Horizontal cracks near the top of brick masonry chimneys are a common but often misunderstood problem. Understanding the forces that cause these cracks is essential for selecting the right repair method and preventing recurrence.
Two Primary Causes of Horizontal Chimney Cracks
Differential Thermal Movement
Every material used in chimney construction expands and contracts at different rates when heated. Brick, mortar, and clay flue liners all have distinct coefficients of thermal expansion. When a fireplace is used, the flue liner heats rapidly while the surrounding brick mass heats more slowly, creating tensile stress along horizontal mortar joints.
| Material | Coefficient of Thermal Expansion | Movement per 100°F Change |
|---|---|---|
| Clay flue liner | 3.0 to 4.0 x 10^-6 /°F | 0.036 to 0.048 in./ft |
| Common brick | 3.0 to 3.5 x 10^-6 /°F | 0.036 to 0.042 in./ft |
| Portland cement mortar | 5.5 to 6.5 x 10^-6 /°F | 0.066 to 0.078 in./ft |
| Firebrick (high alumina) | 2.5 to 3.0 x 10^-6 /°F | 0.030 to 0.036 in./ft |
Differential Settlement
Settlement cracks occur when the chimney structure settles unevenly into its foundation. However, settlement typically produces diagonal or vertical cracking patterns rather than clean horizontal cracks at the top of the chimney.
Why Cracks Appear in Unused Chimneys
- Premature testing: A test fire built before mortar has fully cured
- Moisture intrusion: Freeze-thaw cycles in saturated brickwork
- Mortar wash shrinkage: Cap mortar shrinking away from the flue liner
- Construction washing: Water used to wash down masonry freezes before drying
Preventive Measures
Three critical construction details prevent most horizontal cracking:
Proper Airspace Around Flue Liners
Flue tiles should be installed with a minimum 1-inch airspace between the tile and the surrounding brick. This acts as a thermal break, reducing heat transfer to the outer brick mass.
Flexible Sealing at the Chimney Top
Insert small brick wedges in the gap between top brick courses and the flue liner rather than filling solidly with mortar. This supports the flue while allowing differential movement.
Silicone Sealant at Critical Joints
Rake out the mortar where the flue liner exits the sloped wash to 1/4 in. to 1/2 in. depth and fill with high-temperature silicone sealant. This remains flexible as temperatures fluctuate.
Repair Options
Full Encircling Cracks
If the crack extends completely around the chimney, dismantle the brickwork down to the crack and rebuild from that point. This ensures proper reinforcement and addresses underlying issues.
Partial Cracks
Cut out cracked mortar to 3/8 in. to 1/2 in. depth, clean thoroughly, and fill with flexible masonry caulk. Caulk accommodates future movement better than repointing with mortar.
Conclusion
Horizontal chimney cracks near the top are typically caused by differential thermal movement combined with inadequate provisions for that movement. Proper construction with airspace around flue liners and flexible seals at the chimney top prevents most cracking.