Concrete blocks, also referred to as concrete masonry units (CMU) are used as basement walls as well as retaining and crawl space walls. Regardless of the end use, a concrete block wall must have a supportable footing. A horizontal crack in a basement wall, even if it is only a hairline crack, can be the most significant crack observed and the most costly to repair. The inspector will carefully examine all visible foundation cracks and make recommendations depending on the severity of the crack. A significant crack may cause a wall to fail and be very costly to repair.