Most U.S. rooftops in good repair can take the weight of solar photovoltaic (PV) systems, a three-year study by Sandia National Laboratories has concluded.

“There is a misperception in the building industry that existing residential rooftops lack the strength to carry the weight load of rooftop solar photovoltaic installations,” says Sandia structural engineer Steve Dwyer. “Most existing well-built wooden rooftops can support PV system loads.”

There is a misperception that residential rooftops lack the strength to carry the weight of PV. Image credit: Sandia National Laboratories.There is a misperception that residential rooftops lack the strength to carry the weight of PV. Image credit: Sandia National Laboratories. Sandia took on the job of analyzing rooftop structural strength to address concerns raised in the U.S. Department of Energy’s (DOE) Solar America Cities program. At least one city reported that the primary barriers to adoption of solar were the difficulty and cost of obtaining construction permits for rooftop solar installations because of structural issues.

“I couldn’t believe it was a problem,” says Dwyer, who led the Sandia test team. “Solar PV systems represent little additional weight, and roofs are very strong.”

He says many code officials aren’t familiar with solar technology and lack the training to evaluate how a solar PV system might affect roof structure. So they bring structural engineers into the permitting process, adding time and money for the system owner and the solar contractor. Often, they then deny engineering certification for solar PV installations on wood roofs, declaring the structures too weak.

In two first-of-their-kind studies funded by DOE’s SunShot Initiative, Sandia stressed wood rooftop structures to the point of failure and compared the data with allowable loads identified in the International Residential Code and the National Design Standard. They concluded the actual load-bearing capacity for residential rooftop structural systems is several times higher than the calculated values.

Dwyer says engineers doing rooftop structural analysis often calculate stresses on the basis of an individual beam, rafter or truss. That approach assumes each component of the structure acts alone. “It fails to consider the rooftop system as a whole or consider the load-sharing or load-redistribution effects of a roof system,” he notes. “The result is a conservative analysis that does not accurately represent the roof’s ability to support a PV installation. It’s not a fair assessment.”

Dwyer adds that engineering evaluations are not universally applied across cities and states. “Local governments pick and choose what they accept," he says. "Not everybody uses the same method, so it can be difficult for solar installers and residents to know what to expect. All these issues have posed serious challenges to the solar industry.”

Assuming that engineers were not giving credit for load sharing, Sandia tested a two-by-four by breaking it in half and nailing a piece of sheeting to it to see if it added strength. It did: 35% with nailing and 74% with gluing.

They then built scaled versions of roofs in different lengths with five rafters or trusses 8 to 20 feet long and applied a uniform load over them. “We built bladders of different sizes and used them to put pressure on top of the roof by filling them with air at up to 144 pounds per square foot. We broke every size rafter and the more commonly used trusses, five sets of each,” Dwyer says.

On average, the rafter-based tests demonstrated a 330% excess load-bearing capacity compared to values computed in the National Design Standard. According to Dwyer, this suggests that current rooftop structural evaluations are overly conservative in evaluating the ability of roofs to support additional loading from solar PV installations.

“A well-built home that meets local building standards and has not been adversely modified or damaged should have enough load-bearing capacity to support a roof-mounted PV system,” he concludes.

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