Why That Crack Isn't Just a Cosmetic Problem

You're sitting on the couch, cup of tea in hand, when you notice it. A crack running across your ceiling like a lightning bolt. Or maybe it's that spider web pattern appearing in the corner of your bedroom. Perhaps it's a vertical line creeping down from the window frame like it's making a slow escape.

Your first thought: "Should I be worried?"

Your second thought: "Can I just paint over it?"

Here's the thing – cracks in your walls and ceilings are trying to tell you something. Some are just your house having a bit of a stretch after a hot day. Others are warning signs that something more serious is afoot. And a few are just cosmetic quirks that come with owning a home in New Zealand, where our houses expand and contract more than a yoga instructor at a hot springs retreat.

The good news? Most cracks are completely fixable by a reasonably handy homeowner. The better news? Once you understand what you're looking at, you'll know exactly which filler to grab and which technique to use.

Let's crack on (sorry, had to).

The Crack Identification Guide: What Are You Actually Dealing With?

Before you reach for that tube of filler, you need to diagnose what type of crack you've got. Using the wrong fix for the wrong crack is like putting a plaster on a broken leg – it might look like you've done something, but the problem's still there.

Hairline Cracks (The Most Common)

What they look like: Very fine lines, usually less than 1mm wide. Often appear in random patterns or as single lines.

Where you'll find them: Everywhere. Ceilings, walls, corners, around door frames.

What's causing them: Usually just normal house settlement and the natural expansion/contraction of building materials. In New Zealand, our temperature swings – especially in places like Central Otago where you can go from frost to scorching in the same day – cause timber framing to move slightly. The GIB board moves with it, and hairline cracks appear.

Worry level: Low. These are cosmetic.

The fix: Standard lightweight filler, sand smooth, paint. Easy weekend job.

Spider Web Cracks (Crazing)

What they look like: A pattern of fine cracks radiating outward, like a spider web or cracked eggshell. Usually concentrated in one area.

Where you'll find them: Often in corners, around windows, or in areas that get direct sunlight.

What's causing them: Usually a paint problem rather than a wall problem. This happens when paint was applied too thickly, dried too quickly in the sun, or was painted over a surface that wasn't properly prepared. Can also occur when paint is applied in very cold or very hot conditions.

Worry level: Low to medium. Not structural, but indicates a paint adhesion issue.

The fix: You'll need to scrape off the affected paint, sand back to a stable surface, prime, and repaint. Just filling over crazing won't work – the underlying paint is compromised and will keep failing.

Vertical Cracks Running from Corners of Windows and Doors

What they look like: Diagonal or vertical lines that start at the corner of a door or window frame and head up towards the ceiling (or down towards the floor).

Where you'll find them: At stress points around openings in your walls.

What's causing them: These are stress cracks. When you cut a hole in a wall for a window or door, you create weak points at the corners. As the building moves and settles, these corners bear the brunt of the stress. Very common in New Zealand homes, especially after earthquakes (even the small ones we barely notice).

Worry level: Low to medium. Usually cosmetic, but if they're getting wider over time, worth investigating.

The fix: Flexible filler is essential here. Standard rigid filler will just crack again because the building is still moving at that stress point. Some painters use fibreglass mesh tape over the crack before filling for extra reinforcement.

Horizontal Cracks Along Walls

What they look like: A horizontal line running along the wall, often at a consistent height.

Where you'll find them: Usually where two sheets of GIB board meet, or where a wall meets the ceiling.

What's causing them: In most cases, this is simply the GIB board joint compound cracking along a poorly finished join. The original plasterer might have skimped on tape, used insufficient compound, or the paper tape has lost adhesion over time.

Worry level: Low (if it's a GIB join). Check if the crack follows a straight line at a consistent height – that's your clue it's a join issue.

The fix: You may need to cut out the old tape and re-tape the join properly. Or, if the crack is minor, a good quality filler followed by a skim coat can work. For persistent cracks, paper tape bedded in compound is more reliable than just filling.

Cracks That Form a "Staircase" Pattern

What they look like: A diagonal crack that steps up or down, following a stair-like pattern.

Where you'll find them: Most commonly on brick or block walls, or on interior walls that back onto masonry.

What's causing them: This pattern specifically follows the mortar joints in masonry, which is why it steps. It indicates movement in the foundation or structure. On interior walls, it can telegraph through from external masonry movement.

Worry level: Medium to high. This type of crack suggests differential settlement – one part of your house is moving differently to another. Worth getting a professional opinion.

The fix: Get a structural engineer or experienced builder to assess before you do anything cosmetic. Filling these cracks without addressing the underlying cause is a waste of time and money.

Wide Cracks (More Than 5mm)

What they look like: Cracks you can fit a pencil into. Often with visible depth.

