Cracks After an Earthquake: What They're Really Telling You
Why a crack after a shake is worth a proper look
After an earthquake, most houses in New Zealand come through without any danger to the people inside them. But shaking almost always leaves some mark - a new hairline crack in a wall, a line along a chimney, or a gap that wasn't there before. Most of these marks are cosmetic. A few are warnings. The tricky part is telling the difference without specialist training, which is why insurers and engineers use a consistent way of grading what they see.
How assessors grade earthquake damage
The Natural Hazards Commission Toka Tu Ake (NHC, formerly EQC) sorts earthquake damage into three broad categories in its assessment manual for building claims. Minor damage is cosmetic in nature and usually doesn't need an expert. Moderate damage involves structural or weathertightness issues, generally does need an expert, and the building is usually still safe to occupy or can be made safe with urgent work. Severe damage involves significant structural or weathertightness problems, most often needs an expert, and the building is often unsafe to occupy until it has been checked.
What the crack itself can tell you
The NHC manual also gives some pointers on reading cracks by material, which is a useful starting checklist:
- Plasterboard linings - cracking at sheet joints and around door or window openings is usually non-structural movement, but cracking that runs across a sheet itself can point to structural movement.
- Weatherboards - watch for cracking combined with visible movement of the boards, not just surface marks.
- Stucco and plaster cladding - thicker stucco (over 25mm) and older products are more prone to cracking, and cracks that let water in matter more than hairline surface cracks.
- Brick or stone veneer - "step" cracking that follows the mortar joints in a stair-step pattern usually points to ground movement underneath the foundation.
- Unreinforced masonry (older brick chimneys and walls) - step cracking here is a bigger concern, because unreinforced masonry has little structural integrity and can fail without much further warning.
- Concrete panels - cracking within a panel, or at the joints between panels, along with any misalignment of the panels.
Chimneys deserve a second look
NHC guidance for homeowners checking their place after a quake singles out brick or unreinforced concrete masonry chimneys as a particular hazard, especially where they extend above the roofline. Look for cracks, loose or broken bricks, loose masonry or plaster, and any leaning or twisting. Any of these signs may need urgent attention, and removing a damaged masonry chimney is usually a job for professionals.
When to call a structural engineer
MBIE's guidance on building safety after an earthquake puts the responsibility for confirming a building is safe on the owner, and is clear that structural concerns need professional engineering advice rather than a council visit or a guess. An engineer assesses the building's structural strength, identifies any defects or weaknesses, and checks any earlier strengthening work. The same applies to non-structural elements like ceilings, cladding and services, where advice from an engineer or the right trade is recommended. MBIE also notes that owning an earthquake-prone building doesn't automatically mean it has to be vacated - but structural concerns shouldn't wait for someone else to act on them.
Practical steps right now
- Don't go back into a part of the building that looks unsafe, even to take photos.
- Photograph the cracks and any other damage before any cleanup or repair starts - this helps with insurance assessment.
- Contact your insurer and, where the damage may be covered by EQCover, the Natural Hazards Commission, as soon as practical.
- Note the date and time of the earthquake and which cracks are new versus pre-existing, since this helps an engineer assess what moved.
- If in doubt about whether a crack is structural, get a professional engineering assessment rather than relying on appearance alone.
If you've had a crack appear or widen after a recent earthquake and you're not sure what it means, get in touch with Fil Engineers - we're happy to help you work out what you're looking at.
This article is general information only and is not a substitute for a site-specific engineering assessment of your building.
