Why Chimneys Are Particularly Vulnerable
A chimney is one of the most mechanically disadvantaged elements of any building in an earthquake. It is tall relative to its base, heavy, and constructed from rigid masonry that cannot flex under dynamic loading. When seismic forces arrive — particularly horizontal accelerations — the chimney acts as an inverted pendulum: the base is constrained by the roof penetration, but the mass above it wants to move. Unreinforced brick and concrete masonry cannot accommodate this movement. They crack, shift, and in many cases collapse.
Hamish Armstrong of the Natural Hazards Commission Toka Tu Ake has stated it directly: “Brick and concrete masonry chimneys are particularly vulnerable as they can crack, shift, or collapse” during earthquakes. Every major earthquake in New Zealand over the past 170 years has demonstrated this failure mode. The 2010 and 2011 Canterbury earthquakes produced thousands of chimney failures across the city, and the collapse of chimneys was a significant source of injury, property damage, and road blockage in the immediate aftermath.
Which Homes Are Most at Risk
Homes built before 1970 are the primary risk category. These buildings were constructed in an era before modern seismic design standards were incorporated into the Building Code, and their chimneys were built as structural masonry with no internal reinforcement. Even minor tremors can loosen mortar and create hidden structural weaknesses that are not visible from ground level. The chimney may look stable but have hairline cracks in the mortar at the roofline that significantly reduce its resistance to lateral loading.
A chimney that has survived previous earthquakes without apparent damage is not necessarily safe. Each seismic event degrades the mortar and masonry without necessarily producing visible failure — the accumulated damage reduces the margin available for the next event.
Your Options
The safest approach for a vulnerable chimney is complete removal from the top to floor level — removing the entire above-ground structure and capping the flue opening. This eliminates the risk entirely. A common alternative is to remove the section above the roofline — the most hazardous portion, as it has the greatest height and the least lateral restraint — and replace it with a lighter metal flue extension where the fireplace function is still wanted.
Where immediate removal is not possible, temporary mitigation includes installing restraining straps from the chimney to the structural framing in the roof space, and adding plywood bracing to reduce the chimney’s ability to move independently of the building structure. These are interim measures, not long-term solutions, and should be accompanied by a plan for permanent remediation.
Getting an Assessment
A Licensed Building Practitioner can assess the condition of a chimney and provide advice on the remediation options appropriate for the specific structure. For buildings in areas of high seismic hazard — Wellington, Marlborough, Canterbury, Hawke’s Bay — the assessment should be treated as a routine part of property maintenance rather than an optional extra triggered only by visible damage.
