How it works

Brick is a fired-clay product with a relatively dense outer face and a more porous interior body. When moisture finds a path past the face, through cracked mortar joints, a failed crown, or an unsealed brick, it saturates the porous core. As temperatures drop below freezing, the trapped water expands by roughly 9 percent, generating internal pressure that exceeds the brick’s tensile strength and pops off the face.

Once spalling begins, it accelerates: the exposed core is even more absorbent than the original face, so the next cycle penetrates faster and damages more area. Untreated, a spalling chimney loses structural integrity within a decade. Sweeps document spalling severity during CSIA Level 2 inspections, and the remediation hierarchy runs from stopping the water source, to brick replacement, to full chimney rebuild for severe cases.

DFW context

DFW chimneys see 15 to 25 freeze-thaw cycles per winter, plenty to drive spalling on any chimney with compromised mortar joints or a failed crown. The June 2025 hailstorms added a new vector: hail impacts crack mortar joints and brick faces, opening fresh moisture paths just before the wet fall season. TSE inspections after major hail events frequently document accelerated spalling on chimneys that previously appeared sound.

Related terms

• [Crown](/glossary/crown/)
• [Tuckpointing](/glossary/tuckpointing/)
• [Wash](/glossary/wash/)
• [Crown coat](/glossary/crown-coat/)

Sources

• NFPA 211 (2024), Section 10.4
• ASTM C67 brick testing standard
• BIA Technical Note 7 (Brick Industry Association)