The ribs can be manufactured to project beyond the overall unit thickness (i.e., the unit thickness including ribs is thicker than a typical CMU), or with the rib projection included in the overall unit thickness. In the first case, the net area, and corresponding section properties, will be larger than those published for non-ribbed units, although the effect of this increase is typically neglected in structural calculations. In the second case, where the rib projection is included in the overall unit thickness, the designer should be aware that the actual bearing area, section modulus, and moment of inertia are less than those published for non-ribbed units.
When building concrete masonry walls, mortar is typically placed to all outside edges of the masonry unit. However, with ribbed units, it is difficult to properly tool the mortar due to the projections.