With the advent of computer-aided design, dimensional accuracy in construction documents has become quite important. I recently discovered that the dimensions for modular brick given in Architectural Graphic Standards and elsewhere are incorrect. Three bricks plus three 3/8-inch mortar joints does not equal exactly 8 inches. The actual brick dimension should be 2 19/64 inches. Why is this?
Standard modular brick walls are built with three bricks for every 8 inches. The mortar joints are not exactly 3/8-inch thick. Masons vary the mortar joint size to accommodate dimensional variations in the units. In ASTM C 216, C 62, and C 652, the unit size is allowed to vary. The allowable variation is based on the type of unit. Type FBX has the most stringent requirements for dimensional variability, while Type FBA has no restrictions. Even Type FBX bricks can vary 5/32 inch in 6 to 8 inches. This is because even though all brick may be the same size entering the kiln, the size after firing will vary from unit to unit.