off-Purlin Solar Installation
Gamcorp have been performing structural assessments for the commercial solar industry over the past decade and during this time, our team has completed over 5000 commercial roof installation assessments. Over the past 4 years, our understanding of ‘best practice’ has improved through our on-site physical roof sheet testing and discussions with roof sheet manufacturers. One notable learning has been the strong reliance on internal clips of roof sheets when a concealed roof sheet is engaged when securing the array frame clamps. The strength and quality of internal clips have significant differences in structural performance between the different types of concealed-fixing roof sheets and between different manufacturers. By not taking these differences into account installations can be put at significant risk of dislodging panels.
Over the past 4 years, Gamcorp has been promoting the importance of installing roof sheet clamps on, or at least very close to, the purlin supporting the roof sheeting. This best practice methodology is only being implemented by some installers and framing manufacturers and putting many building owners and solar installations at risk in the event of a significant wind event.
Risks of off-purlin installation:
Although from an installation point of view, off-purlin installation has its own merit. Unfortunately, there are significant structural risks with this installation practice. These risks are discussed below:
Although Roof sheet manufacturers have tested their products for uniform wind actions, Roof sheet cladding primarily works as a non-structural component of a building structure and acts predominantly as an architectural cladding. When the clamp is installed off-purlins, the point load applied on the roof from the clamp can cause a range of damages to the roof sheeting both locally and globally.
Due to the fact that the majority of roof sheets is very thin (approx 0.42mm), its structural stiffness is normally relatively low and this will cause relatively large displacement when a point load is applied on the roof sheet away from the purlin (off-purlin), which is exactly the case when the installation of clamps off-purlin occurs. The large displacement of the roof sheet can cause serviceability issues of the roof sheet as well as potential damages to the roof sheet in the long term.
More importantly, when the clamp is installed off-purlin, due to the relatively low stiffness of the roofing itself as explained above, the roof sheeting at the clamp location will deflect quite a lot under wind actions applied to the roofing through the clamp. Since the wind in nature is fluctuating, the deflection of the roof sheet at the clamp location can go up and down as well. This up-and-down movement will in the long term create loosened clamps over time. This is especially true if the clamp is installed everywhere on the roof, because the movement can be quite different from one location to another due to its unpatterned locations. The differential vertical movement is mainly due to the uneven stiffness of the support underneath. For example, the deflection of the roof sheet when the clamp is installed at mid-span of the roof sheet will be a lot higher than that of the roof sheet when the clamp is installed on top of the purlin or very close to the purlin.
The differential vertical movement in the long term will make some clamps loosen from their original installation and eventually, this clamp might disengage from the ribs of the roof sheet. If this happens, the solar array frame will not be evenly supported due to the failure of this clamp. Afterwards, the clamps surrounding this clamp have to share the additional load taken by this failed clamp. These surrounding clamps have to work harder than they have been designed and eventually, they can fail too. This will certainly generate a progressive failure of the whole system in the long term if proper maintenance is not carried out.
Gamcorp has had multiple discussions with Australian roof sheet manufacturers where they have explained that the primary objective of the roof sheet is to perform as an architectural cladding for the protection of water to the building. The majority of roof sheets have not been designed as a structural element in isolation as the sheet itself has little structural integrity. Although the solar industry has performed some testing for the engagement of clamps to a single rib of the roof sheet, insufficient testing on the global clamp engagement has not been performed and until a significant research study is carried out on this topic, off-purlin clamp installations have high potential risks for dislodgement of panels.
In Summary:
The four clear justifications on why roof sheeting should not be treated as a structural member suitable to accommodate for uplift pressures when being assessed:
Most roof sheets are not considered to be a structural element and are not designed to support point loads.
Roof sheeting is very thin (generally 0.42mm) and therefore has little structural stiffness. This can lead to serviceability issues over high wind events.
Roof clamps placed off-purlin are likely to experience much higher deflection due to the lack of stiffness in the roof sheeting.
Roof sheet manufacturers have explained to Gamcorp that their products are defined as Architectural Cladding and need to be supported by the roof structure directly. It has little structural strength between these direct supports countering uplift.
On-purlin installation can remove all the associated structural risks related to off-purlin installation as discussed above. Although on-purlin installation sometimes can limit the area of installation, it is a must-follow practice since the industry cannot afford any potential structural failure caused by off-purlin installation.
Array Frame manufacturers who are promoting these types of installations have questionable intentions in selling their products to the solar industry who do not understand the risks that the installer will be accountable for if disengagement were to occur.
The structural risks associated with off-purlin installation should be well understood in the industry. We believe that on-purlin installation is the best practice the industry should follow. The clamps should always be located on top of the purlins to ensure a good performance of the whole solar array frame system during its entire design life.
