This blog is co-written by Cristina Senjug, Next Level’s communication manager along with Dr. Karen Liu, Next Level’s green roof specialist who was present when Cristina’s residential green roof was installed more than a decade ago.

Year-round Benefits

In the first part of our winter green roof series, My Green Roof & I Survived Winter! Dr. Karen Liu and I wrote about my residential green roof of nearly 12 years. We described how the hardy, lightweight and “soilless” system made it through the winter and how the sedum vegetation adapted to the cold. In this second part, we will discuss green roof systems’ benefits and how they save energy and help manage stormwater in the winter.

Cool in the summer, Warmer in winter

A National Research Council of Canada study showed that typical extensive green roofs in Toronto reduced heat flow through the roof by 70-90% in the summer and 10-30% in the winter compared to a regular roof.  In the summer, when the sunshine hits the roof membrane, some of the incident heat is reflected but most of it is conducted through the roof into the building, thus the need for air conditioning.  Naturally, added insulation and a reflective roof coating can help reduce the heat gain.

Green roofs reduce the heat gain through the roof  in four ways:  shading, insulation, evapotranspiration and thermal mass.  The plant materials shade the roof to keep it cool.  Water evaporates from the growing substrate and the plants and cools the roof – similar to what sweating does to our skin.  The growing substrate provides some insulation but mostly acts as a thermal buffer to moderate the heat flow – taking up energy during the day and releases it back to the atmosphere in the evening.  This cools our building in the summer.

Green roof traps snow, good insulation

In the winter, the frozen vegetation helps trap snow, a good insulation, on the roof.  As the green roof freezes during winter dormancy, air trapped in the media void space reduces convective heat transfer by the cold winter wind on the roof surface.  The additional thermal mass reduces thermal fluctuations and heat loss from my studio.  I am happy to know that my green roof, despite the plants being in dormancy,  still saves on heating in the winter, although not as much as it cools in the summer.

Spring is on its way! Snow Melt and Runoff

Read part 3 Green Roofs Designed For Low Maintenance 

Stormwater management is perhaps the biggest driver for green roofs in North America right now.  During heavy rainstorms, runoff from roadways and rooftops often overwhelms our storm sewer infrastructure in Toronto, causing flash floods and combined sewer overflow (CSO).  These damage buildings and infrastructure, harm aquatic life and cause beach closures.

The plants and growing substrate in green roofs soak up the rain, slow down the runoff, reduce the peak flow and runoff volume.  The stormwater management potential depends on the green roof buildup such as the type of vegetation, the growing medium depth, incorporation of retention and detention components and the climatic conditions.  A National Research Council of Canada study showed that typical extensive green roofs in Toronto reduced the annual runoff volume by 57%.

Snow on a green roof melts slower, delays runoff

Stormwater runoff is not usually a major concern in the winter as everything is frozen.  However, melted snow produces considerable runoff in the spring and can add burden to our sewer infrastructure.  Green roofs, having larger frozen mass, take longer time to thaw.  Remember how long it takes to thaw a frozen turkey at Thanksgiving?  Compared to a regular roof, the snow on a green roof will melt slower and delay runoff. This delay relieves the burden on the sewer infrastructure during snow melt in spring.

My little green roof is still doing its job helping the environment in the winter despite at a reduced degree.  As I write this blog, I cannot stop thinking about the hardy sedum resting quietly under the snow blanket. They will bounce back to life again with the spring weather as they have the past 12 years – signaling a new beginning.

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