Wind Load Calculations for Solar PV Arrays
Today’s photovoltaic (PV) industry must rely on licensed structural engineers’ various interpretations of building codes and standards to design PV mounting systems that will withstand wind-induced loads. This is a problem, because–although permitting agencies require assessments of the structural attachment of solar equipment to rooftops—the safety and sufficiency of these attachments are not adequately addressed in any codes or standards. The result is a multitude of code interpretations from a range of individuals and groups, often yielding different design loads for the same design specifications.
Solar America Board for Codes and Standards Recommendation
- At present, they recommend basing the structural design of roof-mounted PV systems on the ASCE Standard 7-05 as follows:
- Section 184.108.40.206, main wind-force resisting system (MWFRS), is the recommended starting point for designing the PV mounting structure, with the PV module oriented above and parallel to the roof surface.
- Section 220.127.116.11.1 addresses wind loads on components and cladding. We recommend the use of Section 18.104.22.168.1 and supporting Figures only for the design of the PV module attachment clips and hardware to the structure, and for calculating loads on individual PV modules.
- We do not recommend Section 6.5.15, 22.214.171.124, and Figure 6-21 for the design of PV systems.
- This report provides basic guidance for applying ASCE Standard 7-05 to existing codes and standards for the typical residential application of PV arrays mounted parallel to the roof slope and relatively close (3 to 6 inches) to the roof surface.
- They recommend wind tunnel testing be conducted for the most common rooftop PV installations to verify methods and calculations. The installation types include standoff mounting parallel to the roof, stand-off mounting at an incline relative to the roof, and ballasted installations on flat roofs.
- They recommend that codes and standards be modified to specifically address the mounting of PV arrays to rooftops to eliminate potential barriers to market development in high wind regions.
- They recommend that local jurisdictions and design professionals use the recommendations in this report to ensure continuity in interpreting existing codes and standards.
Thanks for this article. One of my customers is telling me that he wants the stand offs for his solar installatoin on his roof to be 18" high so that he can crawl under the panels if he needs to work on his roof. Here is a photo of his roof. Any thoughts on why doing such a high stand off is a good or bad idea? I am concerned about both the wind loading and the strain on the roof mountings. Thanks for the thoughts on this issue.
If it's seamed metal roof, S-5! clamps (you can use the PV-Kit) are recommended. They can withstand high uplift loads as they can grip on the seams.
I don't quite understand why he would like to crawl under the modules. But he can use Unirac Solarmount Low Profile Tilt Legs that would allow him to lift one end of the array even up to 44".
In this case wind exposure could be an issue which needs to be calculated. He will need a structural engineer to figure it out if he is at a windy site (exposure C and D).
Actually for this type of roof the S-5! will not work. It is hard to tell from this photo, but there is nothing for the S-5! clamp to attach to. We actually suggest using the Chemlink Solar Shoe. CED Greentech offers the Solar Shoe but it is not yet in the online catalog. If you Contact CED Greentech for pricing and availability they will get right back to you.
That is a thru-fastened metal roof (i.e., 5V-crimp type) on that building. These are not structural panels as most stanting seam roofing is. Raising the structure 18" above the roof will most likely increase the forces on the racking system. You shouldn't attach the solar to the roof membrane. It would be best to get to the structural framing under the roof for attachment. The building engineer should let you know what loads can be added to the existing design. In many cases, the underlying roof purlins may not be able to support the loads either and you may need to attach to the main building columns and span roof penetrations. It is advisible to also include the roofing company in any penetration work to maintain any performance warranties on the roof membrane.