Expansive soils contract when dry. When this occurs the foundation of the house and chimneys are vulnerable to movement. In the case of the photo below the foundation has lifted slightly on one side and settled slightly on the other side. The rotation of the chimney ensues resulting in a gap that exists between the caulking and the brick and the flashing.
Jeremy Livermore, S.E., P.E.
Structural Engineer, License #: S5905
Civil Engineer, License #: C71491
OC Structure Check
1133 Camelback St.
Newport Beach, CA 92658