By Jonathan Evans, Chairman and CEO, Ash & Lacy
At Ash & Lacy, we are convinced that a major collapse of a roof-mounted PV installation is inevitable eventually. This could be a setback for the industry, but certainly for the installer concerned.
Undoubtedly, many installations take careful consideration of the loading implications of a PV array; however, there are still jobs in evidence where the considerable weight of the PV system exceeds the roof design capability and also where the fastener and grid system is incapable of withstanding the likely wind uplift and snow loads.
A harsh winter of heavy drifting snow could have a disastrous impact on these installations, the customer and the installer.
Typical PV panels weigh between 12 and 20kg per square metre. This load, plus the weight of the supporting framework, has to be carried by the structural frame below. Therefore a load of at least 0.15kN/m2 is added to the roof, which has probably been designed for a snow load of 0.6kN/ m2 and a nominal service load. This is not normally a problem, as we rarely get ‘design’ snow loads on roofs. However, last winter we did – several times!
The full ‘design’ snow load, combined with the additional static load imparted by the PV panels and their supporting framework, could cause spectacular collapses.
Furthermore, the PV panels can act as a ‘dam’ on a roof, leading to drifting snow, which increases the localised load even more. There is a real danger of overstressing the structural frame of the building, as these build-ups can occur out of sight and during intervals between services. All too often, panels are added to a roof without any reference to the capacity of the structure accepting them. With climate change, we will see challenging weather conditions more frequently.
Avoid Roof Overload
Overloading a roof is a real concern, but there are other issues to be considered. PV panels are generally fitted to a framework which, in turn, is fixed through the cladding into the support below. Care must be taken to avoid water ingress at these points by ensuring that the hole formed by the fixing is properly sealed. It is also vital to make sure that the correct fixings are used and that they adequately anchor the framework to the roof structure. Wind load, especially uplift, can be a serious issue, particularly on exposed sites or on high buildings, where there are no ‘baffles’ from adjacent structures.
The gap between the PV panels and the roof cladding could become a trap for debris, such as leaves, twigs and dead animals. Birds could nest in the warm, protected void. Anything caught in this area will become damp, and will form a ‘poultice’ on the coating below, causing premature failure of that coating.
This is not meant as a warning against the use of PV panels on roofs; it is a strong recommendation to properly consider all aspects of their installation. It is essential that competent roofing contractors be employed to fit PV panels. Such contractors are proficient in all aspects of working on roofs and are conversant with the required safety measures. They also understand the need to properly seal any openings, whether they are for the fixings or for cabling.
Seek Good Advice
Proper advice needs to be sought on the structural capacity of the supporting structure and on the effects of shading on the roof coating. Thought must also be given to the prevention of an accumulation of debris, leaves, twigs, etc. below the panels, driven there by wind and rain. The simple addition of carefully placed flashings and trims can address these issues.
Even for major ground-mounted PV installations, the structural integrity and longevity of the installation relies on several factors. Quality control procedures need to address the possibility of non-conforming metal grids, inappropriate fasteners and poor installation standards. Many of these issues can be addressed with the provision of partially pre-assembled frames. Manufactured under factory-controlled conditions, they are believed to be a big step forward in improving the quality of installed systems; moreover, by taking much of the installation labour time into the factory, the time to the commencement of electricity generation can be reduced. The frames can be manufactured while the site is being prepared and the appropriate ground fixings installed. This also allows the use of certain fixing methods which offer greater security than the standard bolt or self-drilling screw site methods.
PV panels are most efficient when installed at approximately 30 degrees to the horizontal. There are over-roofing systems available for such applications that are totally flexible in their use. They can be custom developed to suit any situation, and are ideal for PV panel arrays, whether they are on the roof or at ground level. For example, a wide variety of bracket depths allows sloping arrays to be installed with minimal effort. It is equally important that fastener systems ensure high-strength, watertight connections in a wide number of applications, as well as being of high quality for ease of use and speed of installation. Having properly taken into account all these factors, the building owner retains the guarantees for the building and its envelope. If the potential pitfalls are not considered and effectively dealt with, all warranties will be void, and premature breakdown of claddings, for example, or even structural collapse of the frame, could become a very unwelcome reality.
A British Board of Agrément (BBA) certification is evidence of the structural performance and longevity of a system. By adopting well-established engineered systems, a company enhances its credibility and improves its chances of building a sustainable long-term future. In doing so, a company must provide detailed design and structural engineering services for its systems, and act as a valuable partner.