Solar panels: installation risks

Light loads

18 May 2017

Installing solar panels on roofs is not without its risks, warns Trevor Rushton 

Notwithstanding the significant drop in feed-in tariffs announced by the UK government in April 2016 – which pay the owners of on-site generation plant for the power they feed into the grid – the market for solar photovoltaic (PV) panels remains competitive.

As global production of solar PV has grown, the cost of the cells and panels has fallen dramatically, while improvements in technology and electricity storage have helped the industry develop profitable models that require little or no subsidy.

According to the Solar Trade Association, there are now more than 1m homes in the UK with PV and solar thermal panels installed – representing 4% of all homes – most of which have had panels retrofitted.

Roof risks

Needless to say, fitting panels to a roof can pose risks, and we are beginning to see an increasing number of errors arising from less than perfect design and quality of work. Problems are not confined to retrofitting existing buildings: new products are continually being introduced, while poor installation of building-integrated systems by individuals who are not familiar with them can result in unsatisfactory performance – wind uplift and leakage being two examples.

Following several reports of damage to buildings arising from inadequate supports, overloading of the structure, impaired waterproofing and wind damage, the body Structural-Safety has released an alert, PV Installations: structural aspects. This makes a number of recommendations, including the following:

  • a structural appraisal should be carried out by a competent person, with proper checks of both the upper and lower surfaces of a roof after installation;
  • particular care is needed in areas likely to experience high wind and snow; and
  • an adequate number and type of fixings must be used – usually at every trussed rafter as a minimum.

Structural-Safety points out that, in addition to the dead weight of the panels, their installation can also lead to problems of snow loading, depending on the type of panel, and wind uplift. As a roof normally works in structural terms by sharing loads over several interacting elements, the provision of panels can result in localised overstressing.

Some risks are obvious: fitting panels over a roof covering that may be on its last legs is unlikely to be a sensible move, but installation on newer coverings can also lead to difficulties.

My firm, Watts, recently reported on a significant claim arising from water penetration of a single-ply membrane roof. In this case, the installer had used secondary fixing screws to secure plastic trays between the retrofitted panels; the screws were very slightly longer than the manufacturer had specified, so when the trays were loaded they penetrated the roof covering in dozens of places. In another case, the reversal of plastic transition tiles resulted in problems of wind uplift between the panels and the adjoining pitched roof.

BRE research

BRE has been conducting research into a number of firefighting issues, and while one might think PV panels present little or no risk, this is not so: firefighters are having to adapt their methods to deal with risks otherwise unfamiliar to them.

It is usually fairly easy to shut off AC power in a building in the event of an emergency. However, PV panels will always be live while light is shining; they can generate potentially high DC voltages that can be more dangerous than AC.

A large array could generate around 1,000V DC, leading to a risk of electric shock in the event of a fire damaging a cable and allowing parts of an exposed conductor to touch a steel frame. Furthermore, the increased roof loads could cause the premature collapse of a fire-damaged structure, while panels could melt, resulting in the release of toxic heavy metals.

According to BRE, some firefighters have reported difficulties in dealing with roof fires as PV panels act as an umbrella to the firefighting water jet, preventing access to the roof structure and resulting in more extensive damage or collapse.

It takes time for users and advisors to appreciate the emerging risks associated with new products. Building surveyors need to be alive to the potential for seemingly harmless technology to cause problems, and to recognise that sometimes things are not as simple as one might believe.

Trevor Rushton is Technical Director at Watts Group

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