Durability of the waterproofing system is key and its life span should, at a minimum, match that of the PV scheme, as well as be able to withstand any additional access requirements for maintenance.
In new build flat roof construction, the availability of roof space should be considered at concept stage to meet any planning conditions or BREEAM requirements, with other rooftop elements and safe access systems designed and incorporated appropriately.
Meeting BREEAM, Planning, ‘Merton Rule’ and LZC Technologies
In new build applications, the sizing of the solar PV array will usually be influenced by low or zero carbon (LZC) technology targets either driven by BREEAM or local planning conditions. The array size will be determined by efficiencies of other elements of the building design and so it is important to establish available roofspace and what can be achieved from this area. The maximum outputs from the array can therefore be established as early as possible and help drive decisions on other building elements.
Combining Solar PV and Green Roofs
In urban areas the inclusion of both a green roof and a photovoltaic system can be a prerequisite for the building, which can bring challenges to the designer on how to locate both within the roof area. Where roof space is restricted the two technologies can compete for position and so layering the green roof and PV array so that they can co-habit the same area is a feasible solution.
Ideally, the PV panels should be raised above the substrate and vegetation, allowing the plants to also grow beneath the panels with sufficient light and moisture levels. The selection of vegetation and growth height is important so that the plants do not create areas of shading on the panels.
The weight loading of different systems and their installation methods can be impactful on the construction where a ballasted PV system on a building in an exposed location can impose loads as high as 160Kg/m² compared to other methods of PV installation which could impose as low as 9Kg/m².
The selection and design of a mansafe system or the parapet height required for safe maintenance access can have an influence as they too can create areas of permanent or fluctuating shade that would impact on the output of the PV panels.
When planning a PV array for an existing building the condition of the flat roof should be taken into account before the mounting method and array location can be determined. These are some key points to consider and clarify before proceeding with the project:
- Can the building withstand additional weight loadings and wind forces? If not, the project should be evaluated to determine whether additional structural support costs are worthy.
- Does the current roof covering warranty or life expectancy match that of the PV array? If not, the roof covering may need replacing before the PV.
- Is the roof covering due for renewal? If so, it may be worth bringing forward the installation so that both are installed simultaneously.
- Will the mounting or ballasted system put the current waterproofing at risk? If the waterproofing system is penetrated by the PV fixing methods, the integrity of the roof could be compromised and the warranty invalidated.
- Is the current waterproofing system able to withstand required access traffic for the installation and maintenance? If not, suitable protection is required.
- Are there shading items that could reduce the array’s efficiency and if so, to what extent? Shading on the PV cells reduces electricity flow and therefore output. If this is the case, the array could be located on a different roof area. The inclusion of a power optimiser can mitigate the efficiency reduction and actually improve efficiencies.
- Does the existing safe access system meet the needs of additional traffic and equipment required to maintain the PV array? If not, the mansafe system may need updating or a permanent barrier may be required, which itself may shade the panels.
- Are there access limitations, such as timing of work or health and safety of the building’s occupants, which would impact costs or installation? A discussion at pre-tender stage can identify challenges early on so a different delivery schedule can be developed.