Deflection checks are a key part of serviceability limit state (SLS) verification for roof structures supporting solar PV arrays. Excessive deflection in purlins or rafters under combined dead, snow, and imposed loads can cause racking system misalignment, panel stress, or water ponding on flat roofs.
For commercial rooftops, engineers check deflection under characteristic (unfactored) load combinations. Span-to-deflection ratios of L/200 to L/360 are typical acceptance criteria depending on element type and construction. Where existing structural members are loaded close to SLS limits, additional dead load from a PV array may trigger a deflection exceedance.