Stair-step cracks are diagonal fissures that follow the mortar joints in brick or masonry walls. According to ACN Construction, these patterns are primarily caused by the following environmental and structural factors:

• Foundation Settlement: This occurs when a foundation moves downward due to organic material decomposing under the slab or construction on poorly compacted soil.
• Expansive Clay Soils: Common in the Denver Front Range, these soils swell significantly when wet and shrink when dry. This cyclical movement exerts uneven pressure on foundation walls.
• Hydrostatic Pressure: When soil becomes saturated with water after heavy rain or snowmelt, it pushes laterally against the foundation. This force often causes walls to bow and mortar joints to separate.
• Freeze-Thaw Cycles: Water that becomes trapped within the mortar joints can freeze and expand, which gradually widens existing hairline cracks.

Because mortar joints are the weakest part of a masonry wall, these external forces cause the wall to crack along the “path of least resistance,” resulting in the characteristic staircase pattern.

• Stair-Step Cracks Explained: Causes, Signs, and Repair Options