A Vegetative Roof or “Green Roof” is an age-old design process that uses a growing medium and plant life as a roof barrier to protect the interior space of a structure. Over time, the technology behind these systems has evolved, as have the purpose and benefits associated with them.
In recent years, vegetative roofs have become increasingly popular because they reduce the amount of storm water runoff from urban buildings that cause environmental problems for nearby waterways and sewage treatment facilities. These systems accomplish this by the absorption and evaporation of periodic storm water. They also delay the time at which runoff occurs, resulting in decreased flooding and stress on sewer systems. Vegetated roofs provide numerous benefits for both the environment and the building owner including environmental conservation, improved storm water runoff, reduced heat island effects, improved thermal performance of the building and noise and sound insulation. They also add a natural element to the appearance of a property and are aesthetically pleasing.
Vegetative roof systems are often installed on top of a traditional roofing system, either flat or pitched and range in depths from as little as several inches all the way up to several feet. Depending on the type of system used and the current weather conditions, they can absorb 60 to 80% of rainfall.
The two basic types of vegetative roof systems are called Extensive and Intensive, and are differentiated mainly by the depth of growing medium, choice of plants and cost.
Extensive green roof systems are typically not pedestrian accessible and are characterized by a relatively small amount of added weight, lower plant diversity, minimal maintenance requirements and lower capital costs. These systems vary in depths between 2 to 6 inches of growing medium and vegetation, and add between 15 and 35 pounds per square foot of additional weight to the roof when fully saturated with rainfall. Due to the lighter weight of these systems they require less structural reinforcement and are often suitable for existing building retrofit projects. Some of these systems are sold with the trays preloaded with seeded growing media which are quicker and easier to install and retain more water than conventional material.
Intensive green roof systems are often pedestrian accessible and are characterized by deeper growing medium, greater weight, increased plant diversity, higher maintenance requirements and higher capital costs. These roof systems vary in depth from 8 to 24 inches and have an increased weight of 60 to 200 pounds per square foot when fully saturated by rainfall. The additional soil depths allow for larger vegetation, including various trees and shrubs. These systems typically require significant structural capacity due to the increased material and weight on the roof.
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