CROSS Safety Report
Safety issues when adding PV panels to existing roofs
A reporter raises concerns over the lack of structural engineering knowledge and dangerous assumptions in assessments for the installation of photovoltaic (PV) panels for a number of public sector buildings and schools.
Key Learning Outcomes
For civil and structural design engineers:
Structural design must comply with Approved Document A
Load effects of snowdrift and wind uplift forces acting on the roof structure due to PV panels should be carefully considered
BRE Digest 489 Wind loads on roof-mounted photovoltaic and solar thermal systems provides very useful design guidance, based on EN1991-1-4 and the UK National Annex (NA) for calculating wind forces
There have been a number of previous CROSS reports on PV panels and these risks were highlighted in a Standing Committee on Structural Safety (SCOSS) Alert in 2016: Photovoltaic installations - structural aspects
For the building team:
On existing roofs make sure that the structural aspects have been considered
Be aware that PV panels can add load to existing roofs
Be aware of the risks of working on existing roofs
Inspection by a competent person should be carried out to ensure the works are installed in accordance with the design intent
Note the CROSS recommendation that there should be guidance upon structural design and installation of PV panels for both new and existing buildings
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A reporter’s organisation have recently been involved in reviewing calculations for the installation of photovoltaic (PV) panels for a number of public sector buildings and schools. They were concerned about the lack of structural engineering knowledge and dangerous assumptions used in the assessments. They have seen/reviewed calculations from multiple organisations, mainly solo practitioners and small building surveying firms.
Insufficient design information and load assessment
A summary of the main issues encountered are:
details of the existing structure and loading arrangement not included in the calculations
details of the proposed PV and loading arrangement system not being provided
calculations excluding the weight of ballast (weights to anchor down panels) and supporting frames when looking at the additional loading applied to the roof
no back checking/assessment of existing structural members, liner trays or purlins, to check they are adequate for the proposed loads
offsetting the roof access loads without consideration of snow loads
in some instances, they have seen the full access load being offset, which raises the question of how someone is meant to install or maintain the PV panels
no consideration of localised snow drift due to PV panels providing new obstructions on the roof
uplift wind forces on the panels are often ignored completely
mixing of design codes, for example using Eurocode loading with British Standard member design and vice versa
no checks on serviceability criteria for the roof, for example to check that no ponding will occur
eroding the partial factors for loading as a way of justification of new loading
They have come across calculations using assumed values with which to offset load. For example, a recent project had assumed that the roof had a ceiling and services allowance of 0.25kN/m2 (they thought this was an industry-standard value) which had then been used to offset loading. There were no existing load plans available to validate the ceiling and services allowance for this project.
They also report that wind uplift forces, which could be critical in non-ballasted installations on lightweight roofs, are often ignored and most engineer's reports only check for gravity load combinations.
Wind uplift forces, which could be critical in non-ballasted installations on lightweight roofs, are often ignored and most engineer's reports only check for gravity load combinations
Another concern raised is the ballast arrangement on the roof frequently being ignored or not properly considered in design. Typically the PV suppliers will concentrate the ballast around the edges due to high uplift forces, while most structural reports ignore this and average the total ballast load over the whole PV installation.
Checks on fixings
They have recently seen several proposed installations where they are proposing to fix in to an existing timber roof with gang nail type trusses. While a global check is often performed on the roof structure, checks on the fixings between the roof structure and PV mounting frames are often omitted. In the reporter's experience they have often found this detail to be critical in terms of fixing layout and edge distances as highlighted in BRE Digest 489 Wind loads on roof-mounted photovoltaic and solar thermal systems
Health and safety file
There have been issues where clients have not been advised on the risks that need to be included in the building's Health and Safety file, for example advising the client that roof access must be limited or that no additional services can be supported by the roof.
They have also noted that some reports contain statements/assumptions which they believe to be unsafe or dangerous, the following were contained in a recent report:
'...given the roof is only loaded with snow for short periods, we do not consider the loadings to be an issue'
'To summarise the structure has a comfortable factor of safety built into its design and a sufficient capacity for any minor overloading scenarios which may occur.
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The fixing details proposed by suppliers / installers is also an issue. I have in the past been sent a pack of project specific calculations and proposed installation details for a PV scheme that detailed a fixing bracket using three 6mm diameter woodscrews aligned perpendicular to the timber truss top chord. The truss chords were only 35mm wide so only one screw out of the proposed three would actually penetrate the chord using this arrangement, and a 6mm screw would not comply with minimum edge distance requirements in any case. Only a 3.5mm diameter screw would comply, and only then if it were inserted dead centre across the width of the truss top chord, i.e. no installation tolerance. So there was simply no way that the proposed fixing bracket could be installed as detailed and nor would it achieve anywhere near the design loading calculated by the specialist, and yet these proposals were purportedly project specific.
Expert Panel Comments
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It is good to see that this reporter’s firm is checking calculations for such installations although worrying to observe the number and extent of the problems found. There have been a number of previous CROSS reports on PV panels and these can be found on the CROSS website along with a SCOSS Alert issued in 2016: Photovoltaic installations - structural aspects.
Design guidance for determining wind loads
BRE Digest 489 Wind loads on roof-mounted photovoltaic and solar thermal systems as referred to above, provides very useful design guidance, based on EN1991-1-4 and the UK NA. for calculating wind forces. Contractors may be familiar with electrical installation, and there are guides on this aspect e.g. Guide to the Installation of Photovoltaic Systems from the Microgeneration Certification Scheme. This guide is not for the structural implications for the roof.
The Scottish Government published Low carbon equipment and building regulations – a guide to safe and sustainable construction – Photovoltaics in 2012 which contains advice on installation.
Industry guidance needed
Given the growing practice of installing panels and the generic faults in design and construction, there should be a structural engineering guide, and possibly a Code of Practice, for the design and installation of such panels. This should cover both new buildings and retrofits on existing buildings. Post installation, there should be a certificate to confirm compliance with guidelines.
Safe working at heights
Note should also be taken of the risks with the manual handling and work at height issues associated with installing and maintaining the panels. There have been a number of incidents of workers falling through or from roofs during such work. Roofs must be left in a state where maintenance can safely be undertaken. HSE in Northern Ireland have a web page on Installing solar panels safely which gives helpful information. The Working at Height Regulations and, by implication, compliance with BS5975 for temporary works are also relevant.
Generic faults in design and installation appear to be widespread, with structural safety and maintenance issues that may require great cost to resolve in future. With increased attention on the green agenda the use, and hence the associated potential problems, with these installations may be expected to increase.