Where you'll find them: Can appear anywhere, but particularly concerning around windows, doors, or running diagonally across walls.

What's causing them: Significant structural movement. Could be foundation settlement, ground movement, drainage issues causing soil to shift, or damage from a seismic event.

Worry level: High. Stop reading this article and call a professional.

The fix: Structural assessment first. Seriously. Wide cracks that appear suddenly, grow over time, or are accompanied by doors or windows that suddenly stick or won't close properly are red flags.

Cracks That Keep Coming Back

What they look like: Any crack you've filled before that reappears in the same spot.

Where you'll find them: The same bloody place you filled it last year.

What's causing them: Either you used the wrong filler (rigid filler on a stress point), didn't prep properly, or there's ongoing movement that no filler will fix permanently.

Worry level: Depends on the crack type. If it's a hairline crack at a stress point that keeps returning, switch to flexible filler. If it's a wide crack that keeps growing, see above.

The fix: Identify why it's returning. If it's a stress point, use flexible filler and possibly tape. If the crack is getting progressively wider each time, that's your house telling you something more serious is happening.

The Right Filler for the Job: A Kiwi Guide

Walk into Bunnings or Mitre 10 and you'll find an entire aisle of fillers. Here's what actually works for each situation.

Lightweight/All-Purpose Filler (Selleys Spakfilla, Polyfilla)

Best for: Hairline cracks, small nail holes, minor imperfections, shallow dents.

Why it works: Easy to sand, dries quickly, perfect for cosmetic repairs where there's no movement.

Why it fails: Zero flexibility. Put this on a stress crack and you'll be filling it again in six months.

Pro tip: Apply slightly proud of the surface – it shrinks a little as it dries.

Flexible/Crack Filler (Selleys No More Cracks, Sika Flex)

Best for: Stress cracks around windows and doors, any crack that keeps returning, joints between different materials.

Why it works: It moves with your house. When temperatures swing and timbers expand, flexible filler stretches rather than cracks.

Why it fails: Not as easy to sand to a perfect finish. Can look slightly different to surrounding surfaces if you're picky.

Pro tip: For the best finish with flexible filler, apply it slightly below the surface, let it cure, then skim over with a thin layer of standard filler for sanding.

Setting Compound (Gib Plus4, CSR Rapid)

Best for: Deeper holes, re-taping joints, larger repairs, building up damaged areas.

Why it works: Sets rock-hard via chemical reaction rather than just drying. Much stronger than lightweight filler. Can be built up in layers without slumping.

Why it fails: Harder to sand (requires more elbow grease). Sets quickly, so you need to work efficiently.

Pro tip: The numbers on these products (20, 45, 90) indicate setting time in minutes. Use 90-minute for large repairs where you need working time, 20-minute for quick fixes.

Caulk (Acrylic Gap Filler)

Best for: Gaps between skirting and walls, gaps around architraves, anywhere two surfaces meet at a corner that moves.

Why it works: Extremely flexible, paintable, easy to apply with a gun.

Why it fails: Can't be sanded. Not suitable for flat surfaces. Will look terrible if you try to use it like filler.

Pro tip: Keep a damp rag handy. Smooth caulk with a wet finger for a professional finish. The goal is to fill the gap invisibly, not create a visible bead.

When to Use Paper Tape vs Just Filling

If a crack keeps returning, or if it's along a GIB board join, adding paper tape provides reinforcement that filler alone can't match.

The process:

1. Widen the crack slightly with a scraper to create a V-groove
2. Apply a thin bed of setting compound
3. Press paper tape into the compound, smoothing out air bubbles
4. Let it dry, then apply two more thin coats of compound, feathering the edges
5. Sand smooth
6. Prime and paint

This is essentially rebuilding the GIB join properly. Takes longer but lasts much longer too.

The Proper Crack Filling Technique (Step by Step)

Step 1: Preparation Is Everything

Clean the crack thoroughly. Use the corner of a scraper or a thin blade to remove any loose material. For hairline cracks, this might seem pointless, but debris inside the crack prevents filler from bonding properly.

Wipe down the area with a damp cloth to remove dust. Let it dry completely.

Pro tip: For cracks that have been painted over before, use a sharp blade to score along the crack edges. This stops the paint from peeling away with your new filler later.

Step 2: Prime Deep or Porous Cracks

For cracks deeper than about 3mm, or on surfaces that are chalky or very dry, apply a coat of PVA primer (watered down 1:4) into the crack first. This helps the filler bond and stops the wall from sucking moisture out of the filler too quickly.

Step 3: Apply Filler in Thin Layers

The biggest mistake DIYers make: trying to fill a crack in one go.

Deep cracks need building up in layers. Each layer should be no more than 3-4mm. Let each layer dry completely before adding the next. Yes, this takes longer. No, there's no shortcut that works.

