Green roofs
Heat islands
In the city, the proportion of dark surfaces (roofs and streets) that absorb rather than reflect energy is large enough to raise the ambient temperature by several degrees when this energy is reradiated. In addition to causing discomfort and aggravating the health problems associated with excessive heat, this phenomenon increases the energy consumption attributable to air conditioning.
Any initiative that limits or eliminates a dark surface exposed to solar rays helps to diminish the impact of heat islands. For example, replacing a dark-coloured roof membrane with vegetation, creating greenspace at ground level, planting roadside trees and installing highly reflective light-coloured roofing materials are all green measures that help reduce the number of heat islands.
Sewage spill
For various reasons, but generally after heavy rainfall, the wastewater in municipal collection systems regularly spills directly into waterways, carrying with it significant quantities of pollutants, including waste collected from city streets (motor oil, leachates, construction materials) as well as a considerable amount of waste from household toilets. In an effort to minimize such spills, which from 2004 to 2006 occurred an average of 1,280 times a year in Montréal, several municipalities now require that buildings have the capacity to retain on site a significant quantity of rainwater for as long as it takes to reduce the initial pressure on municipal systems. The technique usually employed to this end involves the installation of retention basins, relatively costly infrastructures. But it turns out that green roofs, which act like sponges, can also retain significant quantities of water, thus diminishing or even eliminating the need for retention basins. In addition to the basin required by municipal authorities, the Centre for Sustainable Development will be equipped with an extensive green roof installed on top of 100 mm of compost.
Some of the other advantages of green roofs include:
- Building insulation;
- Extension of the life of roof membranes (lower UV impact and less extension-contraction stress);
- Increased amount of inhabitable greenspace in cities.
