Masonry decay, known technically as masonry fretting,is caused by rising damp and salt attack. When your damp proofing course fails, your masonry’s capillaries soak moisture and salts out of the ground. Then, during dryer months, these salts crystallise and expand as the moisture escapes. The expansion can be so forceful that it turns brick and mortar into dust. Obviously, this is devastating to the wall stability, causing them to lean, bow or even collapse.