Green roofs

Components of a green roof

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  Each layer in detail 

  Vegetation

  A typical green roof is partially or completely covered with vegetation, which is a collection of plant life grown in the supporting growing medium.

  Growing medium

  An engineered soil that supports and provides nutrients for plants, retains water, and is designed to be lightweight.

  Filter fabric

  A water-permeable 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 barrier

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

  Waterproofing membrane and underlying roof system

  A barrier that is designed and constructed to prevent water from entering the building. It is part of the waterproof roof system that protects the building interior.

  Thermal insulation

  A layer used to reduce seasonal heat gain and loss through the roof area. Depending on the type of roof system used, insulation may be placed above or below the waterproofing membrane.

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  Layer order

  The order of component layers can vary depending on the roof system. A conventionally insulated roof system places insulation below the waterproofing membrane:

  • Vegetation
  • Growing medium
  • Filter fabric
  • Drainage system
  • Root barrier
  • Waterproofing membrane
  • Supporting roof system (insulation, roof structural deck, and other)

  A protected membrane (or inverted) roof system places insulation above the waterproofing membrane:

  • Vegetation
  • Growing medium
  • Filter fabric
  • Drainage system
  • Insulation
  • Root barrier
  • Waterproofing membrane
  • Supporting roof system (roof structural deck, and other)

  In a protected membrane roof system, the waterproofing membrane is better protected from temperature changes by the insulation. 

  Component layers may be installed individually (built-up systems) or in pre-formed combinations (modular systems).

Types of green roofs

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

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

Information icon Definitions for green roof types and soils depths vary. The ranges mentioned in these sections represent only an example.

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  Extensive roofs

  • Growing medium depth is usually 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 usually 300 mm (12 inches) to 900 mm (36 inches), or more
  • 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 growing medium depth, plant diversity, and other characteristics.

  Growing medium depth usually ranges between 60 mm (2.4 inches) to 900 mm (36 inches).

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  Diagram of layer orders and green roof types

  Information icon A protected membrane roof system is shown here. Select image to review a larger version of the diagram.

  

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

Co-benefits of green roofs

A green roof is a system of components that work together to provide multiple environmental, social, and economic co-benefits.

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 green roof co-benefits 

  • How green roofs compare with other GRI PDF file (123 KB)
  • Common types of GRI tools, key co-benefits, and examples
  • 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
  • Used by trees and plants (transpiration)