In a sixty-year-old brick masonry house, a horizontal crack in the mortar with outward brick displacement is observed. What is the most likely cause?

The presence of a horizontal crack in the mortar accompanied by outward brick displacement in a sixty-year-old brick masonry house is most likely indicative of differential settlement. This phenomenon occurs when different parts of a structure settle at different rates, often due to changes in moisture content in the soil, variations in soil composition, or even the weight of the building itself affecting the foundation over time.

When a building experiences differential settlement, it puts stress on the masonry walls, leading to the types of cracks and displacement observed. The horizontal crack reflects that there are significant forces exerted horizontally, while the outward displacement of bricks suggests that the structural integrity is compromised due to this uneven settlement.

In contrast, excessive sand in the mortar mix generally results in a weaker mortar, potentially leading to cracks, but it would not typically cause the specific pattern of displacement seen here. Defective brick bonding might create structural issues, yet it would not necessarily present with outward displacement as a primary symptom. Improper ceiling joist installation is more related to issues with the roof structure rather than the foundational or wall integrity that would cause the specific cracking and displacement observed.

Thus, the scenario described aligns best with the effects of differential settlement on the masonry.