Solar PV on Flat Roofs Design Considerations Overview

In the UK, solar photovoltaic (PV) is a popular renewable energy and its deployment is rising rapidly across the globe. With recent fluctuations in energy markets and carbon reductions initiatives coming to the fore, the number of flat roof installations will continue to rise as local authorities and businesses look to reduce their carbon footprint and gain energy security for the future.

All too often within the construction industry, photovoltaic specifications focus on energy generation and outputs of the solar system. Any solar designer or specifier should give the same focus to ensuring the rooftop array is installed with methods that have as little impact as possible on the building and its waterproofing and that the array works to its maximum potential for its entire lifespan.

Specifying PV on new buildings

There are numerous reasons for including a solar PV array on a new flat roof. Large commercial buildings can often have the most suitable energy usage profile to benefit from the electricity produced from a rooftop PV array.

Some core reasons for inclusion are:

Embrace the driver to achieve Net Zero.
Meet environmental schemes such as BREEAM and LETI for CO₂ reduction and site based renewable energy.
Satisfy the planning conditions of local authorities.
Provide energy security and save money.

Motives for retrofitting PV on existing buildings

Retrofitting photovoltaic panels brings all the benefits of low maintenance renewable energy generation to an existing building, with the ideal opportunity for the installation to take place when the roof covering is being replaced.

Some core reasons for inclusion are:

Meet sustainability targets for the building and reduce its carbon footprint.
Embrace the driver to work towards achieving Net Zero.
Improve the value and rentability of building stock.
Provide energy security for the future and save money.

Considerations for retrofitting PV

When planning a PV array for an existing building the condition of the flat roof should always be considered before the mounting method and array location can be determined. These are some key points to consider and clarify before proceeding with the project.

Additional dead and imposed weight loadings.
Condition of existing waterproofing system and is it suitable, and impact on existing waterproofing warranties.
If the roof covering is due for renewal.
Fixing method of the array so it doesn’t penetrate the waterproofing.
Shading items in situ that will affect the array’s performance.
Safe access for maintenance of both the solar PV array and roof.
Access limitations for installation.

Sizing of PV arrays

The size of a PV array can be determined by several elements that are often driven by the overall aim of the scheme.

On new build developments, the size of the array will usually be determined by carbon reduction targets or local planning conditions. This can lead to complications if the size of the output required is not applied to the available roof space to ensure that the system will physically fit on the roof and avoid conflict with other roof mounted equipment.

For all installations, these core elements apply:

Where return on investment (ROI) is clients principal focus this can be achieved by designing the system to match the building’s energy profile where as much as possible of the energy produced is consumed by the building.
Avoid shaded roof space, accounting for rooftop plant, access equipment, adjacent buildings, trees etc.
The client’s budget and the project’s financial plan can allow for the inclusion of a PV system though the capital budget may determine scheme size rather than an optimal return on investment or roof size.
Ensure both safe access and fire considerations are accounted for.

Combining a green roof with the solar PV array

Environmental considerations such as carbon reduction, biodiversity loss and storm water management are driving the inclusion of both a green roof and a photovoltaic system for new buildings. This can bring challenges for designers on how to locate both within a limited roof area. Biosolar systems provide the solution through the layering the green roof and PV array so that they can co-habit the same area.

In a biosolar system, the PV panels are raised above the substrate and vegetation, allowing the plants to also grow beneath the panels as they receive sufficient light and moisture levels. The selection of vegetation is important to ensure that vegetation cover is achieved across the roof, but the plants do not create areas of shading on the panels.

Weight loading

The weight loading of different systems and their installation methods should always be considered. A ballasted PV system on a building in an exposed location can impose loads as high as 60 kg/m² which can impact both structural stability and compress waterproofing membranes and insulation. This is compared to other methods of PV installation which could impose as low as 9 kg/m².

It is important to consider the selection of PV modules as these can also not only vary in size but obviously in imposed loads as well.

Fixing methods for solar PV arrays

There are two fundamental options for fixing a PV system to a flat roof, ballasted or mechanical. A ballasted system adds additional weight to anchor the array to the roof whereas mechanical installations cover two key methods, either they are fixed to the deck penetrating the roof covering or they do not and leave the waterproofing system intact.

Non-penetrative mechanical fixing is where the mounting system sits completely separate atop the waterproofing via mounting units, that are held onto the roof through mounting plates and welding or bonding overlying membrane sleeves to the uppermost layer of the waterproof covering.

Penetrative mechanical fixing method embraces two forms where the array is installed either via the creation of a plinth or a proprietary fixing post. Any penetration through the waterproofing membrane should be considered.

Ballasted PV systems require confirmation from a structural engineer that the additional weight and wind load can be accepted into the design. The ballast itself can take different formats and it is important to confirm that the static load created by the designed ballast will be appropriate and sufficient in accordance with the wind load calculation report.

In biosolar roofs the substrate and vegetation of the green roof should provide the ballast to secure the array to the roof. This removes the requirement for penetrations or additional ballast above the vegetation layer. The entire roof area can then qualify as a green roof for biodiversity enhancement purposes.