Photovoltaic solar panels on green roofs: benefits

What are green roofs?

Green roofs, also known as living roofs or vegetated roofs, are systems designed to integrate plant life and gardening techniques on building rooftops. They consist of several layers that not only protect the building but also allow plants to grow without damaging the roof structure.

Green roofs typically consist of the following layers:

Vegetation layer: plants selected for their resistance and adaptability to the local climate.

Substrate layer: growing medium or soil that provides nutrients and support for plant life.

Filtration layer: prevents the substrate from travelling down into the lower layers.

Drainage layer: allows proper water flow to avoid waterlogging.

Waterproof membrane: protects the roof structure from seeping water.

Protection and insulation layer: provides thermal and acoustic insulation and protects the waterproof membrane.

Types of green roofs

Green roofs can be divided into two main types: extensive green roofs and intensive green roofs.

• Extensive green roofs: their main feature is their lightweight nature and low maintenance requirements. These roofs use hardy, low-growth plants such as sedums (also known as stonecrop) and other succulents or cacti. This kind of roof is suitable for small areas and is usually not designed for public access.
• Intensive green roofs: these roofs are heavier and can support a wider variety of plants, including shrubs and small trees. They have a much deeper substrate layer, which makes maintenance more intensive and costly. These roofs are often designed for recreational or aesthetic purposes, turning the rooftop into an accessible garden. 

Benefits of green roofs

The benefits of green roofs are quite extensive. Let’s take a look at a few examples.

• They’re eco-friendly: it's widely known that plants help reduce air pollution and mitigate heat, making them an ideal alternative to combat so-called ‘urban heat islands.’ In addition, they serve as a refuge and habitat for insects and birds, helping improve urban biodiversity.
• They have energy-saving potential: plants contribute to thermal insulation, so having a small garden on your rooftop influences a building’s temperature, reducing the need for heating in winter and air conditioning in summer.
• They act as hydrological systems: plants and soil absorb rainwater, so they function as drainage systems, decreasing the risk of flooding and relieving the burden on urban infrastructure during heavy rainfall.
• An urban oasis: green roofs can beautify a building and act as recreational, leisure, or even cultural spaces for residents and occupants.
• They're cost-effective: although the initial installation can be expensive, over time, green roofs can lead to significant energy savings and lower building maintenance costs, as they protect the structure from the elements.

Disadvantages of green roofs

While, as we’ve seen, green roofs offer many benefits, it’s also important to consider the potential downsides or challenges associated with their installation.

• High upfront cost: installing a green roof requires a significant investment, far more than a conventional roof. These costs include vegetation, substrate, preparation, and installation of multiple layers, irrigation systems, and waterproof barriers.
• Additional weight: these roofs add considerable weight to a building, so not all structures can support the initial load. That’s why a proper engineering analysis is essential to ensure proper roof installation and waterproofing.
• Higher maintenance: even though extensive green roofs require less upkeep, they still need a minimum level of care. Irrigation, fertilisation, and weed control make maintenance similar to that of a traditional garden.
• Pests and biodiversity: while green roofs help attract and protect urban biodiversity, they may also draw pests. It’s therefore essential to properly manage the rooftop ecosystem to prevent infestations that could harm both the plants and the building.
• Regulations and permits: special permits will likely be required, and specific regulations must be met before installing a green roof. This process can be both costly and time-consuming.

Many of these drawbacks can be overcome through proper planning and professional installation, along with a thorough feasibility analysis. Their long-term benefits – both financial and ecological – can far outweigh the initial challenges.

Benefits of combining green roofs with solar panels

Solar panels can be combined with green roofs. In fact, combining both systems can maximise the benefits of both green roofs and self-consumption installations:

• Greater efficiency of solar panels: green roofs help lower the roof temperature. The plants and substrate reduce the heat reflected on the roof, which can increase the efficiency of solar panels, as they perform better at lower temperatures.
• Increased durability and lower risk: the plants provide protection from the elements – heat, radiation, rain, hail, and extreme temperature fluctuations – thereby reducing the long-term maintenance requirements of the solar panels.
• Ecological synergy: combining solar energy and green roofs maximises the sustainable use of roof space. While the panels generate clean energy, the green roof contributes to biodiversity, improves air quality, and manages rainwater.
• Thermal insulation: as mentioned earlier, green roofs provide additional thermal insulation that can reduce the need for heating and cooling in the building. If we add to this the energy savings offered by solar panels, the result is the perfect combination.

Considerations for installation

Before installing a combination of solar panels and green roofs, there are several aspects to consider.

• It is crucial to ensure that the roof structure can support the combined weight of the substrate, vegetation, and solar panels. Therefore, a prior structural analysis is necessary.
• The design must consider the optimal orientation and tilt for the solar panels and ensure access for maintenance – both for the panels and for the plants.
• Compatibility between the two systems is essential. Some green roofs may require specific adaptations to accommodate solar panels, such as irrigation systems that do not interfere with the panels.

In summary, installing solar panels on green roofs can provide a range of additional advantages that improve a building’s energy efficiency and extend the lifespan of both the panels and the roof itself. Some cities and landmark buildings are already implementing these hybrid solutions to maximise the sustainability of their infrastructure and enhance the urban landscape, combining nature and architecture to create healthier spaces.