Four Critical Factors that impact your roof mounted PV array system

How to achieve optimal results from your solar array installation

Four Critical Factors that impact the Return On Investment of your roof mounted solar power system.


The purpose of this report is to make you aware of the risks associated with roof mounted solar array systems and their impact on your building and finances if critical factors are not taken into account. On the other hand, being aware of these critical factors will help to maximise your return on investment and minimise the risk of damage to the structural integrity of the building.

Potential problems arise because each roof mounted solar installation is unique, due to the local wind conditions and the construction and condition of the existing building structure. When these conditions are not taken into account, there is high potential for compromising the efficiency of the solar array system and the structural integrity of the building. There are four critical factors that need to be considered before installing a roof mounted solar system in order to minimise these risks.

If these factors are not considered and a thorough engineering assessment is not carried out, you could be affected in two ways. Firstly, not considering the effects of the local wind conditions could lead to fewer solar panels being installed and thus reducing the power generation. Secondly, by not considering the capability of the building's roof structure too many solar panels may be installed which could impact the structural integrity of the building.

Unless you have a Solar Installation Assessment completed you run a higher risk that the solar installation will not provide the maximum power generation possible or suffer detrimental effects to you building over time. You may also be at risk legally if your installation does not comply with local building code regulations.

In this report, we will highlight the four critical factors that need to be considered to avoid these risks to maximise the ROI and minimise the risk of damage to the building.

By the time you finish reading the report you will understand the critical importance of why a roof mounted solar installation should be assessed and certified by a specialist structural engineer who is experienced in analysing the effect of wind loads on roof mounted solar array systems.

Potential Risks

Rooftop solar panels are considered to be attachments to a building, as opposed to being part of a building and in most Australian States and Territories are exempt from requiring a building permit. (Note: There are exceptions in some locations and is always best to confirm the requirement for a building permit with the local authority.)

However adequate consideration must be made concerning the additional loadings and fixings from the rooftop solar panels onto the roof framing. In other words, the roof frame must be able to support the additional loads associated with the proposed rooftop solar panels and its associated method of fixing. Also importantly they must not adversely affect the safety of the public or occupiers of the building.

A site specific determination will need to be made to ensure the following criteria are satisfied to confirm if the installation is exempt from a building permit, in that the installation:

  • will not adversely affect the structural soundness of the building; and
  • will not adversely affect the safety of the public or occupiers of the building; and
  • will not project beyond the street alignment; and
  • is not work carried out on, or in connection with, a building included on the Heritage Register; and
  • is not work in relation to, and will not adversely affect, an essential safety measure.

Considerations include the potentially different loads associated with types of solar panels to be installed, their extent of application on a roof, their method of fixing/support, the different roof framing types (I.e. conventional or truss), etc.

In Victoria from 4 July 2016, offences relating to carrying out building work without a building permit are extended to land owners, building practitioners and architects.

A land owner, including an owner-builder, is responsible for ensuring building work undertaken on their land – whether by themselves or any other person – is carried out under a current building permit and complies with the Act, the Regulations and the permit, unless a building practitioner or architect has been engaged to carry out the building work.

The penalties for carrying out building work without a building permit range from $75,000 to $150,000 for an individual and $380,000 to $760,000 for a body corporate.

The only way to determine if the solar array installation is exempt from a building permit is to have a site specific assessment completed by a suitable experienced and qualified structural engineer.

Reputable solar array frame manufacturers are members of the Clean Energy Council and comply with their recommendations to have their systems analysed and certified to comply with the National Construction Code and Australian Standards and provide an Array Frame Engineering Certificate. They also provide Installation Manuals to guide installers on the recommended fixing method to the roof. The Installation Manual provides a “one size fits most” method for fixing the array frame to common roof types. They do not consider site specific factors regarding the building where the solar array is to be installed. The structural analysis of the array frame is based on the worst case scenario and as such will be conservative in the number of solar panels allowed on the roof. The purpose of the Array Frame Engineering Certificate is to ensure that the solar array frame is suitable for Australian Conditions. As the array may be installed on a variety of roof types, the certificate does not cover the factors relating to the structural integrity of the roof or the specific building situation and conditions.

Reputable installation companies not only use solar array frames that have been “certified” but also engage specialist engineers to complete a Solar Installation Assessment for each Solar Installation. The Solar Installation Assessment will determine exactly how much of the roof area can be used for the installation of the Solar Array and that the roof structure will be structurally adequate. If the Solar Installation Assessment is not carried out you are at risk of either not taking advantage of the maximum roof area, or compromising the structural integrity of the building.

Factors Affecting the installation

1. Wind Region & Terrain Category

Actions of the wind are the critical factor in the design of solar array frames. As the design wind pressure increases exponentially with wind speed it is critical that the intended installation has the wind speed correctly assessed.

Australia is divided into several regions based on the maximum wind speed expected during peak storm activity. The region associated with a particular building locality can be found from: A structural engineer, building surveyor or certifier.

The following map shows the approximate location of the region boundaries. It should be taken only as a guide and the region verified by one of the above methods.

The terrain around the site of the installation is also taken into account as this has an impact on the overall wind speed. The Terrain Category is designated by a number between 1 and 4 and is determined by the structural engineer.

Generic certification will typically only include terrain categories 2.0 and 3.0 so as to cover most installations. Site specific assessment allows for great efficiency in the design as interpolation can be used to accurately define the location.

