Properties of plaster mortar must be appointed taking into account fracture mechanics of system "aerated concrete masonry – plaster covering", understanding the processes at his hardening and exploitation. Consideration of internal stress in plaster covering and area of its contact with masonry, processes of origin, accumulation and development of cracks at the cyclic and power loadings, is a condition precedent in the construction of theory of fracture mechanics of such system. In the article processes occurring at hardening of plaster mortar, fracture mechanics of system "aerated concrete masonry – plaster covering" are considered. Based on the information about the processes occurring during application and hardening of the plaster covering and fracture mechanics data, compositions of plaster for aerated concrete masonry were projected. The solutions obtained have the required physico-mechanical properties and fracture toughness. Application of these solutions ensures a high durability of the plaster covering and the wall structure.