The paper refers to the importance of test results uncertainty in the assessment of the construction product performance. Uncertainty understood as dispersion attributed to any value which is result of the tests, calculations or other evaluations, occurs at every level of assessment (material, product, construction). Authors presented the various approaches to uncertainty estimation, especially in situation of small number of tests results which is characteristic for testing of construction products. Effects of uncertainty on final assessment of construction product was analyzed using the example of bearing capacity of thin-walled structure obtained by numerical calculation. Different values of material tests uncertainty resulting from different approaches to its assessment was taken into account. It was demonstrated that the difference in the results of strength tests of a material, which falls within the limits of uncertainty, may result in a very significant difference in the evaluation of a structure.