According to Section 4.16.1 of ASCE 7-22 [1] and Section 1607.13.5.1 of IBC 2024 [2], structures that support solar panel systems shall be designed to resist each of the following conditions:

1- The uniform and concentrated roof live loads specified in Table 4.3-1 with the solar panel system dead loads. (EXCEPTION: The roof live load need not be applied to the area covered by solar panels where the clear space between the panels and the roof surface is 24 in. (610 mm) or less.)

2- The uniform and concentrated roof live loads specified in Table 4.3-1 without the solar panel system present.


It means that both situations must be considered, and the structure should be designed to resist each of these conditions. In addition, Section 4.16.3 Open-Grid Roof Structures Supporting Solar Panels, states that structures with open-grid framing and no roof deck or sheathing supporting solar panel systems shall be designed to support the uniform and concentrated roof live loads specified in Table 4.3-1, except that the uniform roof live load shall be permitted to be reduced to 12 psf (0.57 kN/m2). This provision indicates that the condition in which roof live load acts on the structure must always be checked during the design process.


This requirement is stated more explicitly in R324.4.1.1 of IRC 2018 [3], and RS402.2.1.1 of ISEP 2021 [4]:

Portions of roof structures not covered with photovoltaic panel systems shall be designed for dead loads and roof loads in accordance with Sections R301.4 and R301.6. 

Portions of roof structures covered with photovoltaic panel systems shall be designed for the following load cases:

1. Dead load (including photovoltaic panel weight) plus snow load in accordance with Table R301.2(1).

2. Dead load (excluding photovoltaic panel weight) plus roof live load or snow load, whichever is greater, in accordance with Section R301.6.


However, by overlooking or misinterpreting this requirement—stated in ASCE 7-22, IBC 2024, IRC 2018, and ISEP 2021—some engineers effectively omit roof live load from the design of roof systems PV panels. Designing a roof structure based only on the PV dead load (typically on the order of 3–4 psf) while neglecting the applicable roof live load (12–20 psf, as permitted/required by the code provisions) can lead to substantially lighter—but unsafe and noncompliant designs. This issue is particularly more critical in regions with low snow demand and modest wind and seismic actions, where roof live load often governs the design of frames and purlins.


A detailed and comprehensive explanation of this subject is provided in the section titled Roof Live Load at Solar Panels


References:

[1] American Society of Civil Engineers. "Minimum design loads and associated criteria for buildings and other structures." American Society of Civil Engineers, 2022.

[2] International Building Code (IBC), 2024 Edition

[3] 2018 Washington State Residential Code (IRC)

[4] 2021 International Solar Energy Provisions (ISEP)