Equipment innovations and new National Electrical Code criteria have led to wider choices in PV mounting systems. Here’s a guide to help determine what mounting system is right for your modules.
Roof installations are often the most cost-effective option for residential PV applications and therefore typically the first area evaluated on a property, and that’s the focus of this article. However, not all rooftops are appropriate for solar—there may be roof obstacles, such as vents and chimneys, which create too much shading; the underlying structure may not be sound; or the roof’s location and/or orientation may not be suitable. (Look for upcoming articles on ground-, pole-, and awning-mounting systems in future issues of Home Power.)
The mounting product must be able to meet the location’s wind and snow loads. Most building jurisdictions around the country have adopted some version of the International Building Code (IBC), which provides guidelines and methodology for evaluating structures for their ability to withstand expected snow loads as well as the forces from wind and seismic events. Manufacturers of PV mounting equipment often supply an engineering analysis and calculations showing that their product meets certain load requirements. Within this engineering analysis, the manufacturer often supplies tables that summarize the engineering analysis into helpful reference charts. System installations with parameters outside the assumptions made in the report require additional evaluation to determine appropriate design, a service that many rack manufacturers offer.
Load ratings. The first step in evaluating rack options is to collect basic site information necessary to utilize the manufacturer’s load-rating charts. Site details include: building (or array) height from grade; wind exposure category; basic wind speed; snow load; and roof pitch. With this information, the rail span distance can be determined from the manufacturer’s load charts. Wind exposure classifies surrounding terrain into categories depending on the amount of open space around the building. Most mounting products are rated to at least a category C exposure level, which applies to rural locations with lots of open topography around the building area. Basic (or design) wind speed is defined as the greatest 3-second burst of wind speed recorded for the area. This can be obtained by contacting the local building department. Snow load ratings are also available from local building departments. However, in mountainous regions, also take into account snow load advice from locals, since snowfall can vary widely depending on elevation and microclimate. Roof pitch defines the tilt for the PV array and can be measured with an inclinometer.
Most rack manufacturers provide professional engineered structural guidelines for the installation of their product, indicating the product has been evaluated to IBC requirements. Pre-approved product engineering analysis reports, specific to a state or region, provide assurance that a particular mounting product and installation parameters have been reviewed by a third party to meet site-specific load conditions. It can decrease the amount of time and cost associated with this portion of the permitting process.
Local jurisdictions may enforce additional codes. For instance, the 2012 International Fire Code (IFC) has new requirements for setbacks and access pathways that may limit the available roof area for installing PV modules.
Once the necessary structural and engineering criteria have been met, the next step is to analyze the mounting product for ease of installation.