SEAU Membership
by Scott Porter, Membership Committee
As membership chair I get to review new membership and membership upgrade applications. I’d like to welcome all our new members this year. Also, congratulations to any members who upgraded their memberships. That probably means you put a lot of work into passing the PE or SE exams. Your membership in SEAU probably helped you prepare for those exams. Membership in SEAU strengthens engineers. Our annual conference is a great learning opportunity as are our monthly meetings for SEAU members. Beyond these classes, SEAU committees provide opportunities for SEAU members to give service while developing relationships in the structural engineering community and strengthening our profession. So please get the most out of your SEAU membership by getting involved as a member of SEAU and please invite other engineers to be an active part of our organization.
Will Your Roof Support Solar Panels?
By Jared Lee
Roof installed solar panels are becoming increasingly common as the ”Green” movement and renewable energy advance in an attempt to minimize carbon emissions, provide better air quality, and lower energy costs. As a result, homeowners and commercial developers are becoming more likely to use photovoltaic panels (solar panels) on their roofs. In some areas in the state of California, it is a code requirement that a portion of the roof be designed to support solar panels. However, solar panels are frequently installed on roofs that weren’t specifically designed to support them. In this case, approval from a structural engineer is usually required to verify that the roof structure can support the additional loads imposed by the solar panels. At some point you may be designing a building that includes accommodations for solar panels, or you may be asked to determine if an existing structure will support solar panels. Roof mounted solar panels will likely impact the dead, live, snow, wind, and seismic loads on a building.
It is convenient to incorporate the additional loading of solar panels into the design of a new structure. Section 1603.1.8.1 of the 2018 IBC states, “The dead load of rooftop-mounted photovoltaic panel systems, including rack support systems, shall be indicated on the construction documents.” It would be prudent to coordinate with the solar company during the design to ensure your design is compatible with how the system will be installed and mounted. Otherwise, specific design information for the system such as loading considerations and how the panels will be mounted (flush or tilted) should be included in the construction documents.
Installing solar panels on an existing structure that did not account for the loading may be more difficult. Section 1607.13.5 of the 2018 IBC, Photovoltaic Panel Systems, outlines requirements for roof structures that support PV panel systems including dead + live loads and snow drift loads created by the modules. Section 4.17.1 of ASCE 7-16 similarly states “roof structures that support solar panel systems shall be designed to resist… roof live loads specified in Table 4.3-1 with the solar panel system dead loads.” Also see the exception and additional requirement to support live loads without the solar panels present.
The dead load for solar panels is “The weight of the panels, their support system, and ballast” per ASCE 7-16 Sections 3.1.5. A typical uniform load is about 3 psf. However, load from solar panels must be considered as point loads and not a uniform load since the panel load is distributed to individual base mounts. This could be a concern, for example, if the base mounts are attached to every other roof truss. In this case, the dead load of the panels, snow, and wind load would be removed from half the trusses and doubled on the other half in the area of the solar array. Similarly, if solar panels were installed on a standing seam metal roof and were mounted to every other vertical rib, the loading would be doubled at those locations and possibly overload the roof attachment.
Some things to consider for installing solar panels on an existing roof. Is the roof structure in good shape? Has there been water or other damage to compromise the structural integrity? Is it sagging, or was it designed before modern building codes? Will the structure support the additional weight? You could possibly justify the additional vertical dead load if you know what load the roof was designed for. For example, you may be able to sharpen your pencil on the dead load calculation and determine that the actual load of the roof is less than what was used in the design. In Utah it is likely that the snow load was reduced with the adoption of the 2018 IBC. Perhaps you could take advantage of the slippery surface of the glass panels and reduce the roof slope factor, Cs.
Wind loads may be your biggest concern with rooftop solar panels. New provisions for determining wind loads on solar panels on buildings have been added to ASCE 7-16. Two methods are provided. One method (Section 29.4.3) is specific to buildings with roof slopes less than 7° with limitations on panel length, tilt, and height above roof. Wind loads on panels installed per this section may be significantly higher than the design loads for the roof, particularly where the effective wind area is small. The other method (section 29.4.4) applies to all roof slopes but the panels are required to be installed parallel to the roof and less than 10 inches above the roof surface (measured from the flat of the roof panel for metal roofs). A minimum gap of 0.25 inches shall be provided between all panels, with the spacing of gaps between panels not exceeding 6.7 feet. In addition, the array shall be located at least 2h2 (h2=height of solar panel above the roof at the upper edge of the panel) from the roof edge, a gable ridge, or a hip ridge. Wind loads on panels installed per this section tend to be lower than the design loads for the roof [1,2].
Don’t overlook the additional seismic load that the solar panels will add to the building. Section 12.7.2 of ASCE 7-16 states, ”The effective seismic weight, W, of a structure shall include the dead load, as defined in Section 3.1…” Section 3.1.5 states, “The weight of solar panels, their support systems, and ballast shall be included as dead load.” For a house in northern Utah with 25 solar panels, the additional seismic load would be minimal, maybe around 300 pounds. However, it could be more significant on a large warehouse roof.
Our role as structural engineers is to ensure the building is adequate to support any loads imposed by the solar array. The engineer needs to be knowledgeable, resourceful, and perhaps use some ingenuity. Unnecessary reinforcement or additional framing may make a solar project financially unfeasible. Overloading the roof structure could result in failure causing property damage and an unsafe situation. An engineer should always be engaged to avoid common structural issues with roof-mounted solar panels. You are the engineer; you have the power to provide a reliable design and make the future bright!
Sources:
[1] Significant Changes to the Minimum Design Load Provisions of ASCE 7-16
[2] ASCE 7-16 Sections 29.4.3 & 29.4.4
