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What are the real benefits of a biosolar roof?

by Matt Downs

Luke Rootham AIoR, Product Manager for Axter Solar, and Michael Anaxagorou, Sales Director, Axter and member of the board of the Green Roof Organisation, discuss the whys, hows and overall benefits of installing a biosolar roof...

Above: Luke Rootham AIoR, Product Manager for Axter Solar.

Above: Michael Anaxagorou, Sales Director, Axter and member of the board of the Green Roof Organisation.

With sustainability and improving energy performance top of mind for many building owners, a biosolar roof is a practical solution that delivers on both fronts. Green roofs support compliance obligations in areas such as sustainable drainage and biodiversity net gain. Combining these benefits with a solar photovoltaic (PV) system, not only offers energy generation and lower bills, but research also shows that the two together achieve better overall performance. 

Why a biosolar roof makes your PV panels work better
Solar panels naturally heat up as they absorb and process sunlight. As they rise above about 25 degrees Celsius, their efficiency decreases quickly. A green roof will cool the ambient temperature, bringing the efficiency of the panels back up. This is because water will evaporate from the vegetation and the damp earth in a process known as evapotranspiration, negating the urban heat island effect.

A biosolar roof with arrays packed in too closely together won’t result in all of the benefits discussed, which is why GRO (the Green Roof Organisation) recommends that where PV is mounted on a roof, the application and area of the green roof be maximised, and panels should be spaced a minimum of 750mm apart.

Above: A 3D rendition of a biosolar roof system.

Why the solar panels help the natural habitat on your roof
The PV element of a biosolar system creates a diversity in the conditions across the roof. Some portions will be exposed, and others shaded. Water runoff from the panels will collect, creating a mix of damper and drier areas. This creates what is called a habitat mosaic, in which a wider variety of flora can flourish.

The physical presence of the panels also creates wind-shielded areas in which many species will thrive. The overall result is an increase and a greater diversity in vegetation, which will attract other life, such as butterflies and birds, increasing the overall biodiversity as a result of the biosolar combination.

Above: Another image from the Clapham Place project.

Case study: Clapham Place
Clapham Place is a major new development in South London, comprising 62 luxury flats, each with a private balcony or terrace and various other amenities. The project was completed by Regal London in 2021, with Axter supplying the roofing solution, combining Wilotekt Plus hot melt waterproofing and a biodiverse living roof finish, installed by chosen partner TM Roofing. It is fantastic to see nature thriving in the heart of the metropolis as a result of this project.

Case study: Parsloes Park, Dagenham
Contractor Etec Group asked Axter to join the team completing a new public sports centre. For planning reasons, the development needed to meet stipulated minimums of area assigned as a green roof, and also PV output. 

The only way to achieve this was by utilising a biosolar system to ballast the array, in combination with Axter’s HydroSoil Sedum roof in areas where solar isn’t present, which allows a greater area to be covered in substrate and flora, while also delivering the required number of solar units. This ballast approach has the added benefit of anchoring the frame-mounted panels securely without any penetrations that could potentially compromise the underlying waterproofing.

In this case, another supplier of photovoltaics was already on board, and we were able to integrate their solar units seamlessly into our biosolar system, as our frame mounts can be used either with our Crystalline PV panels or to hold any compatible PV that is chosen. 

Axter also supplied Cityflor waterproofing. PV cabling was safely routed in raised cable trays mounted on protective supports that maintain drainage, do not encourage the build-up of roof debris, and protect the waterproof membrane from damage. Roofing work by Axter-accredited installer Hambro Roofing was completed in October, with the centre expected to open soon.

Maximising the potential of your roof
With high energy prices, the benefits of installing solar units are clear, generating renewable energy that can be used to reduce bills and even sold back to the grid, while also improving the carbon footprint of the building. 

Green and brown roofs also bring many benefits, adding beauty and biodiversity to your building, and with the right system, they also help manage drainage, by reducing and delaying stormwater runoff. Biodiversity net gain and sustainable drainage will both become mandatory for most new construction next year, and green roofs can contribute to meeting these requirements. 

Rooftops are an underutilised resource. A quarter of all available space in our cities is in the form of our roofs – just think what could be achieved in terms of energy savings and increases in biodiversity if all suitable roofs were transformed into energy-producing natural habitats.


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