Water is leaking through mortar joints and you need to repair them. Which method works the best? This question was faced in the repair of the Science Building at Northeastern Illinois University in Chicago. Its masonry walls were very permeable. Many repair methods were tested, measuring the water permeance of the masonry walls before and after making the trial repairs. THE TEST To measure the water permeance of the walls, a 12-square-foot test chamber was mounted on the wall. Water was sprayed against the wall to simulate rain, and the chamber was pressurized to simulate wind. The amount of water entering the face of the wall in 3 hours was taken as the permeance. THE REPAIR METHODS The trial repairs and testing were performed on the masonry walls of two rooftop penthouses. The walls of each penthouse were divided into six test areas, creating a total of 12 test areas. Four methods were examined: tuckpointing; covering the entire wall surface with grout; surface grouting joints only; and clear waterproof coatings. THE RESULTS All the repairs reduced the water permeance of the masonry to some extent. The three clear waterproof coatings were the least effective. Surface grouting the joints in the walls was a more effective repair. Coating the entire wall surface with grout and conventional tuckpointing with tooled joints were the most effective treatments.