The fault tolerance of simulated vertical and horizontal panel joints was assessed in a laboratory study in which the degree of watertightness of defective joints was determined by subjecting the joints to a range of simulated wind — driven rain loads, loads that were consistent with those that might be found atop tall buildings in a severe storm event. The joints were 20 mm wide and consisted of a one-component polyurethane product and closed cell backer rod. Small deficiencies (cracks), ranging in size between 2 and 16 mm in length, were purposely introduced in the jointing product, thus permitting water to pass through these openings. The joint was extended up to a maximum of 10 % of joint width so that the nature and size of the crack could be varied. Consequently, joints with cracks of varying length and size were subjected to water spray rates ranging between 1.6 and 6 L/(min-m2) and pressures of up to 2 kPa. The watertightness tests were carried out using a purposely developed test apparatus described in detail in the subsequent section.