PV Plus operates in alignment with the GRO BIOSOLAR Best Practice Design Guide. This has been developed by the GRO Technical Committee and specifically the GRO BioSolar Working Group. This working group comprises GRO Manufacturer, Supplier, and Contractor members with extensive expertise in solar and green roof systems.
What are the Benefits of BioSolar?
Combining green roofs with solar photovoltaic (PV) panels, commonly referred to as a BioSolar roof, offers numerous advantages. Green roofs contribute to managing surface water run-off, capturing CO2, and improving the efficiency of PV panels through the evaporative cooling effect of the vegetation.
It is important to distinguish BioSolar roofs from PV support frames that rely on ballast, such as paving slabs or stones. These are not designed to support vegetation beneath the panels.
When designing a BioSolar or green roof, careful consideration must be given to ensure safe access and maintenance for all roof components.
BioSolar Design Considerations
The support frame system must be designed and tested in compliance with the relevant standards, based on the type of metal used for the framing system:
- BS EN 1991-1-4:2005 and BS EN 1999-1-1:2007, Eurocode 9 – Design of Aluminium Structures, or
- BS EN 1993, Eurocode 3 – Design of Steel Structures.
When specifying a BioSolar roof, it is crucial to seek bespoke design advice from a BioSolar PV support supplier. Early communication and coordination among all project stakeholders are highly beneficial to ensure a successful outcome.
The design process should include the following deliverables:
- Layout Plan: A detailed layout showing the positions of solar supports and panels on the roof, along with the spacing between panel rows.
- Wind Loading Report: A report using local weather data to calculate the ballast loading requirements for the solar PV supports. Calculations should align with BRE Digest 489 and BS EN 1991-1-4: Actions on Structures.
- Ballast Loading Plan: A specific plan indicating varying ballast loadings if a zonal arrangement is necessary.
BioSolar Bases
Each support base must be positioned on a stable and appropriate surface, such as a waterproof membrane, a Water Flow Reducing Layer (WFRL installed over inverted insulation), a suitable void former, or a drainage board with sufficient compressive strength. These bases are specifically designed to integrate into the green roof system and should never be installed directly on substrate or aggregate layers. Adhering to the manufacturer’s installation guidelines for the selected system is crucial for optimal performance.
Structural Wind Load Calculations
Wind load calculations must be performed in accordance with BS EN 1991-1-4:2005 – Eurocode 1: Actions on Structures, Part 1-4, and should align with the guidelines outlined in BRE Digest 489. These calculations ensure that the green roof buildup has sufficient dry weight to meet the ballast requirements for securing the solar PV supports.
Ballast loading should be based solely on project-specific wind load calculations, and the saturated weight of the green roof should not be factored into these calculations. For inverted roof systems, a minimum dry weight of 80 kg/m² is recommended to prevent insulation flotation.
BioSolarDesign Guidance
The solar panels must be arranged with adequate spacing between rows to enable safe and practical maintenance of both the PV panels and the green roof. The minimum required spacing should be obtained from the BioSolar system supplier and should align with Health and Safety Executive guidelines.
Fire Considerations for BioSolar Systems
For BioSolar roofs, the standard fire prevention measures used in green roof design, such as vegetation-free margins, minimum substrate depths, and appropriate substrate specifications, remain applicable. These requirements are outlined in the GRO Green Roof Code.
Solar panel supports generally do not require vegetation-free margins unless their upright sections penetrate the waterproofing membrane. Additionally, on larger roofs, a one-meter-wide fire break should be included at intervals of 40 meters, as recommended in the GRO Green Roof Code.
All electrical work for the installation and commissioning of PV panels must comply with MCS guidelines (MCS Certified | Confidence in home-grown energy). Any cabling associated with the PV system that runs on the substrate surface or within the green roof layers should be enclosed within a secure conduit.
BioSolar Maintenance
All green roofs require regular maintenance, and BioSolar roofs are no exception. To ensure the long-term success of a BioSolar system, a robust maintenance regime is essential. PV Plus offers specialised solar maintenance services, ensuring that both the green roof and PV system operate efficiently and safely.
The GRO Green Roof Code highlights the need for proper maintenance to prevent the build-up of organic matter, which could pose a fire risk. GRO recommends that BioSolar roofs be maintained at least twice a year as a minimum standard.
During maintenance visits, the following tasks should be performed:
- Vegetation Management: Ensure vegetation remains below a height that could cast shade on the solar PV panels.
- Clearance: Check all fire breaks during each maintenance visit and ensure they are cleared of vegetation or combustible materials.
- Substrate Gap: Maintain a minimum gap of 200mm between the lower edge of the PV panels and the substrate surface.
- Biomass Removal: Remove old biomass, including seed heads and dead vegetation, annually—typically during autumn.
- Weed Control: Remove invasive weed species.
- Shading Prevention: Remove individual plants that may shade the panels.
- Substrate Depth: Verify that substrate depths remain consistent with the original wind loading requirements.
- Vegetation Repair: Reseed any bare areas as necessary.
- PV Panel Maintenance: Clean and inspect the solar PV panels.
Records of each maintenance visit, detailing the work completed, should be provided to the building owner or the entity responsible for building maintenance. Maintenance of the solar PV system must comply with MCS and manufacturer guidelines. If solar PV panels require cleaning, the cleaning materials must be verified as safe for use on the green roof. Similarly, any fertilisers applied to the green roof should be assessed to ensure they do not impact the performance of the solar mounting system or PV panels.
Failure to perform regular maintenance can reduce the performance of both the green roof and the solar PV array. It also increases the risk of fire and other safety hazards.
BioSolar Irrigation
If an irrigation system is installed on the roof, it is crucial to avoid over-irrigating the vegetation beneath and around the PV panels to prevent excessive plant growth, which could lead to shading issues.
