It is at the interface between brick and mortar that most of the leaks in masonry walls occur. Sound, crack-free brick and mortar do not leak, but hairline cracks at the interface of the brick and mortar often do. How do you prevent these cracks? By using a mortar that holds the brick together tenaciously. Many architects and engineers require only that the mortar conform to ASTM C 270, Mortar for Unit Masonry. This is good, but it's not enough. To prevent cracks, the specifier must match the mortar type to the brick. He should specify mortar using ASTM C 270, monitor the quality control of the field made mortar using C 780, and have masonry prisms tested according to E 447, E 518, or C 1072.