Q There are multiple horizontal cracks on the interior concrete masonry in a four-story residential building that my company is working on. The masonry walls have 4-in. face bricks and 4-in. CMUs behind. The face bricks extend from the foundation to the roof without any relief angles.

Most of the horizontal cracks are occurring near the top of the building. In many areas, the wall is cracking in line with the lintel at the head of the windows. In other areas, horizontal cracks are occurring randomly in the wall. The owner noticed the cracking when he removed the gypsum wallboard and insulation as part of interior remodeling.

What is causing these cracks and how can they be repaired?

A It is difficult to know the origin of the cracks without conducting an investigation. However, from what you described, it seems that the cracks could be related to differential growth of the masonry.

Bricks begin to expand the moment they are removed from the kiln. The units chemically react with moisture from humidity or moisture absorbed from rain and melting snow. This expansion is permanent and cannot be reversed without refiring the brick.

Concrete masonry, on the other hand, shrinks over time. These units are generally at their largest when installed. Concrete masonry shrinks during drying as it acclimates to the environment.

Because the brick veneer is not supported by intermediate shelf angles, the differential moisture expansion accumulates with the height of the building. In an 8-in. wall there is only a narrow space between the brick veneer and concrete masonry, and this area is most likely filled with mortar droppings. Mortar in the collar joint can cause the two wythes to become locked together, preventing independent movement.

Therefore, as the brick masonry expands and the concrete masonry shrinks, the brick and precast floor planks supported on the concrete masonry crack. Such cracking can occur anywhere along the height of the wall, although they tend to be more common and larger at the upper floors.

Since both the moisture expansion and concrete masonry shrinkage occur early in the life of the building, I would recommend trying to tuckpoint the concrete masonry on the interior for the full depth of the interior wythe. This action reestablishes support in the concrete masonry. Since the building is 20-years-old, it is unlikely that significant additional cracking will occur in the future.