What Does a Bad Foundation Crack Look Like? Signs You Can't Ignore

If your house is shifting, creaking, or showing cracks in the walls, it’s easy to brush it off as normal aging. But not all cracks are the same. Some are harmless. Others? They’re warning signs your foundation is failing-and time is running out.

What a bad foundation crack looks like

A bad foundation crack isn’t a thin, hairline line you can barely see. It’s wider than a credit card. It’s jagged. It runs diagonally across a wall. It’s growing. And it’s often paired with other symptoms like sticking doors, sloping floors, or gaps between walls and ceilings.

In New Zealand, where soil moves with rain and earthquakes, foundation cracks are common-but not always dangerous. The key is knowing the difference between cosmetic damage and structural failure.

Bad foundation cracks are typically:

• Wider than 5mm (about the thickness of a pencil lead)
• Diagonal or stair-step-running up or down along brick or block walls
• Running through multiple levels-from the basement to the second floor
• Widening over time-you can measure it with a ruler and check again in 3 months
• Accompanied by other signs like doors that won’t close, windows that stick, or floors that slope noticeably

These cracks often start at corners of windows or doors, or where the foundation meets the first floor. If you see a crack that looks like a staircase going up the wall-each step a different level of brick or concrete-that’s a classic sign of differential settlement. The ground under one part of your house is sinking more than another.

Why foundation cracks happen

Foundations crack because the ground beneath them moves. In Auckland, that’s mostly due to:

• Clay soil expansion-when it rains, clay swells like a sponge. When it dries, it shrinks. This constant push-pull stresses your foundation.
• Poor drainage-gutters clogged, downspouts pointing at the house, or flat yards that pool water near the foundation.
• Tree roots-large trees like pohutukawa or kauri can suck moisture from the soil, causing it to dry out and shrink under your home.
• Earthquakes-even minor tremors can shift footings over time, especially in older homes built before modern seismic codes.
• Old or weak materials-homes built in the 1960s-80s often used thin concrete slabs or unreinforced masonry that can’t handle modern loads or soil movement.

It’s not about the weather alone. It’s about how your home was built, what’s around it, and how water moves-or doesn’t move-away from it.

Types of cracks: which ones matter

Not every crack needs a repair crew. Here’s how to tell the difference:

Vertical cracks

These run straight up and down, usually along a wall or column. They’re often caused by concrete shrinkage as it dries. If they’re less than 2mm wide and haven’t changed in a year, they’re usually fine. But if they’re wider than 5mm or show signs of movement, they could mean your footing is settling unevenly.

Horizontal cracks

These are serious. They run side to side, often near the bottom of a basement wall. They’re caused by soil pressure pushing against the wall. If you see a horizontal crack, especially if the wall is bowing inward, your foundation is under stress. This isn’t a DIY fix. It needs professional underpinning or wall reinforcement.

Diagonal or stair-step cracks

These are the red flags. They follow the mortar joints in brick or block walls, stepping up like stairs. They’re caused by uneven settling-part of the house sinking more than another. This is the most common sign of a failing foundation. If you see these on the outside of your home, they’re likely connected to internal cracks.

Cracks with water seepage

If water is dripping through a crack, or you see white chalky residue (called efflorescence) around it, moisture is getting in. That’s not just a structural issue-it’s a mold and rot risk. Water weakens concrete, rusts steel rebar, and destroys insulation. A crack that leaks is always a bad crack.

What to do if you find a bad crack

If you spot a crack that fits the bad-crack profile, don’t wait. Don’t paint over it. Don’t ignore it because "it’s always been there." Here’s what to do next:

1. Measure it-use a ruler or caliper. Mark the start and end with tape. Check it again in 30 days. If it’s wider, it’s moving.
2. Check for other signs-do doors stick? Does a ball roll on its own across the floor? Are there gaps between baseboards and walls?
3. Take photos-date them. These help professionals track progression.
4. Call a foundation specialist-not a general builder. Look for someone certified by the New Zealand Foundation Repair Association. They’ll use laser levels, moisture meters, and sometimes ground-penetrating radar to assess the damage.

Most repairs are fixable if caught early. Underpinning, slab jacking, or installing steel piers can stabilize your home. But if you wait too long, you could be looking at $50,000+ in structural repairs-or worse, a home that’s unsafe to live in.

What bad foundation cracks look like in different homes

Foundations vary by era and construction type. Here’s what to watch for:

1950s-70s brick veneer homes

These have a brick outer wall with a timber frame behind. The brick doesn’t carry weight-it’s just a shell. But if the brick is cracking diagonally, it’s because the timber frame underneath is shifting. That’s bad. The fix isn’t repointing the brick-it’s fixing the frame.

1980s-2000s concrete slab homes

These are common in newer suburbs. Cracks in the slab itself? If they’re less than 2mm and don’t extend beyond the edge of the slab, they’re usually fine. But if the crack is wider, or if the floor above is sloping, the slab is settling. This often happens when drainage was ignored during build.

Older stone or unreinforced masonry homes

Found in Ponsonby, Newton, or Onehunga. These homes were built without steel reinforcement. Their cracks are more common-and more dangerous. A stair-step crack in a stone wall? That’s not normal. It means the wall is separating. These homes need expert assessment before any renovation.

What you shouldn’t do

Don’t:

• Fill cracks with caulk or epoxy alone-this hides the problem, doesn’t fix it
• Paint over cracks-paint doesn’t stop movement
• Assume your home inspection covered it-most standard inspections don’t test for foundation movement
• Wait for a big earthquake to "prove" it’s bad-damage accumulates slowly
• Try to fix it yourself with DIY kits-foundation repair isn’t a Home Depot job

These mistakes delay real fixes and cost more in the long run.

Prevention: how to stop cracks before they start

Good drainage is the #1 defense against foundation cracks.

• Keep gutters clean and downspouts extending at least 2 meters from the house
• Grade your yard so it slopes away from the foundation (at least 5cm per meter)
• Plant trees at least 3-5 meters from the house
• Install a French drain if your yard is flat or clay-heavy
• Check for plumbing leaks under the house-dripping pipes can soften soil

These steps won’t stop earthquakes, but they’ll stop 90% of preventable foundation damage.

Frequently Asked Questions

Next steps if you’re worried

If you’ve spotted a crack that matches the description above:

• Take clear, dated photos from multiple angles
• Measure the crack’s width and mark it
• Check for movement in 30 days
• Call a foundation specialist with experience in Auckland soils
• Don’t delay-foundation problems only get worse with time

The sooner you act, the cheaper and easier the fix. A small crack today could cost you half your home’s value if ignored for five years.