Finding practical and effective ways to implement sustainable development remains a significant challenge in Canada. But green roof benefits, along with those of vertical gardens, can have big impacts on residential, institutional, industrial and commercial buildings.
They provide such benefits as better air quality, reduced greenhouse gas emissions, improved storm water management and long-term economic advantages.
Green roof installations are different from freestanding planters in that they are applied as part of the roofing system. Green roof technology includes: roof structure and possibly some insulation; waterproofing membrane, often with root repellent insertion; a drainage system, sometimes with built-in water reservoirs; landscape or filter cloth to contain the roots and the soil; specialized growing medium; and plants.
The barrier between the plants and any roof penetrations, parapet walls or flashing is crucial to prevent roof penetration and allow excess water to run off the roof.
Green roof types
There are basically two types of green roof systems: extensive and intensive. Extensive green roofs are characterized by their low weight, low capital cost and minimal maintenance. Intensive green roofs have a greater soil depth and more plantings with higher maintenance requirements.
A vertical garden is essentially a living cladding system with many of the benefits of a green roof. With these gardens, plants grow on, up or against a building’s façade. Suitable plants include a wide variety of perennial and annual vines, as well as espaliered trees. Vertical greening has more potential impact, with greening of a building’s façade often encompassing four times the area of the roof and even more for a high-rise building.
Understanding the benefits
Canada Mortgage and Housing Corporation funded research, with in-kind support from Environment Canada, to gain a better understanding of the benefits of green roofs and vertical garden technologies and barriers that prevent more widespread adoption in Canada.
Finding new ways of using roof and wall space can generate added economic impetus and make cities more liveable by providing significant amounts of accessible outdoor recreation or amenity space close to work and home.
Vertical gardens block movement of dust, while green roofs have a moderating effect on thermal air movement and trap airborne particulates. Studies have shown that treed urban streets have substantially less dust compared to those without trees. Both green roofs and vertical gardens further contribute to reducing pollution by absorbing gaseous pollutants.
These systems have a beneficial impact on moderating the heat gain and loss of buildings, as well as on humidity, air quality and reflected heat. In conjunction with other green installations, these technologies can play a role in altering the climate of a city as a whole. A healthy urban climate could be achieved by greening only five per cent of all roofs and walls within a city.
Widely implemented, these technologies can provide effective methods for reducing greenhouse gas emissions by shading buildings, improving insulation values and reducing higher urban temperatures caused by the expanse of reflective surfaces in urban areas, known as the “urban heat island effect”.
Higher temperatures increase atmospheric instability, which in turn can increase the chance of rainfall and severe thunderstorms. They also affect air quality, as heated air stirs up dust. Strategically placed vertical gardens can help cool air and slow it down, by creating turbulence in vertical airflow.
Another significant benefit of green roofs is their ability to retain storm water. Typical storm water systems in urban areas have resulted in a number of problems, such as water contamination, sewage overflows, drops in local water tables, water temperature increases, severe flooding and erosion.
Green roofs and vertical gardens provide viable alternatives for environmentally appropriate storm water management. Studies show that green roofs absorb 75 per cent of the precipitation that falls on them. Runoff occurs over several hours, thereby reducing the risk of sewage overflows and flash floods.
There are also economic cost benefits for building owners. The green roof and vertical garden systems provide energy cost savings due to increased insulation, extend the life span of roof membranes and vertical surfaces due to improved protection, provide sound insulation, increase aesthetic appeal and, potentially, improve property values. Green roofs protect roofing membranes against ultraviolet radiation, extreme temperature fluctuations and puncture or physical damage from recreation or maintenance.
Vertical gardens likewise provide protection from ultraviolet radiation, driving rain and wear and tear caused by moisture and temperature differentials. They also decrease the effect of wind pressure, which can improve the air tightness of doors, windows and cladding. Green roofs can be used for other advantages, such as recycling wastewater and in water-based heat exchange systems.
Paula Gasparro is manager of business development for multi-unit mortgage insurance at the Canada Mortgage and Housing Corporation (CMHC). CMCH has been Canada’s national housing agency and a source of objective, reliable housing information. Gasparro can be reached at 416-250-2731 or [email protected].