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Ballast for solar on flat roofs – do you really need it?

Most flat roof solar installations are ballasted. It means, the solar installer will put ballast stones below the solar panels to keep the solar installation in place, and not be lifted off the roof in case of extreme winds. In most cases, Over Easy Solar’s VPV unit eliminates the need for ballast.  


What exactly is ballast? 



Ballast for a solar installation is “something heavy” that gives additional weight to the installation, to counteract for lift and push forces generated by wind. In most cases, concrete blocks or stones are used, mainly because such stones are easily available, easy to stack on a pallet, and relatively easy to handle for the installers on the rooftop.  


How much ballast is necessary for a flat roof solar installation? 

Depending on the type of installation, the building geometry, and the wind zone, the necessary amount of ballast varies a lot. The need for ballast is especially high on small installations (like a private house or garage), on tall buildings and in areas where you have strong winds. It also depends highly on which mounting system is used – south-facing solar panels elevated at a distance from the rooftop, for example, gives high wind loads and high need for ballast.


In the best cases, conventional installations may need only 5-10 kg of ballast on the outer rows of panels in an installation. The worst cases may need over 100 kg of ballast for every panel in the installation. For a private house, we could be talking about several tons of ballast added to one small rooftop. For a higher and larger commercial rooftop in a windy area, we are talking about tens of tons.  


Why should you care about ballast for a solar installation? 

For the owner of the solar installation, ballast seems not to be a very big problem. It only lays there, right? But there are some important things you should keep in mind:  

  1. What is the load bearing capacity of the roof? Not only is the structural strength important, but also the strength of the roofing membrane and the underlying insulation. Both can take damage from a heavy installation, especially if the weight is concentrated on a small surface area.  

  2. What is the maintenance need of your roof? If you think it is likely that there will be some maintenance work on the roof in the next 30 years, which demands that you remove the solar panels, ballast may be a challenge as it complicates the work on the roof (imagine finding the ballast plan for a solar installation in 15 years from now!).  

  3. For the installer, ballast is not desired because it introduces more work and logistics with the installation. A wind-exposed solar installation will therefore be more expensive, and installers may be reluctant to take on such projects. Sadly, some installers will also forget the ballast or even manipulate the calculations or the mounting system itself, sometimes resulting in accidents during strong wind events. 


How much ballast is needed for Over Easy Solar’s VPV unit? 

In 2023, we published the results of our wind certification. After thorough testing both in digital models and in a physical wind tunnel, we have now proven that our technology is ballast free for most flat roof buildings. Meaning, you can save money on ballast in your solar system and save your roof the strain of a heavy installation. 


As for other installations, a calculation for each project is needed. Our PV engineers will help you plan your installation step by step, customizing it to the needs of your roof. 


Innovative, vertical solar panels on a green sedum roof.
A 45 kWp solar system at Løren skole in Oslo, with zero ballast needed.

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