Under the leadership of GRÜNSTATTGRAU, the position paper for value-added roof space utilization was written – a guide for decision-makers, the real estate industry and municipalities.

The aim of the paper is to provide basic information and data for a value-added-oriented system decision when changing or extending the use of existing and new roof areas. In collaboration with 20 experts, including RENOWAVE.AT, different variants of roof space utilization are classified and evaluated in terms of their potential to reduce CO₂ emissions.

The position paper emphasizes that different types of use such as green and solar roofs do not compete for space, but rather have a kind of symbiotic relationship with each other. The combination of these uses offers the greatest added value for residents and nature. Experts from various fields, including photovoltaics, vegetation technology, green building, water management, environmental medicine and public health, contributed to the paper. It illustrates that the use of roof surfaces has many advantages, from energy generation and the use of daylight to solar or wind power and the greening of the roof cladding:

• Energy production through the installation of PV, wind power, heat pumps or the like
• Energy savings due to additional insulation effect and cooling capacity through evaporation of water
• Retention of rainwater on site
• Reduction of precipitation peaks and savings on wastewater tax and sewers
• Reduction of surface temperatures
• Protecting the roof waterproofing and increasing the service life of the roof
• Binding of dust and air pollutants
• Yield increase of solar and photovoltaic systems mounted above
• Promotion of biodiversity
• Binding of CO₂
• Use of daylight through skylights

Vienna’s roof surfaces offer potential for greening buildings and using solar energy. The green and solar roof potential register shows an overlap that enables effective multiple use. This innovative approach can be successfully applied not only in Vienna, but also in other Austrian municipalities and cities.

CO2 savings through individual use of roof surfaces

The CO₂ savings effect of solar thermal energy is calculated by comparing the heat generated with the average Austrian district heating mix. This results in CO₂ savings of 78 kg CO₂e per m² and year, which corresponds to a total CO₂ reduction of 3,870,544 tons of CO₂e per year.

The CO₂ savings effect of photovoltaics is calculated by comparing the green energy produced with the average Austrian energy mix. Photovoltaic systems have a CO₂ saving effect of 33 kg CO₂e per m² and year, with a total CO₂ reduction of 1,642,230 tons of CO₂e per year for the economic potential without façades.

Green roofs can bind up to 10 kg of CO₂ per m² and year, which corresponds to a CO₂ reduction potential of 885,600 tons of CO₂e per year for Austria’s entire roof area.

Multiple use and climate protection potential

The synergy of building greening and solar technologies increases the efficiency and yield of the existing areas. Selected findings on multiple use:

Solar green roofs for increased energy yield: The evaporation of 60-75 percent of the total annual precipitation by plants increases the yield of roofs mounted over solar and photovoltaic systems by up to 5 percent.

Roof greening as a carbon sink: An extensively greened roof area can bind up to 23.6 kg CO₂/m² over 50 years.

Green roofs mitigate the effects of climate change – compensation for sealing: Retention of rainwater on site reduces the frequency of overflow events in the combined sewer system by up to 99 percent. This leads to a longer service life of the infrastructure, reduces costs for erosion control and minimizes flooding.

Green roofs mitigate the effects of climate change – heat reduction: Due to their higher albedo (reflection of solar radiation), green roofs have a surface temperature that is up to 25° lower than that of bitumen and gravel roofs. This leads to a reduction in the average air temperature by 0.2 to a maximum of 1.5°C, saves cooling capacity and extends the service life of the roof thanks to more moderate temperatures between day and night.

Effects on climate, health and well-being

From a public health perspective, the use of green roofs offers a number of positive aspects that can have an impact on the health of the population:

• Improving air quality
• Noise insulation and sound level reduction
• Oasis of peace and “cool spot”
• Improving biodiversity
• Improving the urban microclimate
• Improving people’s physical and mental health
• Improved lettability and saleability of properties and units
• Use of roof areas in relation to food security and community gardens

Proposals for adapting to the climate crises

Roof space utilization plays a crucial role in adapting to the climate crisis. This includes the use of solar energy to reduce CO2, the integration of rainwater harvesting systems to save water, the promotion of urban farming and community gardens for biodiversity and the creation of habitats for wildlife. Raising public awareness is also of great importance in order to promote sustainable roof use through education, subsidies and training.

You can find the full position paper here.