Certificate of Structural Adequacy – Solar Panel Installation

Certificate of structural adequacy for solar panel installation attached to building roof and parallel to roof surface. (eg. flat against roof). The solar panel installation has to be entirely within the scope of AS1170.2 for determination of wind pressure coefficients for both the solar panels and the adjacent building roof. The roof to which the panels are attached shall be simple beams.

For solar panel installations outside the scope of AS1170.2, and with a support structure other than simple beam: the fee would start at Technical Effort Level 7

NB: The reality is that determination of wind pressures on the surface of solar panels is beyond the scope of the Australian wind loading code AS1170.2. There is some guidance in the book Wind loading of structures by John Holmes. There is also literature to be found around the internet, on the testing of solar panels in various countries, including tests by NASA, unfortunately current and relevant research outside Australia is typically not accepted by local city councils.

If solar panels were mounted flush with the roof surface, then the pressure normally generated on roof surface would occur on the solar panel. However, solar panels are not mounted flush, they are mounted on rails which span the crests of the roof cladding. There are two consequences of this:

  1. Turbulence at the edges of the solar panel
  2.  Air flow under the panel

As a result the actual pressures are unknown. The typical judgement is to adopt the pressure that would otherwise be experienced on the roof surface. This is a reasonable judgement with respect to serviceability in most regions, but may be unconservative with respect to ultimate strength.

Solar panels mounted at an angle to the surface are even more problematic. These panels are not equivalent to a monoslope free roof (canopy): the height to span ratios of a canopy are not compatible with those of a solar panel. {For that matter these canopy height to span ratios are not compatible with those of a small canopy like a bus shelter.}

Even if the  pressure coefficients for free roofs were adopted to get pressures on solar panels, such would only be suitable for free standing solar arrays. Solar panels attached to a house or carport roof structure also modify the airflow over the roof of the structure to which they are attached. Turbulence occurs at free edges, and something also occurs in the void behind the solar panel: it could be stagnation of air flow or more turbulence. Turbulence can either magnify uplift pressures or magnify down draft pressures.

Therefore solar panel installations should not be claimed to be compliant with the Australian wind loading code AS1170.2, the installations can only be judged as structurally adequate in terms of  the designer/assessor’s learned opinion and their personal concept of fitness for purpose. NB: At ultimate strength loads, materials deform permanently. When the loads are removed the structure will be permanently deformed and typically cease to be fit for purpose and will need repairing or replacing. At serviceability level loads, materials are expected to deform elastically, and when the load is removed the structure should more or less return to its original state. There are a whole spectrum of serviceability levels, since deformation can stop a system from functioning at differing levels of acceptability. The solar panels therefore should have been load tested and the specification should define the pressures at which the panel will remain operational.
I recently read preview for AS4055:2012, it states that: References to the differential pressures on photo voltaic solar panels in AS/NZS1170.2 were included. I have done a search of the pdf version of AS/NZS1170.2:2011 and cannot find any reference to solar panels. Further checking and found that there was an amendment AS/NZS 1170.2-2011 Amdt 2-2012 Structural design actions, which adds a section D6 for solar panels mounted parallel to roof surface. To be clear it only covers panels mounted parallel to the roof slope of enclosed buildings. It does not provide pressure coefficients for solar panels mounted on roofs at an angle to the roof slope, nor does it provide coefficients for individual solar panels or solar arrays. So I reemphasise that the h/d ratios for monoslope free roofs are not compatible with solar panels nor with small canopies the size of bus shelters.  Testing is required for individual panels and arrays else authorities have to accept judgements. Testing has been done for individual panels, arrays and mounting on various buildings but not in Australia, the only testing I have been aware of in Australia is: Investigation on Wind Loads Applied to Solar Panels Mounted on Roofs by Cyclone Testing Station at James Cook University. This report I have largely ignored because as far as I could tell it wasn’t concerned with solar panels, it was focused on increased pressure to house roof, and I’ve read a lot more reports relevant to solar panels.


[17/09/2016] : Original
[13/10/2016] : Added note about AS4055