World Green Roof Day

A Growing Movement

Towns and cities globally are going green to adapt to climate change. One of the strategies that is becoming popular is the concept of green roofs, which transform urban rooftops into vibrant, living ecosystems. Today is World Green Roof Day, which is a celebration of green roofs all over the world and the benefits they bring to people and nature. This blog post will explain what a green roof is, and some of the benefits they hold, and will delve into Ottawa’s High Performance Development Standards. 

What is a Green roof?

Isometric vector showing two buildings with green roofs growing trees.

A green roof is a contained area of plant life such as trees, shrubs, crops, or grasses, planted on top of a human-made structure. They can be found anywhere from public structures such as government-owned buildings, to private structures such as apartment buildings and commercial buildings. Green roofs offer a natural escape in the city, soaking up harmful runoff and reducing the “urban heat island” effect. They bring nature to urban rooftops, while also capturing rainwater and cooling the air.

Typically, a green roof will incorporate high quality waterproofing and root repellent system, a drainage system, filter cloth, a lightweight growing medium, and the plants themselves. There are three major types of green roofs:

  1. Extensive green roofs, which are shallow in depth, require no irrigation and typically will have smaller plants like grasses or moss.
  2. Semi-intensive green roofs, which are deeper, require moderate levels of irrigation and have larger plants.
  3. Intensive green roofs, which are the deepest type of green roof, have a high capacity for irrigation and can hold the largest plants such as trees and large shrubs.
Picture showing a roof in a city scape, covered in green grass.
Photo Credit: Landscape Ontario

Benefits of Green Roofs

Energy Use

One of the biggest considerations for green roofs is the reduction of energy use. In summer months, the roof act as heat insulators, reducing the energy required for air conditioning systems. They also reflect heat that would normally be absorbed into the building with a black or grey rooftop. Research conducted in Ottawa at the National Research Council of Canada’s field roofing facility also showed a 10% decrease in energy use for heating over the winter. These savings in energy requirements amount to savings in the operating costs for the buildings.

Flat isometric illustration concept. electrical energy with solar panels

Heat Island Effect

The heat island effect occurs in cities when large amounts of buildings that absorb heat are placed together. The combined effect on the buildings causes noticeable differences in the air temperature around those buildings. Green roofs counter this by shading roof surfaces and through evapotranspiration. The shade from the plants blocks the sunlight from reaching the roofs surface, and evapotranspiration further cools the roof by using heat to evaporate the water from the plant surfaces. 

Pollutant Removal

Plant life does wonders when it comes to filtering pollutants from the air and water. Vegetation removes pollutants and greenhouse gas emissions from the air through dry deposition and carbon sequestration and storage. It also filters stormwater, removing C02 and precipitation-borne pollutants and cools the water more effectively than a standard roof surface. 

Vector graphic of stalks of wheat in three colours; golden, golden mixed with green, and green.

Food Production/Amenity

Depending on the size of the building and the type of green roof installed, green roofs hold potential for local food production. Community gardens can fill spaces typically unused, which will help tackle the growing food insecurity crisis seen globally. They can also provide beautiful landscape and useful green spaces, providing a park-like environment that residents/tenants of a building can enjoy.

Practical Tips for Implementing Green Roofs

In understanding the benefits of a green roof, you might be wondering how you can work towards green roofs in your buildings. While newer buildings are easier to design with a green roof in mind, there may be more difficulty when it comes to retrofitting existing or older buildings. There are several considerations that you need to think about, and the system may not work for every building.

First, the structural capacity of the building needs to be factored into any decision/design to make sure it can support the added weight of a green roof system. This involves close collaboration with structural engineers and architects to verify the feasibility of your business. Furthermore, local climate and weather conditions play a crucial role in determining the success of a green roof. Factors like temperature fluctuations, rainfall patterns, and wind exposure will influence both plant selection and irrigation systems. Moreover, consideration needs to be given to optimizing natural light access for the plants. Lower buildings surrounded by skyscrapers will need to account for the time that the sun is blocked from the roof. Finally, the budget of the project will play a large role in the feasibility of your project.

Green roof on the Ottawa War Museum
Canadian War Museum - Green Roof. Photo Credit: Flynn Companies

With climate change upon us and the world shifting towards more ecologically friendly measures, cities and developers will need to work hard to keep up with trends and science. Cities need to be built to be more climate resilient while keeping energy use low. Green roofs are an excellent way to combat the effects of climate change through their natural filtration and cooling effects.

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