Basement walls and floors of new constructed buildings are sometimes exposed to water that exerts pressure from the outside due to the existence of high level of ground water. Therefore, oxidized and modified bituminous membranes are applied as external or internal tanking for waterproofing. In such applications; the membranes are strained excessively due to the effects of the mechanical agents that are generated from the construction, and meanwhile they are exposed to water pressure that is originated from the groundwater. The watertightness integrity of these membranes are determined according to the existing test methods which can only measure the resistance of unstrained membranes to water pressure. The aim of this paper is to propose a modified test method which evaluates the performance of the strained bituminous membranes under hydrostatic pressure. In this test method; the effects of the mechanical agents on the membranes which are the deformation of the membranes under compressive pressure and the excessive strain of the membranes over the cracks are simulated in laboratory conditions. Then, the conditioned test samples are exposed to hydrostatic pressure with a modified testing equipment and the watertightness integrity of strained test samples under hydrostatic pressure are evaluated with a moisture/water sensor.