THE WEAKEST LINK? PROBLEMS WITH PV

According to the NHBC, increasing solar PV installations are leading to more problems with leaking roofs. Redland’s Kevin Ley suggests a reason why and considers the company’s position with regard to its own PV offering.

Thursday 15, December 2011

A recent report by the NHBC identified a growing problem with leaking roofs associated with the increased use of roof-mounted solar PV systems. This has not come as a huge surprise as there are currently no standards to define appropriate fixing systems for UK weather conditions.

As a manufacturer, our concern is clearly that this issue could lead to all PV systems being tarred with the same brush when properly designed systems will function without any problem.

While there are plenty of PV systems to choose from, the majority rely on products designed for climates other than the UK. Any roof mounted PV system will be exposed to strong winds and driving rain and it is therefore essential that it can both resist the wind forces and safely transmit these forces back to the building structure.

There has been considerable focus on the quality of the PV products themselves, with MCS certification giving assurance of the performance of the PV panels and certification of the contractors doing the same for the proficiency of the installation.  The critical “missing piece” of the jigsaw is a focus on the method of fixing the PV panels to the roof without compromising the primary function of the roof.

With a lack of European or British Standards to regulate the mechanical installation it is left to the manufacturer or installer to provide guidance, generally based on ad hoc testing and/or experience. Typically this experience is taken from Europe where the wind loadings, in particular, will be very different, and this is where problems can arise.

Integrated PV systems, typically specified as part of a roof replacement project in social housing or by housebuilders for the street-facing elevation, are likely to cause fewer problems. Redland’s integrated Solar PV tiles are designed and tested to work with specific Redland tile products and will therefore function as part of the whole roof system. It is when products from different manufacturers are combined that there can be a problem with the integration of the system as a whole.

On-top PV panels present a different set of challenges. These may be specified on cost grounds and will often be the choice for rear elevations where the look of the roofline is less significant.

The most common fixing system for on-top solar PV is a hook which penetrates between the tiles and fixes to the rafters, or occasionally the battens, of the roof structure. It is the design of the hook itself which can be the cause of leaks.

Inadequate design can lead the hook to deflect under wind load, lifting surrounding roofing elements. Alternatively, it may increase the gap between the tiles or the hook can interfere with adjacent tiles causing them to break under wind or snow loads.

An additional issue is the fixing to the rafters: rafters in UK houses tend to be narrower than those used in Europe, with result that fixings designed for German houses, for example, may actually split and damage UK rafters rather than providing a firm anchor.

There is a solution on the horizon with the MCS bringing out a new standard, MCS 012, which will include consideration of the mechanical fixing method as part of the product certification process.

The introduction of this much-delayed standard will come as a welcome relief to manufacturers like ourselves who are deeply concerned with the reputation of Solar PV technology. Correctly installed, and using appropriate products, PV technology should not compromise the weather-tightness of a roof system either in the short or long term.

In our view manufacturers who have delayed introduction of products until they have proved their performance in rigorous wind-load tests have been disadvantaged by the lack of proper regulatory attention to this critical area of product performance. Our own on-top solar PV offer is a good example. We have focused attention on the fixing hook, going back to the drawing board to design a product that we are happy to guarantee within our pitched roof systems. The sooner all manufacturers are on a level playing field when it comes to product quality, the better.