If a masonry wall is well constructed with proper workmanship, how necessary is the cavity behind the outer wythe? Will the wall leak if the joints are tightly constructed?

Even with the best workmanship, some water penetration should be expected to occur into and through the veneer wythe during wind-driven rains because the bond between the mortar joints and brick units is not watertight. Although the brick and mortar joints are relatively impervious to water penetration, some small voids or bond separations almost always occur at the interface between the two. There are many factors that affect the rate of water penetration, including the amount of compaction during tooling, material properties of the brick, compatibility of the brick and mortar, moisture content of the masonry units during placement, and weather during the installation of the masonry.

Bricks units with a very low initial rate of absorption (do not readily absorb water that falls on the bedding plane) occasionally develop a small film of water between the brick unit and the mortar during placement (often called “floating”). On the other hand, brick units that have a very high initial rate of absorption and a low moisture content at the time of installation often draw so much water out of the mortar that the top surface of the joint is dry and as a result does not develop good intimacy of contact.

Hot, dry, windy days increase the evaporation rate of water from the mortar and also can lead to a product that is dry and stiff on the top surface. By stringing less mortar during construction, the time between placing the mortar and the units is shortened, which reduces the magnitude of this problem.

Developing good joint compaction depends on proper tooling. A concave tool, which applies pressure to the joint, should be used when the joints are thumbprint hard (maintain a thumbprint on the front surface). Tooling the mortar joints too early or too late leads to less compaction and a less watertight joint.

However, even if everything is done perfectly, some water penetration likely occurs at the face of the wall. For this reason, it is essential that the cavity be free of mortar or debris that can restrict drainage and that flashings be properly installed at the base and top of the wall.