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dec home > wsmd home > erp > green infrastructure > green stormwater infrastructure > green roofs

Green Roofs

 

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Vegetated roofs, or green roofs, are conventional rooftops that include a thin covering of vegetation allowing the roof to function more like a vegetated surface. The overall thickness of the vegetated roof may range greatly depending if it is an intensive or extensive design, they typically contain multiple layers consisting of waterproofing, synthetic insulation, non-soil engineered growth media, fabrics, synthetic components and foliage. In addition to stormwater runoff reduction, green roofs also provide the benefit of additional insulation on the roof, the cooling effect of evaportranspiration, and a reduction in urban heat island effects from numerous black roofs in a concentrated area.

Heritage Aviation, Burlington

( Heritage aviation green roof, south burlington - Design by: Wagner Hodgson Landscape Architecture - Photo By: Susan Teare)

There are two different types of green roof: intensive green roofs, and extensive green roofs. The differences between the two are described below.

Intensive Green Roofs

The intensive green roof uses planting mediums that have greater depth than the extensive green roof. This deeper soil allows intensive roofs to accommodate large plants and dramatic plant groupings. Another term for these green roofs is “rooftop garden.” Intensive green roofs require more maintenance because of the plant varieties they will support. Vegetable and herb gardens fit in well on these rooftops and to a degree their care is slightly easier than ground level gardens since fewer pests and weeds find their way to the higher elevations. All plants will have fertilizer and water needs and many will require clipping and pruning. These green roofs tend to stay more attractive in dry weather and are very often irrigated.

The planting medium in intensive green roofs starts at 6 inches (although you will see some wiggle room in various definitions) and really elaborate designs may exceed a couple feed. Once the plants are installed and the soil is moist these rooftop green spaces can weigh as much as 150 pounds per square foot. The irrigation and drainage systems have to operate at peak efficiency to reduce the chance of overloading the roof’s structure.

Extensive Green Roofs

Extensive green roofs are the simplest to install and are very often added to existing roofs. Depending on the source you look at these roofs may add 10 to 35 pounds per square foot to a roof’s load. Generally, extensive greenroofs can be constructed on roofs with slopes up to 33%, and can be retrofitted onto existing structures with little, or most often, no additional structural support. The average weight of a fully saturated minimum extensive greenroof is 17 pounds per square foot, which is comparable to the weight of gravel ballast placed on many conventional roofs. These roofs are not intended for recreation, or to accommodate the weights of people, larger shrubs nor trees. Extensive greenroofs are less costly due to single or double layer construction.

The planting medium in extensive green roofs ranges from 1.6 to 6 inches deep and while deeper systems have been installed they are not favored as much as the shallower systems. Drought-tolerant sedums (succulent plants) and grasses are the typical plants used since they are shallow-rooted and use little water. Plant diversity on these roofs is kept low to simplify care and to be sure all plants have similar moisture requirements. During dry times these rooftops may turn brown, only to revive with rain.

 

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