Green roofs & solar

The ultimate combination

"Double your green on one roof"

Green Roofs and Solar PV (Photovoltaic) Panels have become important tools for the sustainable design and construction community. [As they provide many benefits like tools for storm water management, reduction of urban heat island effect, and improve air quality and energy efficient, many cities in North America, such as Toronto, are mandating them by law].

In high density cities, the roof tops are the best option to install Green Space or Solar PV Panels. However, quite often they are fighting for the same roof space. This raises the question: Why not use them together on the same roof? Could the two systems work together to enhance the benefits that each system provide or would the solar panel affect the performance of the green roof or vice versa?

In order to find answers to these questions, a three year research test was conducted. During the test, three Solar PV panels where installed on the test roof. Panel 1 and 2 were installed on a bituminous waterproofing membrane, one on a low and one on a high mounting frame. Panel 3 was installed on an extensive green roof with a high mounting frame. The efficiency of photovoltaic panels depends on their temperature. The greater the temperature of the panel, the lower the level of efficiency. In practice, the temperature of the PV panels increases considerably due to solar radiation. This is compounded by the hot surface of the roof, for example dark bituminous waterproofing membrane, white reflective roof materials or a gravel roof ballast materials, which can easily lead to temperatures of up to 90 °C.

The three year research test has shown that the temperature of Solar PV Panel 3, installed on the green roof, remained closest to the air temperature, while the Solar PV panels on the bitumen membrane were considerably warmer. The result is a difference of 4% higher efficiency annually between the temperature of Solar PV Panel 3 on the green roof and that of Solar PV panels 1 and 2 on the bituminous waterproofing membrane.

Another benefit of combining solar and photovoltaic is that the green roof can be used to ballast the solar array against wind uplift. While traditional systems are ballasted with heavy concrete blocks (resulting in high point loads) or are fastened to the roof deck (resulting in penetrations through the waterproofing membrane with a higher risk of leakage), with the Green Roof, the solar array can be anchored to the green roof assembly, avoiding high risk penetrations through the waterproofing membrane and high point loads. Engineered wind calculations are required to determine the amount of ballast weight required.

In 2013, the city of Toronto added a biodiversity guide component to the green roof by-law. Green roofs have the opportunity to create habitat and enhance biodiversity in the urban fabric of the City. There are three design factors that have been linked to the creation of biodiverse green roofs. First, is the variation in depth, topography and composition of growing media. As the depth of growing media increases, the opportunity to promote biodiversity also increases, simply because a greater range of plant species and plant types can be accommodated. Secondly, is diversity in vegetation. Maximizing the diversity of plant species and plant life forms has many benefits, increasing the opportunities for pollination, food, shade, nesting, perching and nutrients for flora and fauna. Thirdly is to create niche spaces for organisms. The use of structures is a simple approach that can be used to manipulate and increase the utilization of the roof as habitat, all of which help to create different microclimates and microhabitats, which may lead to greater species diversity.

The combination of green roof and solar is sometimes called BioSolar. The ‘Bio’ element of the Biosolar focuses on the provision of biodiversity on green roofs through the use of solar panels. Solar panels on green roofs can, in fact, through good design, provide nice spaces for a more diverse native floral community, which in turn benefits biodiversity, especially pollinators. The solar panels create shade and block the rain on particular areas on the roof. This creates a pallet of diverse growing conditions and microclimates for a wide variety of plant species and microorganism.

As you can see, using solar photovoltaic panels and green roof on one roof is the ultimate combination. They complement each other and are not only beneficial for each other, but also for the building, the owner, the city and the surrounding environment. You have “double green” on one roof.


Jelle Vonk is the business manager of ZinCo Canada Inc. He has a Bachelor of Science degree in Landscape Architecture and Land Development from VanHall Larenstein University of applied sciences in the Netherlands. He has over 15 years of green roof experience in Europe and Canada and has worked on many green roof projects including the Vandusen Visitors Centre in Vancouver, the Waterpark RBG head office in Toronto and the Bank of Canada in Ottawa.