What is the likely cause of a horizontal crack in the mortar of a 60-year-old brick masonry house?

A horizontal crack in the mortar of a 60-year-old brick masonry house is most commonly linked to differential settlement. This type of crack often results from variations in soil conditions or changes in moisture content, which can cause one part of the foundation to settle more than another. As the building shifts due to this settlement, stresses can develop in the masonry and mortar, leading to cracking.

Differential settlement can occur for several reasons, including erosion, changes in water drainage, or the natural settling of soil over time. In older homes, it is not uncommon for the foundation to experience some settling as it ages, especially if the original soil was not properly compacted or if changes in landscaping have altered drainage patterns.

While options related to mortar mix and brick bonding might seem viable, they typically lead to different types of structural issues. For instance, excessive sand in the mortar could ultimately affect its strength but is less likely to directly create horizontal cracks. Similarly, defective brick bonding generally causes vertical cracks rather than horizontal ones. Improper ceiling joist installation pertains more to issues with the roof structure than to ground-level foundation and wall integrity. Hence, the presence of a horizontal crack strongly suggests the involvement of differential settlement.