Ultimate limit state (ULS) design ensures that structural elements have sufficient strength to resist the most adverse combination of factored design actions. For rooftop solar PV, ULS checks are performed for all relevant load combinations under Eurocode 0 (EN 1990), including combinations of dead load, PV array load, wind load, snow load, and imposed maintenance load with appropriate partial safety factors.
A structural member fails ULS checks when the design effect of actions exceeds the design resistance calculated from section properties and material strength. Structural engineers present ULS utilisation ratios: a ratio below 1.0 means the element is adequate; above 1.0 indicates structural inadequacy requiring redesign, strengthening, or array load reduction.