INSTALLATIONROOF MOUNTED


typically specified to accommodate service loads such as suspended ceilings, air conditioning and such like. So, subject to survey to confirm the design specification, these usually require minimal modification to take PV (unless the service load has already been utilised).


18


attach directly to the roof purlins/rafters. Angled brackets accommodate the replacement of the tile and as long as the tile is not cracked, care is taken to avoid damage to the insulation, and any underlying weather membrane, then this part of the install should be relatively straightforward. However, when installing the modules (which are heavy) onto rails attached to the mounting brackets the potential to damage other tiles or even fall off the roof is an ever-present danger. It is not good enough to simply put up a ladder and happily step onto the roof. HSE (Health & Safety Executive) are increasingly concerned and taking an interest, so prosecutions for bad practice will follow.


Agricultural awareness


Now in agriculture or industrial installations things inevitably get more complicated – the installations are normally far larger and the roof types typically very different from a domestic house. From asbestos cement through fibre cement to metal sheet – corrugated or seam-crimped – to flexible membranes. All of these roof types/surfaces have their own challenges and when it goes wrong taking down tens of kWs of PV modules to fix a leak is no easy (or inexpensive) matter. The first challenge for these building types is of course the roof loading. There are two main standards for these type of buildings in the UK – industrial falls under BS5950 while agricultural BS5502 class 1, 2 and 3. Buildings constructed to confirm with BS5950 are generally OK (though this still should be checked) as this standard assumes people will be permanently occupying the structure and under all weather conditions. They are also


Agricultural builds (one hopes) to BS5502 standard can be a different matter altogether. The three classes are rated for various levels of human occupancy as follows: class 1 for a lot of the time; class 2 occasionally, class 3 hardly ever (virtually a temporary building). There is no real regulation in this sector in England and Wales (in Scotland the builder has to warrant to the standard) and many buildings of this type are simply not to any standard. Take some of these buildings following a few years of livestock use (with the resulting bio- mass fallout) and even well-designed roof structures can be significantly weakened through corrosion. The recent snowfalls over the winter saw many thousands of agricultural buildings collapse and that was before PV was put onto their roofs. So while agricultural buildings have worked well for farmers in keeping building costs down, in many (if not most) cases they will not take the weight of PV without significant structural work to increase their load capacity.


Warranty details


So having confirmed loading rating (or strengthened the existing structure) the next issue is the roof skin itself. Perhaps the biggest challenge on these various roof types mentioned earlier is fibre-based sheets, be these fibre cement or the older asbestos cement. These roof surfaces are fine when being fitted new but after that are meant to be left alone for life (25 years plus). Indeed they are explicitly not rated for any load bearing after fitting. So recent pictures of a large PV installation being “rested” onto the fibre cement roof [picture] with roofers standing (be it on boards) directly onto the roof surface was not a happy day for our friends at the HSE (who are one must remember there simply to save lives). By its very nature and design fibre cement is breathable to stop condensation. The manufacturers completely disown warranty if you put ANYTHING onto the roof surface that stops this. Further, extra fixings through the roof panels have a similar effect to the warranty. So these nice stand-off fixings/bolts are fine but they cannot go onto the purlins through newly made holes! Even when using existing holes the thermal expansion of these


www.solar-pv-uk.com Issue III 2011


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