Green Roof Learning Center

Generally speaking, there are two types of green roofs: Extensive and Intensive.

  1. An extensive green roof is one that is more purpose-driven on the performance of water retention and thermal regulation.
    1. It is usually lower maintenance and is not meant to have foot traffic from people.
    2. Lower weight load on the roof (15-50 lbs/sq. ft)
    3. Thinner soil depth of 3-6 inches
    4. Limited plants species option (drought-tolerant sedums and grasses for shorter roots)
    5. Less aesthetically pleasing
  2. An intensive green roof is one that is still purpose-driven on the performance of water retention and thermal regulation but, is more interactive.
    1. It is usually higher maintenance and is meant to have foot traffic from people (walkways, benches, etc.,)
    2. Higher weight load on the roof (50-150 lbs/sq. ft)
    3. Thicker soil depth of 6 inches or more
    4. Wide variety of plant species option (succulents, herbaceous perennials, shrubs, trees, etc.,)

Anatomy of a Green Roof

Figure 1: Credit to Vijayaraghavan K.
  • Vegetation: The top layer of a green roof consists of vegetation. Chosen plants should be based on the type of roof, surrounding climate, and condition of the building. (These plants are described in further detail in the green roof plants tab)
  • Growing Medium: The growing medium is the soil or material that all the vegetation will grow on top later. The medium type, mixture and depth area all depend on the plants chosen for the green roof. Typically the medium must be able to hold a lot of moisture since green roof environments tend to be more windy and dry so mulch can be used to assist with this aspect.
  • Root Barrier: When there are deep-rooted plants such as trees and shrubs, a root barrier (foil or plastic) prevents roots from reaching the membrane on the bottom layer. This plastic can be made of HDPE (High-Density Polyethylene) which is very durable and can help retain soil moisture. This is important because a punctured membrane could potentially cause roof leaks and decay.
  • Drainage: A drainage layer is installed to remove excess water from a green roof. The drainage systems should be designed to ensure stormwater can be used by plants for an extended period of time without an oversaturation of the entire green roof system.
  • Insulation:  The insulation prevents the weight of the green roof from crushing and impairing the membrane.
  • Membrane Protection: This is a protective layer (light concrete, insulation, thick plastic, copper foil, or etc.,) that ensures that the membrane cannot be punctured to prevent deterioration.
  • Membrane: The bottommost layer of a green roof which separates the green roof above from the structural supports below.
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