefits.
There are two main types of green roofs (i.e. roofs planted with vegetation): extensive and intensive. Extensive green roofs are shallow, usually less than six inches of substrate, and because of root zone limitation do not traditionally support a large diversity of species. Intensive green roofs are more like rooftop gardens with deep substrate (from six inches to several feet) and a wide variety of plants. Intensive green roofs are beautiful but most buildings are not designed to withstand the additional weight loading so they are therefore limited to new construction. The shallower extensive green roofs much better meet existing building standard load requirements and are therefore utilized more often. Because of this, extensive green roofs are the most commonly used form of green roof.
To break it down further, there are two categories of extensive green roof systems: continuous and modular. Continuous systems are "built in place" on the roof with layers designed to work together to provide the right environment for plants. Modular systems are self-contained trays, which can be variable in size, and have relatively shallow depth. When planted with groundcover plants, it is difficult to tell in the middle of summer that it is a modular system.
Green roofs are installed to meet various objectives, including slowing stormwater runoff from rooftops and reducing peak storm flows, filtering stormwater runoff and reducing ambient temperatures compared to conventional roofs (i.e. reducing the Urban Heat Island Effect). Research has compared the hydraulic and thermal performance of various green roof types to some commonly used roofing surfaces. Benefits for the building are energy savings via additional insulation and evapotranspirative cooling, extending the life of the roof membrane by reducing temperature fluctuations and shading from ultraviolet light as well as being aesthetically pleasing. See the Michigan State University Green Roof Research Program for a list of environmental and building-related benefits.
Although the initial cost of a green roof installation currently is higher than that of a conventional roof, the lifetime costs are roughly comparable due to longer roof lifespan and energy cost savings attributable to green roofs. Conservative estimates claim that the roof membrane life will be doubled when a green roof is installed. In addition, decreases in stormwater runoff from green roofs may allow for scaled-down and more affordable conventional post-construction stormwater infrastructure (i.e. smaller storage requirement for stormwater detention pond).
An example of a green roof on a reinforced pergola for a residential application. Photo by Jennifer Bouselot.