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As solar energy becomes a more affordable choice to power our lives, hardware costs are less frequently the biggest hurdle to overcome in installing rooftop photovoltaic (PV) panels. Homeowners and solar installers in some locations are required to have a structural engineering analysis of the home’s rooftop, resulting in higher costs and delays. While a structural engineering analysis can help ensure that a rooftop can support the solar PV system, new research indicates that the caution employed in the analysis significantly underestimates the actual roof strength.  

A structural analysis is not universally required, and rules vary by location – even among neighboring municipalities; this makes it difficult for solar installers who work in multiple jurisdictions. For homeowners, the step may add  hundreds of dollars to the cost and a week or more to the installation. If the engineer conducting the analysis requires site visits and revisions, costs can grow rapidly and the home owner may scrap the PV project entirely.

A technical report from the Energy Department’s Sandia National Laboratories could reduce permitting costs by providing something the solar industry has needed: data showing the actual loads that various wooden rooftops can carry. The report, funded by the SunShot Initiative, is the result of a first-of-its-kind study that tested wood rooftop structures by stressing them to the point of failure and compared this data with the allowable loads laid out in common building codes.

Tests revealed that wooden rooftops have a much greater capacity to support loads than previously thought. Overly conservative building codes and engineering analysis methodologies significantly underestimate the strength of wooden roofs. More importantly, the study details how the typical structural engineering analysis misrepresents a roof’s ability to support a PV system.

Most well-built wooden rooftops may be sufficient to support PV system loads. This report gives permitting officials, installers, and engineers the information they need to support safe and cost-effective solar rooftop installations. Learn more about the study by visiting Sandia’s website.

SunShot investments are driving innovations that have contributed to the significant drop in the cost of solar in the United States. Since the beginning of 2010, the average cost of solar PV panels has dropped more than 64% and the cost of a solar electric system has dropped by about 50%. However, non-hardware costs — often referred to as “soft costs” — account for as much as 64% of the total cost of new solar systems. This study was funded by the SunShot Initiative and was conducted in partnership with the University of New Mexico.