Mortar is designed to be slightly weaker structurally than the masonry it is bonding. This goes for all types of masonry, including brick, concrete block, or stone, and the logic behind it is that any cracks caused by movement in the wall are easier and less expensive to repair at the mortar joints.

    A crack in the mortar is called a “bond break” and, because they allow water into a wall and the subsequent deterioration over time, finger-size pieces of mortar will eventually begin to fall out or the mortar will crumble away unless repaired. 

    Occasionally, we see cracks that run though both the masonry units and the mortar, like in the photo below. This happens when the mortar strength exceeds or is close to the rating of the masonry, and can also be due to an occasional defective block or brick.

    While this explains why cracks usually follow mortar lines, it does not answer the question of what causes the movement in the wall in the first place, which is more complicated. Here’s a list of several of our other blog posts about understanding different types of crack patterns: 

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• Why is my stucco cracking?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize serious structural problems in a house?

• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

        Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.