green roofs

National Institute of Building Science

PECO Main office Building, PhiladlephiaVegetative roofs, also known as green roofs, are thin layers of living vegetation installed on top of conventional flat or sloping roofs. We have chosen to use the word “vegetative” rather than the word “green” in this guide because a non-vegetative roof could be considered to be environmentally “green” without being vegetative. For example, due to it being white and therefore mitigating heat gain within the building and reducing heat island contribution, a white non-vegetative roof might be considered as being “green” or environmentally friendly. In other words, “green” has too broad of a connotation to be clear for use in this guide, and we recommend that the industry adopt the nomenclature “vegetative,” rather than the overly broad “green.”

A. Features

All well-designed extensive vegetative roofs include subsystems responsible for:

  • Drainage: Vegetative roof drainage design must both maintain optimum growing conditions in the growth medium and manage heavy rainfall without sustaining damage due to erosion or ponding of water.
  • Plant nourishment and support: The engineered medium must be carefully designed to provide for excellent plant growth, no wind scouring, and proper water holding capacity.
  • Protection of underlying waterproofing systems: Vegetative roof assemblies must protect the underlying waterproofing system from human activities (including the impact of maintenance) and biological attack, and solar degradation. A capillary break immediately above the membrane is required for most membranes.
  • Waterproofing systems: Waterproofing is critical for protecting the structure from water intrusion.
  • Insulation systems: Insulation is critical for saving energy.
Generic Extensive Green Roof on a Concrete DeckFigure 2. Generic Extensive Green Roof on a Concrete Deck
(Image courtesy of American Hydrotech)
Generic Extensive Green Roof on a Steel DeckFigure 3. Generic Extensive Green Roof on a Steel Deck
(Image courtesy of American Hydrotech)

A wide range of methods can achieve these functions. For instance, drainage layers may consist of plastic sheets, fabric or synthetic mats, or granular mineral layers. Similarly, the physical properties and performance characteristics of growing media (engineered soils) and plant materials may vary with the climate, plant community, or engineering requirements. Figure 2 shows a generic cut-away of a common type of vegetative roof assembly that utilizes a lower granular drainage layer in combination with an upper growth medium or substrate.

There are many potential benefits associated with extensive vegetative roofs. These include:

  • Controlling storm water runoff
  • Improving water quality
  • Mitigating urban heat-island effects
  • Prolonging the service life of roofing materials
  • Conserving energy
  • Reducing sound reflection and transmission
  • Improving the aesthetic environment in both work and home settings
  • Mitigation of wildlife
  • Cost/benefit.

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