Steven Vincent, Major Projects Manager at Eco Green Roofs (EGR), in conversation with Architecture Today’s Technical Editor John Ramshaw, discusses best practice design for the innovative SolarNature roof system from SIG Design & Technology (SIG D&T – now AccuRoof) and EGR.
When it comes to environmentally responsible roofing solutions, specifiers and their clients tend to think in specific terms: a green roof, a blue roof, or perhaps a solar roof incorporating photovoltaic panels. But there is a new typology emerging, which combines the advantages of both green and solar roofs in a single integrated and accessible system. Among the leaders of the UK ‘biosolar’ market is SIG Design & Technology, which in partnership with Eco Green Roofs, designs, supplies, installs, and maintains the innovative SolarNature system. So, how does the product work? What are the main features and benefits? And how should architects approach the design and specification and processes? Steven Vincent, Major Projects Manager at Eco Green Roofs, discusses these questions and more with Architecture Today’s Technical Editor John Ramshaw.
What is SolarNature and what are the benefits of the system?
SolarNature is a complete roofing system, including the waterproof layer, green roof build-up and integrated photovoltaic installation. It addresses two fundamental issues. First, and somewhat counterintuitively, PV panels provide more electricity if they are kept cool. This is best achieved by surrounding them with vegetation, such as on a green roof. Second, a ‘traditional’ roof-mounted PV array does not allow the plants on a green roof to thrive as it produces too much shade and not enough water.
The demand for biosolar systems, such as SolarNature, is currently being driven by the need to achieve both carbon net-zero and biodiversity net gain. This is particularly important in urban locations and on public projects, such as schools, where existing green spaces are often built over to provide additional facilities. We have limited roof space so why not maximise every single square metre to achieve the best insulation, attenuation with a layer of blue roof, biodiversity gain with a green roof, carbon zero with PV, and recreational planted spaces.
So how does SolarNature differ from traditional PV installations on green roofs?
Traditional PV systems are mounted close to the surface of the roof and are laid fairly flat, i.e. shallow incline. This means that on these green roof applications the vegetation only survives around the panels because nothing can grow beneath them. By contrast, SolarNature increases both the distance from the underside of the panel to the top of the roof and the angle of the panel itself, creating more generous spaces beneath that can accommodate both healthy plant life and biodiversity.
The panels themselves are supported by the green roof build up. This incorporates mounting frames within the green roof reservoir board, which helps drive the rainwater back under the solar panel. The growing substrate provides the necessary ballast, and we carry out wind mapping to provide the correct depths required to compensate for the wind uplift. Corners and perimeters have a bigger vortex to contend with, so the substrate is deeper in these zones.
Central to the SolarNature system is an innovative drainage reservoir board located at the base of the green roof. Water, which is held in ‘cup-like’ containers, within the board, evaporates as the roof gets warm, soaking the filter layer and soil above – just like the water table. Excess rainwater in the board is directed to the perimeter where it is drained away through the roof gullies/outlets. Any trapped water can evaporate back up through small perforations in the board, feeding the soil substrate above.
How should specifiers approach the design process?
The specification process might best be described as a ‘dance’ between several closely related factors and early engagement with SIG D&T is highly recommended to save both time and cost. SIG D&T also adopts a holistic approach to roof design, whatever type of system is being used, ensuring that key issues relating to fitness-for-purpose, safety, buildability, and warranties are considered from the outset.
At the beginning of the process, the client and design team will need to know the carbon net-zero and biodiversity net gain targets they need to achieve from the roof(s). This will have a direct effect on how many PV panels are needed, their size, installation orientation and angle, the spacings between them (to allow for maintenance access and the system to function properly), and the amount of ballast required (relating to the size of the PV units).
Next, the design team will need to consider the size and position of all the services on the roof that reduce the available area for the SolarNature installation. These can include penetrations, soil and vent pipes, lift over runs, risers, rooflights, hatches, cable pathways and HVAC equipment.
A detailed roof plan will then be handed over to the PV provider who will work to the agreed format in terms of the carbon/biodiversity split for the installation, before using SIG D&T and EGR spacing recommendations and running shading modelling. This will result in a layout drawing featuring panel sizes and orientations. We use this to run wind mapping analysis, which produces a ballast weight map for each module base.
The map is based on the dry weight of the growing medium, which ensures adequate load in dry conditions. SIG D&T and EGR then send the saturated system weights to the project team to ascertain viability for the building’s structural frame and roof. The project team will then reconsider the carbon zero and biodiversity net gain targets, against a range of factors, including roof weight and costs relating to the size of the biosolar system, installation and ongoing maintenance, before agreeing the final specification.
How should installation of the system be carried out on site?
SolarNature can be installed by a fully trained SIG Design & Technology Accredited Contractor (DATAC) or EGR. The contractor will work from a detailed setting out plan, provided by SIG D&T and EGR, to ensure that the spaces between the PVs are in accordance with the design specification.
How should the system be maintained post installation and is there a warranty available?
SIG D&T has a rolling warranty that lasts for the life of the roof – providing the system is properly maintained. The maintenance schedule usually involves biannual inspections/services, which can involve strimming any vegetation that is blocking the PV panels, topping up soil levels and nutrients, and checking that the system is working properly. The maintenance visits effectively guarantee the installation for the following year.
