Green Roofs: Best Management Practices
"Green" roofs are layers of living vegetation installed on top of buildings, from small garages to large industrial structures. They help manage stormwater and contribute to improved water quality by retaining and filtering rainwater through the plant’s soil and root uptake zone. The water that does leave the roof is slowed, kept cooler and is filtered to be cleaner. Green roofs can also further insulate the building, reducing cooling and heating costs.
Key considerations for implementing green roofs include the structural and load-bearing capacity of the building, plant selection, waterproofing, and drainage or water storage systems. The quantity of rainfall retained or detained by a green roof can vary. For small rainfall events little or no runoff will occur and the majority of the precipitation will return to the atmosphere through evaporation and transpiration. It has been estimated that green roofs, in comparison to conventional roofs, can reduce cadmium, copper and lead in runoff by over 95 percent and zinc by 16 percent; nitrogen levels also can be diminished.
In addition to the stormwater benefits, green roofs extend the life of roofs two to three times. They can help preserve habitat and biodiversity in an otherwise sterile urban environment. Green roofs can also improve air quality by helping to reduce the “urban heat island” effect. Finally, they can provide garden areas and attractive views for other buildings.
The two most effective types of green roofs are:
Extensive systems, with 2 to 4 inches of soil, 12 to 40 pounds per square foot of roof area, short plants with shallow root systems, and easy maintenance. Extensive systems absorb stormwater and provide insulation. Intensive systems are similar to gardens on the ground. They have a minimum of 6 to 12 inches of soil,80 to 150 pounds per square foot, host deeper-rooted plants, including shrubs and trees, and require more maintenance. This type of roof provides more stormwater benefits, including insulation, water filtration, storage and increased habitat opportunities.
In both new building designs and rehab opportunities, the load-bearing capacity of the roof may dictate the most appropriate type of system. Green roofs are appropriate in most of the properties in the City including residential, commercial, industrial and institutional properties.
Once a green roof is well established, maintenance requirements are usually minimal. Maintenance requirements may include inspection of the roof membrane and drainage flow paths. Some watering may be required during the first few years when root systems are getting established. Depending on the dimensions of the planting, some weed removal may be necessary as well. Of course, the more complex the garden, the more it needs to be maintained like a typical garden.
In the United States, green roof costs -including everything from waterproofing to plants -tend to range from $18 to $25 per square foot, depending on how intensive the system is. The initial capital and ongoing maintenance costs of a green roof are offset by some long-term cost savings -most notably roof maintenance and replacement and utility costs. A vegetated roof, on average, can be expected to prolong the life of a conventional roof by at least 20 years because the vegetation prevents the roof from being exposed to ultraviolet radiation and cold winds.