Which foil roofs are suitable for photovoltaic systems?
Look out over the roofs of a town and you’ll quickly notice just how many different types there are. It’s not just the way they look; they also work in different ways, with various slopes and materials. You can see what this means for installing rooftop photovoltaic systems in this and our further blog articles on special roof types (e.g. bitumen roofs etc.).
There are basically two types of roof: sloping and flat. While inclined roofs are traditionally found more in residential suburbs, flat roofs frequently top buildings in industrial parts of town. The reason? A flat roof is the most cost-effective option for roofing large areas in the majority of cases. They also permit the integration of a roof terrace or special lighting concepts such as light domes.
What are the differences between foil roofs?
A foil roof is a cost-effective form of flat roof. Unlike a bitumen roof, which consists of a mixture of various organic materials, a foil roof is made from different plastics. The qualities of the foils largely depend on the plastics used to manufacture them: polyvinyl chloride or polyolefins. Both these foils have specific advantages for a flat roof.
- The universal plastic, PVC (polyvinyl chloride), is used as standard in most cases, mainly because of its low cost. However, the plasticiser in this plastic evaporates over time and can adversely affect the service life of the roof covering. It is also harmful to health.
- Roof membranes made from polyolefins (FPO), on the other hand, are a modern, flexible alternative. They last longer than PVC because they contain no plasticisers. At the end of many years of use, this plastic is also much easier to recycle. Installation is less harmful to health because the seams joining the foil are heat welded. FPO in a roof membrane cross section prevents root damage, which is one of the reasons behind its widespread use, including as an effective base layer for green roofs.
Other differences between foil roofs relate to their insulation material (polystyrene foam, rock wool). Every “building with a use” must be insulated in accordance with the German energy conservation regulations (EnEV). This document also applies to industrial roofs, such as production halls and warehouses.
- Polystyrene foam is one way of insulating foil roofs. A big advantage of this material is its low self-weight, which is helpful for photovoltaic installations. However, polystyrene foam has inferior insulation properties and is combustible, which is why rock wool is most frequently used in practice.
- Rock wool has excellent thermal and sound insulation properties. In contrast to polystyrene foam, rock wool is incombustible. Its higher self-weight and its distinct lack of rigidity are disadvantageous for roof use. These factors must be considered in photovoltaic installations, but do not fundamentally preclude them.
What must be considered for a PV installation on a flat roof?
Our Energy Systems team specialises in the installation of rooftop PV systems. You can have your roof assessed by our experts to see whether it is suitable for solar modules.
The assessment looks for any existing damage to the roof, which must be rectified before the PV system is installed. In this case, we work with long-term partners who can perform the repairs. The plans must take into account any items that might interfere with the installation of the rooftop system, such as lightning protection, smoke and heat extraction fixtures or light domes. The latest advance involves using a drone to take photographs, which can be processed to create a 3D model of the flat roof.
In addition to examining any damage and interfering rooftop objects, the technicians measure the “friction coefficient” during the assessment of the foil roof. The friction test is done to exclude any risk to the overall stability of the system due to e.g. strong winds. This static friction or “stiction” depends on the weight of the whole PV system (solar modules, substructure and ballast) and the interaction of the various materials involved. The test is done on site at various positions, in both the dry and wet condition.
What safety aspects must be considered for the installation of photovoltaic systems on a foil roof?
In accordance with the flat roof guidelines, flat roofs such as foil roofs must have a fall of at least 2% to ensure, for example, that water can drain from the roof during heavy rain. There are also some hidden dangers: to prevent the aerodynamically optimised substructure from sliding off the roof, it is normally weighted down with ordinary, inexpensive paving slabs.
Flat roofs must also be safe for people to work on. Therefore we maintain an appropriate edge distance with our rooftop PV system installations. To prevent falls from height, we often use anchor points, which are required from a height of two metres. These are fixed anchor points to which personnel attach a (steel) rope to ensure their safety during roof work. An even better solution is to allow for integrated roof fall safety measures at the planning stage for a new photovoltaic system.
We would be delighted to go through the possibilities of rooftop PV systems on foil roofs with you! Keep up with the news and find out more about PV on different roof types.
Picture: Dirk Brandecker