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Installer Insight: Rooftop Structural Analysis

Posted by Guest Installer In Wednesday, February 10 2010 under: Solar Power Info, Installer Insight, solar installation, Solar PV Panels
The Importance of Structural Analysis for Roof-Mounted PV Arrays
Chris Fidler, President
Primal Energy Systems LLC
Most racking manufacturers provide excellent resources for designing a safe and well-engineered support structure for roof-mounted solar panels. But the racking is only half the equation when it comes to mechanical design – the other half is the building structure. All racking installation manuals contain a disclosure that reads something like, “the installer is solely responsible for ensuring the building structure can support the array under all loading conditions.” A load is any force that is applied to the structure. Structures must be able to support dead loads (the weight of the roof deck, shingles, racking, and panels) and live loads (wind and snow). Most roofs have no problem handling the additional dead load of the racking and panels, which usually add about 5 pounds per square foot. However, an array can significantly change how wind and snow loads are transferred to the structure. When wind blows against a roof with no array, the wind pressure is uniformly distributed on the surface of the roof. This uniform pressure is then transformed to a uniform load along the rafters. The uniform load on the rafter results in a certain amount of stress and displacement. Engineers and architects assume a uniform wind loading condition when selecting the size and material of the rafters. Too much stress can cause a failure of the rafter and too much displacement can cause bowing and cracking of attached surfaces, such as drywall on a cathedral ceiling. When an array is added to a roof, the uniform wind pressure is absorbed by the panels and is transferred to the rafters by the racking attachment points. These point loads create a completely different loading profile on the rafters. The resulting stress and displacement may be higher than what the building codes allow.
The following is a real-life example of a job that required structural reinforcement. The customer’s home was located in Richmond, VA where local wind design loads are 90 mph and snow design loads are 20 lbs per sqft. The house was built in 2006 and the roof structure was made from 2 x 4 engineered trusses on 24” centers. The racking was attached to every other truss, which meant that the uniform load that originally was carried by every truss was now transferred by point loads to every other truss. Analysis of the new loading profile showed that the stress would exceed the allowable limits for southern pine. The problem was solved by adding a sister 2 x 4 to each truss that had a racking attachment point. This example is only an illustration of one particular problem and solution. Failure to properly analyze the roof structure in this case may have resulted in a roof failure down the road. Installers should not automatically assume that the roof structure is capable of handling the different loading conditions presented by the addition of a solar array. Partnering with a locally licensed Professional Engineer is a best practice that can not only guarantee safer installations but also provide a competitive advantage.
Chris Fidler is a mechanical engineer and president of Primal Energy Systems LLC, a PV design and installation company located in Richmond, Virginia.
Solar installer Chris Fidler talks about the importance of structural analysis when designing a roof-mounted solar panel installation.

The Importance of Structural Analysis for Roof-Mounted PV Arrays

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