RILEM Test Method II.4 provides a simple means for measuring the rate at which water moves through porous materials such as masonry. The test can be performed at the site or in the laboratory and can be used to measure vertical or horizontal water transport. Water permeability measurements obtained in the laboratory can be used to characterize un-weathered, untreated masonry. Measurements made at the site (or on samples removed for laboratory testing) can be used to assess the degree of weathering that the material has undergone. Test Method II.4 can also be used to determine the degree of protection afforded by a water-repellent treatment. A description of the equipment and procedure for conducting this test is provided in paragraphs below. The theoretical basis for the method and interpretation of test data are discussed.