The purpose of planting vegetation on rooftops is seen as a way to reduce the heat effect and reduce energy costs. Rooftop gardens have the potential of lowering energy usage for heating and air-conditioning as well as reducing rainwater runoff, but their effectiveness is not well established.
In order to survive on a rooftop, vegetation has to to survive high winds, prolonged exposure to the sun and unpredictable water availability. To resist these harsh environs, many green roofs are planted with sedum, often a non-native species, that can survive wind and long periods without rainfall. But what about when sedums are native (obviously, they are somewhere)? Or what about greening with other native species plantings?
The cities of Chicago and Portland, have each developed hundreds of green roofs over the last decade, and both cities have implemented programs to monitor the success and cost/benefits of green roofs. The climates are very different, so the green roof requirements are different, but they have been successful in both cities.
Thin (4" or less), light-weight (50 lbs/cu ft saturated) engineered soils are often used for green roofs because of weight limitations. These soils are engineered so they do not retain water so as to limit additional weight on the roof structure. Because of the physical limitations of the soil (i.e., limited-to-no organics, limited capacity to retain water), sedums and other succulents are often used because those species are the best suited for those environments. Other plant species which are adapted to poor soils with limited water typically compensate by having large and/or deep root systems; on a thin-profile green roof, however, these types of plants do not do well because they cannot establish the root systems they need to survive.
Since the 2008 launch of its Ecoroof Incentive Program, the City of Portland has seen the installation of hundreds of vegetated roofs. But in 2012, the City gained a whole lot more green roof footage in a single project: the new Hayden Meadows Walmart in North Portland. This is the second green roof project for Walmart who introduced a similar project to Chicago in 2008. Walmart voluntarily monitored the performance of the green roof. The roof drainage system was purpose-built to separate runoff from the green and non-green roof areas and enable precise comparisons to be made between the two roof areas. Flow meters, installed in each manhole, record total flows and flow rates from each roof section. The entire roof is outfitted with a weather station and data logger to measure temperature, solar radiation, humidity, rain, and wind speed and direction to monitor the conditions on both the vegetated roof and the white Energy Star® rated membrane. Additionally, a moisture probe was installed in the green roof media. The study was the largest and most intensive of its kind in the world.
Improvements in the rate and volume of runoff were apparent as soon as the green roof was installed. Because of the study we now have conclusive evidence that the size of a green roof affects its performance. The scale of a green roof allows it to operate at or above its expected performance rate. This benefit also relies on the non-modular design in which moisture slowly migrates through the green roof toward the outlet. Longer ‘residence times’ in the green roof result in greater moisture uptake, delayed runoff, and enhanced growing conditions for the foliage plants. The larger the green roof the greater the distance runoff needs to travel to reach the roof drains thereby further delaying the peak rate of runoff. There is no other BMP that more closely matches pre-development conditions than a large scale green roof. A decrease in the temperature of the greened roof was also observed, compared to the adjacent white roof.
Perhaps one of the key elements in deciding on a green roof is to identify its purpose. Is it an ornamental green roof, used in combination with decks, seating, etc.? Is it intended to recreate native habitat? Or is it a thin-profile sedum roof, used as a means to slow water runoff and reduce the heat island effect? Each of these green roofs has different technical requirements and will have different characteristics once they have been established. Failure of one type of roof may not mean failure for another. If you're thinking of a green roof, the City of Portland has published an Eco-Roof Guide to help you: http://www.portlandoregon.gov/bes/article/331490