Portland cement plaster may be applied over the surface of concrete block walls for a variety of reasons. It may be to enhance the appearance of the building, to level out irregularities of alignment of the blocks, to achieve uniformity and continuity of surface where concrete blocks abut other construction materials, to provide alternative attractive textures or additional resistance to intrusion of moisture. Concrete blocks provide an excellent base for cement plaster because of the solidity and integrity of block walls and freedom from some of the problems associated with wood framing.
Application of cement plaster over concrete block walls contributes to resistance of walls to absorption or intrusion of water, because of greater density and slower rate of absorption of moisture into cement plaster, compared with concrete blocks. In addition, the membrane of plaster is extremely beneficial in deflecting rain.
Masonry walls, like concrete, provide a stable base for plaster because of virtual freedom from shrinkage, expansion, warping or subsidence; any or all of which may occur in wood framing.
Plaster that is bonded directly to blocks generally does not suffer from shrinkage cracking which may occur over open frame construction. Also, plaster applied over a block base generally is not affected by structural stresses which may cause cracks in plaster over wood framing. Stress from a problem in the concrete block base wall could be transferred to an overlying plaster membrane, however.