Confirming the Strength of Residential Roof Structures for Solar Installations

Test Setup Photo

Test Setup

The solar photovoltaic (PV) industry has historically lacked sufficient data related to rooftop load for solar installations. This gap causes misperceptions about the strength of existing rooftops, and, in many locations across the United States, leads to decisions to conduct structural analyses prior to solar permitting. These analyses often use an overly conservative methodology that underestimates the roof’s load-carrying capacity, thus delaying or even blocking the PV installation. This extra step is also not universally applied across states or municipalities, so it can be difficult for solar installers to know where and when this step will be needed.

To address this issue, Sandia National Laboratories and the University of New Mexico teamed to conduct research under which numerous wood rooftop structures were stressed to failure. The research tests provided actual rooftop load capacity data and compared it to the perceived load-carrying capacity in building codes. Test results indicate that the actual load-bearing capacity for residential rooftop structural systems is several times higher than published values. These results provide a new tool and set of data for consideration in evaluating rooftops for solar PV installations. On average, rafter-based tests demonstrated a 330% excess load-bearing capacity, as compared to values computed in the National Design Standard.

Results from this work are available in a fact sheet and two Sandia reports, Empirically Derived Strength of Residential Roof Structures for Solar Installations and Structural Code Considerations of Solar Rooftop Installations.

This work was funded by the SunShot Initiative of the U.S. Department of Energy’s Solar Energy Technologies Office.

 

SolarStructuralGraphic

The system effects of the rafter, sheathing, and nailed or glued connection add strength to the rooftop structure. These system effects are often not considered in structural rooftop analyses for solar PV.