Vegetative roofs can be "too light". Materials used to build highly lightweight systems—16 pounds per square foot or lighter—can potentially be harmful to long-term plant health and viability.
Fremont, CA: The vegetative roofs offer a wide range of advantages and incentives for facility owners and their tenants. These benefits include easily accessible green environments, habitats for birds, butterflies, and insects, and even completely new spaces, such as playgrounds at city schools or childcare facilities. They help to reduce the urban heat effect of the island by giving shade and removing heat from the air through evapotranspiration, which inevitably leads to a cooler air temperature above the roof surface. Green roofs can also act as excellent tools for stormwater management.
These problems are discussed below as general considerations where green roofs are either designed or already built.
The weight of the vegetative roof will range from 16 to 60 pounds per square foot, depending on the depth of the growth medium, the form of planting and the cumulative weight of other additions, such as pavers or water features. Often ask the manufacturer or contractor to provide you with the full saturated weight of the overall green roof system.
Although the water-holding capability of green roofing media is of great benefit to stormwater management, it adds considerable weight.
Vegetative roofs can be "too light". Materials used to build highly lightweight systems—16 pounds per square foot or lighter—can potentially be harmful to long-term plant health and viability. For example, while a volcanic rock is light and fluffy, not all plants can thrive. Why waste time and money building and maintaining a vegetative roof just to make the plants die after just two years?
Structural changes are not required for vegetative roof installations over most concrete decks; very rarely for metal roof decks and often for wood decks. These considerations will direct the choices made by designers – architects, landscape architects or other building and design practitioners – about the scope of the vegetative roofing scheme, including the depth of the media and the types of plantings. For example, a proposed "intensive" vegetative roofing system with a depth of 8 or more inches will need to be changed to a semi-intensive or comprehensive system to meet weight and load parameters.