Face brick delivered to a recent project varied up to 1/4 inch in length. The mason laid the brick with head joints as small as 3/16 inch and as large as 1/2 inch to overcome the size variation. The appearance is not objectionable (most joints are consistent), but how about future performance?
Head joints in running bond masonry generally contribute little to the strength of walls. But they can be the cause of excessive water penetration. In my experience, narrow head joints result in more water leakage problems than do wide joints. This is because narrow joints often aren't filled completely and are difficult to tool properly. Wide joints generally are not a problem unless the joints are very wide. I recommend the width of head joints be at least 1/4 inch to permit proper tooling and a maximum 3/4 inch to limit shrinkage cracking. The width of bed joints does have a significant effect on wall strength. For example, 3/4-inch-wide mortar bed joints produce only half the flexural and compressive strength of 3/8-inch-wide mortar joints, according to Tables 5-6 and 5-12 of Recommended Practice for Engineered Brick Masonry, published by the Brick Institute of America (BIA). Thus, the width of bed joints should be at least 1/4 inch to permit proper tooling and no more than 1/2 inch to provide adequate strength. ASCE 6-92, Specification for Masonry Structures, provides for head joint widths from 1/8 to 3/4 inch and bed joints from 1/4- to 1/2-inch thick.