Question: The exterior wall of our building has several cracks in the wide mortar joints. The contractor evidently made all joints the same width as the extra-wide joint containing the 3/8-inch-thick shelf angle (the dark slot shown in photo). Do wide mortar joints inevitably suffer cracks?
Answer: Masonry consultant Colin C. Munro of Batavia, Illinois, says cracks are much more likely to occur in very wide mortar joints. Joints typically should be 3/8 to 1/2 inch wide, depending on the size of the brick. To lay much wider joints, the mason must use a stiffer mortar, probably one that contains less water and less lime. The result: not only is there more mortar (and more mass), but there's also a greater proportion of cement in the mortar. This high-cement mortar undergoes more drying shrinkage and thus more shrinkage cracks occur. The problem is worst when brick with a high initial rate of absorption (IRA) also is used. The brick will absorb water, making the mortar even drier.