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Green Roofs

Green roofs, also known as living roofs, vegetated roofs or eco-roofs, consist of live vegetation established on top of buildings. The plants and growing medium (soil or other material) rests on top of a synthetic waterproof membrane protecting the building from penetration of water, water vapour and roots.

Green roofs range from owner-constructed designs on sheds and homes, to more elaborate systems installed on large buildings. Green roofs are one of many green technologies implemented as part of both green building and low impact development practices.

Information on green roofs, including specifications can be found in the following resources:

How do green roofs work?

When it rains, the plants and growing medium on the green roof capture and help filter rainwater. Most rainfall that lands on the green roof will eventually return to the surrounding atmosphere by evaporation and through regular plant processes. As a result, the volume and speed of stormwater leaving the site is reduced.

Green roofs retain enough moisture for the plants to survive periods of low precipitation, yet are capable of draining extra moisture when required. During extreme periods of low precipitation, green roofs need to be irrigated.

Green roof plants are chosen for suitability on the following criteria:

  • Drough-tolerant requiring little or no irrigation after plants have established.
  • Self-sustaining without the need for fertilizers, pesticides or herbicides
  • Able to withstand heat, cold and high winds
  • Very low-maintenance, needing little or no mowing or trimming
  • Perennial or self-sowing

Suitable plants also have growth patterns that allow them to thoroughly cover the soil. A variety of species is normally planted to ensure a self-maintaining community which appeals to animal and human users alike.

Types of green roofs

There are two types of green roofs: extensive and intensive. Each type of green roof has its benefits and challenges, and come with a variety of factors to consider before construction. Green roofs can be placed below, at, or above grade; but, in all cases the plants are not planted in the ground but on the roof itself. This technology can only be used on roofs that have the strength to bear the added weight.

Extensive systems
An extensive system is the simplest form of green roof and is suitable for renovation or retrofit projects on existing buildings. These vegetation blankets consist of soil and plants that are placed on the existing roof with a waterproof liner and root barriers underneath as required. In general, extensive systems have a lightweight design, a thin layer (approximately 50 to 150 mm or 3 to 6 inches) and are usually not designed for public access.

Intensive systems
Intensive systems are more complex and used for new construction, where weight loads can be ensured and where the green roof is integrated into the buildings infrastructure. As they are often used as public spaces, intensive systems require much more structural capacity to accommodate deeper soil, a wider range of plants, shrubs and trees and irrigation systems.

Benefits or green roofs

Green roofs offer a number of hydrologic, atmospheric, thermal and social benefits for the building, people and environment:

  • Reduces the volume and speed of stormwater runoff leaving the site
  • Improves the quality of water that returns to the storm system and watersheds
  • Reduces ambient air temperatures and increases humidity levels in the surrounding areas (reducing urban heat island effect)
  • Filters and binds dust particles, filters airborne toxins to improve air quality
  • Increases energy efficiency (heating in the winter and cooling in the summer)
  • Increases building roof membrane life by protecting the underlying components against ultraviiolet and thermal degradation
  • Provides habitat for a diversity of wildlife species
  • Increases aesthetic appeal and useable green space for tenants
  • Increases property value
  • Absorbs external noise pollution