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 media (usually not soil but rather light-weight material) rest on top of a synthetic waterproof membrane and root barrier protecting the building from penetration of water, water vapour and roots.

Green roofs range from owner-constructed designs on sheds 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 evapotranspiration. As a result, the volume and speed of stormwater leaving the site is reduced.

When properly designed and landscaped, 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 may need to be irrigated.

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

  • Drought-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 growing medium. 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 comes with a variety of factors to consider before construction. For both cases of green roofs the plants are planted on the roof itself. This technology can only be used on roofs that have the strength to bear the added weight.

In general, and where possible, a green roof in the Calgary region should have a depth of at least 150 mm for the purpose of stormwater management to permit adequate flexibility in the type and variety of vegetation that can be incorporated, and to ensure greater survivability of plants.

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 accommodated and where the green roof is integrated into the building's infrastructure. As they are often used as public spaces, intensive systems require much more structural capacity to accommodate the deeper growing media, a wider range of plants, shrubs and trees and irrigation systems.

Benefits or green roofs

Green roofs offer, to varying degrees as a function of its composition, 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.
  • With appropriate growing media composition, 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 (specifically, cooling in the summer).
  • Increases building roof membrane life by protecting the underlying components against ultraviolet, thermal degradation and hail.
  • Provides habitat for a diversity of wildlife species.
  • Increases aesthetic appeal and useable green space for tenants.
  • Increases property value.
  • Absorbs external noise pollution.
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