For the construction of a storeroom using concrete masonry units, it's essential to align the dimensions of the structure with the actual sizes of the blocks and the intended mortar joints. On the site in question, there's a stockpile of concrete masonry blocks with a nominal size of eight by eight by sixteen inches, which are to be used in the construction of the storeroom.

The site engineer has laid out a plan for the storeroom with external dimensions of twelve feet in length and eight feet in width. To align with these dimensions, the plan specifies the use of nine blocks for the length of the walls and six blocks for the width. However, the actual size of each block is seven and five-eighths inches in height and width and fifteen and five-eighths inches in length, with a three-eighths inch allowance for the mortar joint. When combined with the mortar, this will provide an effective block length of sixteen inches.

This calculation ensures that when laid end to end, the blocks will fill the planned dimensions without the need for cutting, which facilitates a more rapid construction process and achieves a professional finish. Accordingly, the internal dimensions of the storeroom walls will measure ten feet eight inches along the length and six feet eight inches along the width once construction is completed. This methodical approach to the use of materials ensures efficiency in building and a clean aesthetic outcome for the storeroom.