Municipalities across Canada and the world are beginning to mandate green roofs, which are roofs covered with any form of vegetation—trees, grass, etc. Think hobbit huts, but bigger. 
 


Stayin’ alive

A roof that is covered entirely with vegetation is at least ten per cent more expensive to construct than a traditional roof, but can double its lifespan. The increased construction costs can be recouped within two years, according to the City of Port Coquitlam. In the long run, a green roof benefits the tenant, the city’s services and the environment.

In the rainy Greater Vancouver Area, the sewer system can easily be overwhelmed, particularly in the winter months. The Vancouver Public Library recently installed a green roof, significantly reducing its runoff. From July to February 2003, the new roof’s runoff was measured. During that period, runoff was reduced by 48 per cent. 

Throughout the day the temperature fluctuates, which causes roofs to expand and contract. This continual expansion and contraction reduces a roof’s lifespan. Since green roofs absorb and reflect heat, they significantly increase a roof’s lifespan. Some experts have said that green roofs rarely reach temperatures above 27°C, while black asphalt roofs often reach temperatures around 71°C during the summer.  

Beating the heat

Generally, vegetation within a city cools down the surrounding area. Daniel Roehr, a professor at UBC’s School of Architecture and Landscape Architecture, has cited studies showing that for every 100 square metres of urban parks, the city’s temperature is reduced by 1°C. Larger parks such as Central Park reduce temperature by as much as 2–5°C.  
 


A cooler roof eases the burden on the electricity grid as it mitigates the heat island urban effect, which causes cities to be around 1–3°C hotter than their surrounding suburbs.

“By creating a vegetated surface, green roofs reduce outdoor air temperature and the urban heat island effect through transpiration from plants, shading, and increased reflectivity (albedo) of the roof. Additionally, because a vegetated roof is significantly cooler than a non-vegetated roof on hot days, less heat is transferred through the ceiling into the room below, reducing indoor air temperature and thus the demand for air conditioning,” Earth Pledge, a consulting firm that advocates sustainable practices, states on their website.  

Saving energy

According to Roehr, buildings that have 60 per cent of their roof covered reduce their overall CO2 footprint by 15 per cent. A different study found far greater energy savings: a 2003 report by Liu and Baskaran claimed that green roofs would reduce a building’s energy consumption by 75 per cent.  
 


However, these optimistic claims were not found in other documents. In 2005, Alcazar and Bass monitored the effect a newly installed green roof had on a building in Madrid. Over the course of a year, the building’s energy consumption was reduced by a paltry one per cent. 

 

Whatever the actual savings, any reduction in buildings’ energy usage is beneficial. Buildings account for 30 per cent of energy use and 27 per cent of Canada’s green house emissions.  
 
 

But can a green roof deter the success of other green technologies?  

One would think that photovoltaic panels and vegetation destined for a roof would compete for space, but a green roof actually improves a photovoltaic panel’s efficiency, since they operate more efficiently under cooler conditions. 

Quirky benefits

There are other benefits too: Migrating birds that travel along narrow paths have begun using green roofs as temporary sanctuaries. “Green roofs act as small airports for migrating birds,” said Roehr.

At the same time, humans can grow food on their roof tops (as of yet, there are no reports of birds eating the residents’ food). Food production further increases a roof’s retention of water.  
 


Types of green roofs

Green roofs can be broken up into two types: intensive and extensive.  

 

Intensive roofs have a growing medium—such as soil—of at least 20 centimetres, along with irrigation and maintenance. These roofs typically have small trees such as Japanese Maples. They often double up as gardens for nearby residents.  
 


Extensive green roofs, on the other hand, are far simpler operations. Their growing medium is no more than 15.2 centimetres deep. Commonly, drought-resistant plants are selected for these roofs in order to reduce the need for upkeep. The Vancouver Public Library, for example, selected Blue Fescue, which is a drought and heat tolerant plant. 

Moving forward

In Vancouver, less resistant plants also thrive. Ward Teulon is Vancouver’s self-dubbed City Farm Boy. Teulon sells rooftop garden beds and garden boxes. From beans to asparagus to squash, he grows year-round in Vancouver. After a few financially difficult years, his one-man operation has become barely “profitable” thanks to numerous volunteers, he said.  

Teulon sees growth in Richmond, where currently farmland sits unused. But other than working with Richmond’s municipal government to utilize the land, he does not generally get involved with government or politics.

But Richmond and Port Coquitlam are taking steps toward increasing the number of green roofs within their boundaries. In 2006 and 2008, Richmond and Port Coquitlam mandated green roofs for buildings whose surface area was above a certain threshold. Port Coquitlam’s green roof bylaw mandates green roofs for commercial and industrial buildings. As such, these roofs are typically intensive and not open to the public. However, the city has allowed businesses to opt out of implementing a green roof if it can be proven that a green roof would interfere with the building’s business operations. 

Maureen Connelly, who works at BCIT, heads up a program that specializes in the construction and landscaping of green roofs. She applauds the two municipalities’ decision.

“It’s an indicator of what’s to come,” Connelly said.

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