For shallow cracks, one application is fine – but apply it slightly proud of the surface to account for shrinkage.

Technique: Push the filler into the crack using a flexible filling knife held at about 45 degrees. Then scrape across the crack at 90 degrees to remove excess and level the surface.

Step 4: Let It Dry Properly

Read the product instructions. Then add extra time. In Central Otago's dry climate, filler dries faster than expected. In Dunedin's humidity, it takes longer. Winter? Even longer.

Pushing ahead before filler is fully cured leads to cracking, sanding problems, and visible repairs under your paint.

Step 5: Sand Smooth

Use 120-grit sandpaper for initial sanding, then 180-grit to finish. Sand in circular motions, feathering the edges so there's no visible ridge between the filler and the wall.

Run your hand over the repair. Can you feel it? If yes, keep sanding (or apply another thin skim coat if you've sanded through).

Pro tip: Sand with a light behind you, angled across the wall. This shows up every imperfection. If it looks smooth under raking light, it's actually smooth.

Step 6: Prime Before Painting

Filled areas are more porous than surrounding paint. Skip the primer and you'll see every repair as a dull patch once you paint.

Apply a coat of sealer/primer over your repairs before your topcoat. This evens out the porosity and ensures uniform paint coverage.

Climate Considerations for Kiwi Homes

Central Otago's Temperature Swings

Our region sees some of the most dramatic temperature changes in New Zealand. Houses literally expand and contract with the seasons – and sometimes within a single day. Stress cracks are more common here than in more temperate regions.

What this means for filling: Flexible filler is your friend. Rigid products that work fine in stable climates will fail repeatedly at stress points here.

Coastal Humidity

If you're near the coast (Dunedin, the Peninsula, anywhere with sea exposure), humidity fluctuates more, and your drying times will be longer.

What this means for filling: Allow extra drying time between coats. Don't fill on humid days if you can avoid it.

Earthquake Country

New Zealand's seismic activity means even small, barely-felt earthquakes gradually stress building joints. If you've felt a decent shake recently, expect some new cracks to appear as the house settles.

What this means for filling: Monitor cracks after seismic events. If they're widening significantly, get them assessed before you fill.

When to Stop Filling and Call a Professional

I'm all for DIY – it saves money and there's satisfaction in fixing your own home. But some situations need professional assessment:

Call a professional if:

• Cracks are wider than 5mm
• Cracks are growing noticeably over weeks or months
• You see staircase cracking on brick or block walls
• Multiple cracks are appearing throughout the house simultaneously
• Doors or windows have started sticking when they didn't before
• Cracks are accompanied by visible bulging or bowing in the wall
• You're seeing cracks in concrete foundation walls
• The crack has any moisture or staining around it

A structural engineer or experienced builder can tell you:

• Whether the cracks are cosmetic or structural
• What's causing the movement
• Whether repairs are needed beyond cosmetic filling
• If there are issues that need fixing before you sell (important for those pre-sale touch-ups)

Preventing Future Cracks

Some cracks are inevitable – houses move, materials age. But you can minimise new cracks:

Control moisture: Ensure good drainage around your house. Water pooling near foundations causes soil movement, which causes cracks.

Maintain consistent temperatures: Dramatic temperature swings stress building materials. Keeping your home at a relatively stable temperature (not too hot in summer, not freezing in winter) reduces thermal expansion/contraction.

Fix problems early: That small crack at the window corner? Fix it properly now, with flexible filler and maybe tape. Ignore it and it'll become a big crack.

Good paint prep: Paint that's properly applied to well-prepared surfaces is less likely to develop crazing cracks. Check out our guide on prep for more on this.

The Bottom Line

Cracks happen. In a New Zealand home, dealing with occasional wall cracks is just part of the deal – our seismic activity, our climate swings, and our timber-framed construction guarantee it.

The key is understanding what you're looking at:

• Hairline cracks: Fill with standard filler, sand, paint, move on with your life
• Stress cracks around openings: Use flexible filler, possibly tape, accept you might need to redo it eventually
• Persistent cracks at joints: Re-tape properly with paper tape and compound
• Wide or growing cracks: Stop, assess, and get professional advice before doing anything cosmetic

Get the diagnosis right, use the right product, and take your time with the technique. Your walls will look better, your repairs will last longer, and you'll save yourself the frustration of filling the same crack for the third time.

And if it all seems like too much hassle? There's no shame in calling someone who does this every day. Sometimes the smartest DIY decision is knowing when to hand it over.

Dealing with cracks as part of a larger painting project? If you're in Queenstown, Little Dog Decorating handles everything from crack repairs to full interior repaints. For Wanaka homeowners, Painters Wanaka offers professional wall preparation and painting services designed for Central Otago's unique conditions.