The influence of visual assessment grade on the residual flexural capacity of 46 existing precast concrete ribbed panels from different agricultural buildings was studied. Before the tests the panels were assessed on a 6-point rating scale according to visually distinguishable corrosion deterioration. All panels, the ultimate load of which was lower than the control load, received grade 0 on the visual rating scale. Consequently, attention should be paid to panels where the concrete cover of longitudinal reinforcement has spalled (grade 0) which could be a sign of decreasing load capacity. The majority of panels with grade 0 exhibited larger deflections under load than panels with higher grades. Of the 46 panels tested flexural ductile failure was noticed at 36 panels.