Photovoltaic systems are experiencing a real boom - on private homes as well as on large commercial buildings. But while the modules on the roof convert solar energy into electricity, something often goes unnoticed: The sub-roof plays a decisive role in longevity, efficiency and protection. Depending on the type of installation, there are different requirements for sealing, temperature resistance and building physics. In this article, we show you what you should pay attention to when installing a sub-roof under a photovoltaic system and how to avoid typical mistakes.
On-roof vs. in-roof - two systems with different requirements
The choice of installation method for a photovoltaic or solar system has a direct impact on the sub-roof. We distinguish between two approaches to systems:
- Roof-mounted installation: common solution with low additional load
Here, the solar installation is mounted over the existing roof covering. This means that the traditional roof cladding remains intact and continues to act as the first water-bearing layer. This solution is particularly popular for retrofitting and new builds, as it is technically easy to implement - provided that the fixing points do not damage the waterproofing layer. The use of system-compliant brackets is essential here. If the on-roof installation is carried out professionally, there are no additional requirements for the sub-roof. - In-roof installation: Aesthetically pleasing - technically challenging
With in-roof systems, the solar modules themselves serve as the roof covering. Here, the solar modules take on the function of weather protection. This brings design advantages, but places higher demands on the building physics - especially for flat pitched roofs. This is because in-roof systems are often not completely rainproof. As a result, the sub-roof becomes the relevant sealing layer and must be water-repellent, robust and temperature-resistant - and this is where things get challenging.
Temperature resistance of the subroof
Photovoltaic systems can generate high temperatures when exposed to direct sunlight. Therefore, the requirements of PV systems for the temperature resistance of the sub-roof can vary. The Technical Commission for Pitched Roofs of the Swiss Building Envelope has carried out various measurements and determined that a temperature load of 80° C is sufficient if the design complies with the standards.
For sub-roofs with normal to increased loads, we offer the Underroof membrane sky-2 and for extraordinary demands the Underroof membrane SL plus.
Not only the sub-roof membrane is exposed to this temperature load, nail sealing tapes must also fulfil these requirements. Both products, the Nail sealing tape Nb duo and the Nail sealing tape S can be installed under PV systems.
Constructive measures to increase efficiency
The sub-roof itself is not the only decisive factor - design measures also contribute to the sustainable operation of photovoltaic systems. A temperature <80° C under the PV modules is important to keep the efficiency of the modules as high as possible. For this reason, it is also recommended to increase the rear ventilation area by at least 15 mm compared to the list in the SIA 232/1 "Pitched roofs" standard (Art. 2.2.9.2).
Conclusion
A functioning and correctly designed under-roof system is the invisible but essential basis for all photovoltaic systems - especially for in-roof installations. The combination of well thought-out planning, physically optimised products and professional execution ensures safety and increases the efficiency of the system - and therefore also its long-term cost-effectiveness.
Our team will be happy to provide you with further information or to arrange an appointment. Counselling available.
