The majority of clay masonry walls built today are cavity walls. Water that penetrates the exterior masonry will be handled by the cavity and flashing at the base of the wall. There are, however, many walls still being built with single-wythe concrete masonry units. Won't these be prone to leak?
Single-wythe concrete masonry walls will be more prone to leakage than brick cavity walls. Unless the exterior surface is protected, water will penetrate these walls. Conventional CMUs also will readily absorb water. Water can soak through the thickness of the concrete masonry walls and appear on the interior face. It is for this reason that most single-wythe concrete masonry walls are either coated with an elastomeric coating system or covered with plaster or some other system that will stop water from reaching the wall surface. Clear water repellents are also sometimes used; however, in many cases, these treatments are not effective because they cannot bridge cracks or large mortar separations. As with coating systems, water repellents will weather and, therefore, must be reapplied periodically.
Another common approach is to use an integral water repellent in the concrete masonry. Walls using integral water repellents and face-shell bedding can function very similar to cavity walls if properly designed and constructed. Because water is not absorbed, it will drain through the cores of the units. If the cores line up properly and are not blocked, water penetrating the mortar-bond interfaces will flow down through the cores similar to water flowing in a cavity wall. In these walls, it is extremely important that flashings be properly detailed at the head of all openings and at the base of the walls, to direct any water that penetrates the concrete masonry back toward the exterior. As in a cavity wall, it is important that these flashing joints be thoroughly sealed and be continuous around corners.