What is the intended size of a mortar joint for a CMU?

The intended size of a mortar joint for a Concrete Masonry Unit (CMU) is typically 3/8 inch. This dimension is preferred because it allows for proper adhesion between the masonry units and sufficient space for the mortar to fill any irregularities in the surfaces of the units. A joint size of 3/8 inch balances the need for strength and stability while also accommodating slight variations in the unit dimensions.

A joint size larger than 3/8 inch, such as 1/2 inch or 5/8 inch, may lead to an increased risk of cracking under stress or may compromise the structural integrity of the wall due to excessive mortar volume. On the other hand, a joint that is too small, like 1/4 inch, may not provide enough room for the mortar to properly adhere or compensate for masonry unit irregularities, leading to potential structural issues or diminished performance. Therefore, the 3/8 inch joint size is widely accepted as the optimal choice for CMU construction.