The satisfactory performance of a sheet membrane waterproofing system, during its useful life, relies solely on the permeability of the waterproofing membrane, when construction and workmanship are correctly implemented. In field conditions, the membranes are subjected to various stresses, such as those brought about by building loads and lateral soil pressure that may strain the membrane, particularly in deep basements, and adversely affect its permeability. In this paper, a new performance-based laboratory test method is proposed to assess the vapour and water permeability of strained waterproofing membranes under hydrostatic pressure. The novelty of the method is that for a given basement configuration (height and depth), the effects of strain arising from tensile stress and compressive pressures acting on the membrane are simulated on the test specimen and the vapour and water permeability is measured in laboratory conditions. The test method was applied to two types of bituminous membranes assuming that each type of membrane was placed in one to six-story high basements. The test results confirmed that at some strain levels that simulate field conditions, to which membranes can be subjected, the strained membranes transmitted vapour under hydrostatic pressure. The tests also revealed that the test apparatus was effective in measuring the vapour and water permeability of the specimens under hydrostatic pressure. (C) 2004 Elsevier Ltd. All rights reserved.