Uncertainty is undoubtedly an inevitable part of programming process. A lot of effective factors in the water resources’ performance are not deterministic while their values are surely needed for the design and implantation of water systems. Since an optimal design of water distribution systems is in demand of a just evaluation of the costs, efficient operation of such systems calls for a proper evaluation of their efficiencies. Hereby, the performance of pipe networks, considering simultaneous uncertainty of nodal demands and pipe roughness is assessed in this research. The flexibility of the designed Two-Loop water distribution network along with simultaneous probabilistic changes of nodal demands as well as pipe roughness coefficients are evaluated, calculating a deterministic efficiency factor in Monte Carlo simulation method. Moreover, the variation of nodal demand and pipe roughness coefficient in the considered network is assessed. The results indicate that in a state of simultaneous changes in demand and in roughness coefficient, the Two-Loop network is able to supply consumers with the desired demand and at the required pressure, only approximately in one third of the cases.