What is considered the weakest pattern bond in masonry?

The stack bond is regarded as the weakest pattern bond in masonry due to its lack of horizontal joint reinforcement. Unlike other bond patterns, the stack bond is characterized by each layer of bricks being aligned directly over the layer below, which results in a vertical alignment of joints. This arrangement can lead to increased vulnerability to lateral forces, such as those caused by wind or earthquakes.

In contrast to the stack bond, other bond types, like running bond or Flemish bond, incorporate staggered joints in their design, which enhances their structural integrity. Staggered joints allow for better distribution of load and increase the overall bond strength of the wall. The American bond, which also features some staggered joints, is stronger than a stack bond for the same reasons.

Thus, while the stack bond might be aesthetically pleasing in certain applications, its structural limitations render it the weakest option among the various masonry bond patterns.