Australian Wind Regions

Terrain Category Descriptions

TC 1 - Exposed open terrain with few or no obstructions

TC 1.5 - Open water surfaces subjected to shoaling waves, such as near-shore water, large unenclosed bays on seas and oceans, lakes, etc.

TC 2 - Water surfaces, open terrain, grassland with few, well-scattered obstructions having heights generally from 1.5 m to 10 m.

TC 2.5 - Terrain with few trees, isolated obstructions such as agricultural land, cane fields or long grass, up to 600 mm high.

TC 3 - Terrain with numerous closely spaced obstructions 3 m to 5 m high such as areas of suburban housing.

TC 4 - Terrain with numerous large 10 m to 30 m high closely spaced obstructions such as large city centres and well-developed industrial complexes.

2. Building type, use & location

The height, length and breadth of a building impact how the wind acts on the roof and therefore has an impact on the solar panel installation. The type of building type, how it is used and the probability of an extreme wind event happening must all be considered when completing the assessment to ensure that the optimal result is achieved.

3. Specific Array Installation Factors

The configuration of the solar array proposed to be installed on the roof will ultimately determine if the roof structure will be satisfactory. Tilt mounted panels will impose a greater force on the roof structure than flush mounted panels. The size of the solar panels, the orientation of the panels (portrait or landscape), and the distance they are placed to the edges and ridge line of the roof all have an impact on the suitability of the roof to support the array.

4. Roof Structure

The final step in determining the suitability of the roof for the proposed solar array installation is to analyse the roof structure taking into account the existing loads plus the new loads imposed by the solar array. Information about how the roof is constructed and the materials used is required so the engineer can complete the assessment. Ideally this information would be obtained from the Structural Roof Framing drawings, but when these are not available a site inspection will be required. The inspection is required to obtain all the relevant structural details of the roof and should be conducted by someone experienced in building construction. Obtaining accurate details about the roof is vital to the assessment. Without the right information, the assessment cannot be completed or may result in incorrect results.

Impact of Solar Installation Failures Can Be Extensive

The generic Array Frame Engineering Certificates typically exclude high wind pressure areas of the roof for installation, which can result in up to 60% decrease in the number of solar panels that can be installed on the roof, resulting in lower efficiency/power/pay back of the solar array system. By assessing the structural capacity of the roof it may be possible to install solar panels in these high pressure zones and thus increasing the capacity of the solar system.

If the roof structure is not considered when installing a solar array system, you risk affecting the structural integrity of the roof. The roof structure (purlins/rafters) of existing buildings are very unlikely to have been designed with the intention of the installation of solar array frames.

The effect of wind pressures acting on the solar panels introduces additional loads on the roof structure. The dominant force that is acting on the solar panels is lift. The effect of the wind passing over the solar panels makes the panels act like an airplane wing wanting to lift upwards. The weight of the array frame and solar panels helps to counteract this force but it is much less than the lifting force of the wind pressure. Unless the roof structure is substandard, the effect of the additional forces are unlikely to cause the roof to fail, but they can have other substantial implications for the building (eg. deflection, cracking of plaster, noises, etc.) or to the solar installation itself.

Wind damage to roof-mounted solar photovoltaic arrays on commercial building due to improper installation. Courtesy SolarPro |June/July 2012 / BEW Engineerin

The main problem is that guessing that the solar installation should be okay without having a proper engineering assessment conducted is fraught with danger. A clear example of what can go wrong is cited at where the company installed the solar panels on a roof in Brisbane without a proper assessment with the result that high winds in a storm led to panels being dislodged, with one ending up quite a distance away. It was lucky that no person or other property was damaged by this event. On a commercial site, the potential for property damage and collateral damage is multiplied.

A 2013 storm produced wind forces that overturned roof-top solar panels on a church building

Depending on the roof type, up to ⅔ of the roof area could be excluded as an installation area for the solar panels if a general certification has excluded high pressure zones on the roof. This is the result of a “one size fits most” approach that has been taken for general installation certification, and where any site specific factors are ignored. By not utilising the maximum roof area where possible the cost effectiveness of the solar array installation is compromised.

The Australian Building Regulations as yet do not require the structural assessment of either the solar array frames or the roof structure. There are many solar installations that take place without these checks being performed. The effect of the solar installation on the building is unknown unless the structural assessment is completed. The least you will achieve from the assessment is peace of mind in the knowledge that the installation is suitable for your building.

National Renewable Energy Laboratory, the primary research and development centre for renewable energy, states that only 25% of commercial and residential roof tops are suitable for solar panels.

Without having a structural assessment to certify the installation, you take a real and considerable risk that the installation may not be suitable, putting both the property owners and installers at risk of legal action in the event of injury or property damage being incurred.

Solar Installation Assessment

The Solar Installation Assessment gathers installation specific information about the site location, roof structure and proposed solar panel layout. A specialist engineer will then determine the optimal panel layout suited to that building and provide a Solar Array Installation Engineering Certificate specifying how and where the solar panels should be installed.

When the solar installation company provides a Solar Array Installation Engineering Certificate you can be assured that they have taken the time to ensure that all aspects of the installation have been considered.

To ensure you achieve the optimal results from your solar installation you must:

1. Use only reputable Solar Installers. A CEC-accredited installer/designer.

2. Ensure you are provided with an Array Frame Engineering Certificate and a Solar Installation Roof

Assessment Certificate

Before giving any installer the go ahead to commence work on your solar array installation, you should insist on seeing the Solar Array Installation Engineering Certificate for your building or structure on which the array will be installed.