Components of a green roof

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  Typical design

  Vegetation and growing medium

  A typical green roof is partially or completely covered with vegetation. It is supported by a special engineered soil known as a growing medium. This medium is designed to be lightweight and has other useful properties.

  Underlying layers

  This includes a filter membrane, a drainage system, and a root repellant or waterproofing membrane.

  Construction base

  It is constructed on top of a structure like a building or a parkade.

  Additional rooftop components

  Related components may include thermal insulation, vapor control, and structural supports.

  Layer order

  The order of component layers can vary and may be installed individually (built-up systems) or in pre-formed combinations (modular systems).

  Green roof types: Layer by layer comparison

  Information icon Select image to review a larger version of the infographic.

  

  Download green roof build-up by type infographic PDF file (635 KB)

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  Each layer in detail 

  Vegetation

  A collection of plant life grown in the supporting growing medium.

  Growing medium

  An engineered soil usually designed to be lightweight and to support other green roof design objectives. Plant roots anchor in the growing medium.

  Filter membrane

  A barrier placed over the drainage layer to keep the growing medium materials from entering and potentially blocking the drainage system.

  Drainage system

  A system to collect and transfer water from the roof to an appropriate location in a safe and effective manner.

  Root repellant membrane

  A physical or chemical barrier that prevents plant roots from damaging the waterproofing membrane.

  Waterproofing membrane

  A barrier that minimizes moisture or leaks from entering the building.

  Thermal insulation

  A layer used to reduce seasonal heat gain and loss through the roof area.
   

Types of green roofs

Green roofs are generally classified by their growing medium depth as:

• Extensive
• Intensive
• Semi-intensive (in-between variation)

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  Extensive roofs

  • Growing medium depth is 150 mm (6 inches) or less 
  • Supports lower plant diversity, including moss, succulents, herbs, and grasses
  • Water retention and urban heat reduction may be relatively lower
  • Usually inaccessible or limited access
  • Direct human interaction is likely not supported
  • Irrigation system is likely not required
  • Fully saturated weight, cost, and maintenance are relatively low
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  Intensive roofs

  • Growing medium depth is more than 150 mm (6 inches)
  • Supports greatest plant diversity, including a variety of shallow vegetation, bushes, shrubs, and small trees
  • Water retention and urban heat reduction may be relatively higher
  • Usually accessible
  • Direct human interaction is likely supported
  • Irrigation system is likely required
  • Fully saturated weight, cost, and maintenance are relatively higher
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  Semi-intensive roofs

  These hybrid roofs can include both extensive sections and intensive sections with associated ranges of soil depth, plant diversity, and other characteristics.

Co-benefits of green roofs

A green roof is a system of components that work together to provide co-benefits like:

• Managing rainwater
• Reducing urban heat
• Enhancing biodiversity and habitat
• Providing or complementing amenity space
• Increasing access to nature

Blue-green roofs (review diagram) also use shallow water reservoirs located below a growing medium. This reservoir can also help irrigate the plants. 

Depending on the growing medium depth, choice of plants, and other design features, certain co-benefits can be created or emphasized in our community.

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  List of co-benefits of green roofs

  • Improves water quality
  • Improves air quality
  • Reduces runoff and flooding
  • Enhances resilience to climate change (rainwater, carbon, urban heat) 
  • Enhances biodiversity and habitat
  • Supports urban agriculture
  • Increases access to nature and improves education, health, and well-being
  • Provides recreational, play, and amenity space
  • Reduces sewer infrastructure costs
  • Lowers heating and cooling costs for buildings
  • Increases property value
  • Improves aesthetic appeal

  Learn more about co-benefits and strategies advanced by GRI

How rainwater detention and retention work in grey infrastructure and green roofs

Pipes and other grey infrastructure move rainwater and sewage away from private property. Sometimes this involves rainwater ‘detention’.

GRI systems like green roofs and natural assets like trees and plants also use rainwater ‘retention’. The use of this water has many co-benefits, and may delay the need for grey infrastructure upgrades. 

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  Rainwater detention

  This occurs when water is held back and later released. It slows down water but does not change the amount of water released into the sewer system.

  Examples of how water is detained:

  • Soils
  • Green or blue-green roof growing medium
  • Blue-green roof shallow water reservoir
  • Detention tank (may also be used with a green roof)
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  Rainwater retention

  This occurs when water is used for another purpose. It reduces the amount of water leaving a site and released into the sewer system.

  Examples of how retained water is used:    

  • Releasing into the air (evaporation)
  • Seeping into the ground (may be used with a green roof)
  • Used by trees and plants (transpiration)