The present study deals with the ultimate strength of stiffened plates subjected to spatially distributed mechanical properties and compressive load. Normally, mean values of mechanical properties based on tensile tests are used to validate the numerical assessment with experimental results. However, mechanical properties may vary within a single specimen. To investigate the impact of that, random fields of yield stress and Young modulus are employed together with the FE method. The variations of mechanical properties are estimated based on tensile tests. Different parameters of the random field are analysed, showing that the structural response will be different when compared with the stiffened plate with constant mechanical properties. Finally, it was revealed that the variation of yield stress has a significant impact, and Young modulus uncertainties play a secondary role. It is concluded, that the spatial variation of mechanical properties need to be considered when validating the numerical assessment with experimental measurements.