Green Roofs and the Urban Heat Island Effect

urban heat island

Penticton is in full festival swing and my wife and I have been enjoying walking downtown for many events. Canada Day was gorgeous and hot! But it wasn’t just the sun getting us, we could feel the heat radiating off the sidewalk. This reminded me of one of the biggest reasons I became passionate about urban agriculture; the urban heat island effect.

The urban heat island effect is experienced when the urban temperature is warmer than surrounding rural areas due to the storing and release of heat from manmade structures. Urban areas are built with heat-absorbing materials like asphalt roads and roof shingles. Overnight the city releases all this stored heat, only to trap it again the next day. Cities also create their own heat from sources like electrical devices and gas engines. Many factors contribute to urban areas being 5-10 degrees Celsius warmer than surrounding rural areas. “The land in more developed areas such as downtown Vancouver… is significantly hotter than in more suburban areas like Langley or Maple Ridge ⁠— by up to 17 C in some cases” according to a June 2019 CBC article.

For all the ways urban districts have been built to be heat sinks, there are just as many ways to redesign them to be flourishing, healthy microclimates. Increased urban vegetation has a multitude of positive impacts. Trees absorb water and release it back into our environment through a process called evapotranspiration. This lowers the ambient air temperature. Additionally, the shade trees provide cools both the air and ground. In regards to your budding vegetable garden, your plants will likely benefit from the warmer urban environment as it lengthens the growing season. In this way, you are both using and reducing the urban heat island effect.

Unfortunately, the built environment only amplifies the existing heat, and all the impermeable surfaces cause runoff which puts pressure on the city’s wastewater systems. “Linking urban heat island concerns with stormwater management planning is a leading example of integrative climate change adaptation planning,” says a 2014 UBC study. Extreme weather events include both drought and flooding, and cities’ Official Community Plans need to centre climate adaptation techniques.

Green roofs are one way people are attempting to combat the urban heat island effect. These typically come in one of two forms. The original green roof, and still the more favoured, utilizes hardy ground cover vegetation that lives in a growing medium rather than soil and requires very little attention. These roofs can last 40-50 years, far longer than a conventional roof. The other kind of green roof looks and acts much like a regular garden. However, these can have more specific structural requirements as the associated plants and soil weigh much more.

Both styles intrigue me and have a place in our adaptation efforts. While you can opt for a green roof yourself, it’s not in the cards for many of us, especially renters. However, you can petition your local government, and participate in open houses addressing the Official Community Plan, like the City of Penticton recently hosted. Local government changes are a necessity in creating climate resilient communities. The cities of Toronto and Portland each have bylaws requiring or incentivizing developers to commit to green roofs. Why not Penticton?


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