Cracks in your GIB plasterboard walls or ceilings can be alarming, but they're one of the most common issues NZ homeowners face. Some cracks are purely cosmetic and easy to fix yourself; others can signal something more serious going on behind the scenes. Understanding what type of crack you're dealing with is the key to fixing it properly — and making sure it doesn't come back.
Why Do GIB Walls and Ceilings Crack?
Plasterboard (commonly known by the brand name GIB) is a rigid material fixed to a timber or steel frame. When anything causes movement in that frame — or in the plasterboard itself — the joints, stopping compound, and paint are the first things to show it. The most common underlying causes include:
- Seasonal timber movement: NZ's native and plantation timber framing expands and contracts with changes in humidity and temperature, especially in the first few years of a new build.
- Building settlement: All new homes settle as the ground compresses under the load. This is normal, but it stresses joints and corners.
- Poor stopping or jointing: If the original plasterer didn't use the correct tape, compound, or technique, joints are more likely to crack over time.
- Moisture and water damage: Leaks from roofs, plumbing, or condensation can soften stopping compound and cause cracking or bubbling.
- Structural movement: Subsidence, earthquake damage, or inadequate framing can cause more serious cracking.
- Impact damage: A knock to the wall can crack the surface compound without damaging the board underneath.
Common Crack Types and What They Mean
Not all cracks are equal. Here's a quick guide to help you identify what you're looking at:
| Crack Type | Appearance | Likely Cause | Urgency |
|---|---|---|---|
| Hairline crack at joints | Thin line along plasterboard seam | Timber movement, normal settlement | Low — cosmetic |
| Corner cracking | Crack running from door/window corner at 45° | Framing movement or settlement | Medium |
| Taped joint lifting | Tape bubbling or peeling | Poor original installation or moisture | Medium |
| Wide or stepped cracks | Gap wider than 3–5 mm, uneven edges | Structural movement or subsidence | High — get advice |
| Cracks with staining | Brown or damp edges around crack | Active water leak | High — find the leak first |
| Ceiling sag with cracking | Plasterboard bowing downward | Moisture damage, fixing failure | High — safety risk |
How to Diagnose the Problem Step by Step
Step 1: Check the width and pattern
Use a ruler or coin to measure the crack width. Hairline cracks (under 1 mm) at plasterboard joints are almost always cosmetic. Cracks wider than 3 mm, or cracks that are stepped (one side higher than the other), need closer investigation.
Step 2: Look for moisture
Run your hand along the crack. Does the area feel damp or soft? Check the ceiling above for roof leaks, and check nearby walls for plumbing pipes. Any moisture must be resolved before you repair the crack — otherwise it will return. If you suspect a plumbing leak, this is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be fixed by a licensed plumber.
Step 3: Check if the crack is active
Mark each end of the crack lightly with a pencil and date it. Check back in 4–6 weeks. If the crack has grown, it's still moving and you need to find out why before repairing.
Step 4: Inspect the framing
If you have access to the ceiling space or wall cavity, look for signs of damaged, wet, or poorly fixed framing. Sagging ceilings with multiple cracks can indicate fixing failure — the screws or nails holding the plasterboard to the frame may have pulled through.
Step 5: Consider recent events
Has there been an earthquake, heavy rain, or recent plumbing work? NZ sits on an active fault system, and even minor seismic events can open up existing weak points in stopping compound.
What You Can Safely Fix Yourself
For hairline to small cracks (under 3 mm) that are dry, stable, and clearly cosmetic, a confident DIYer can tackle the repair:
- Widen the crack slightly with a scraper or utility knife to give the filler something to grip.
- Apply a flexible filler such as a premixed jointing compound or a product like Sika's flexible gap filler for minor movement joints. Avoid rigid fillers on joints that may move again.
- Embed paper jointing tape over any cracked plasterboard seam before applying compound — this bridges the joint and prevents the crack reappearing through paint alone.
- Feather out the compound over a wide area (at least 200–300 mm either side of the crack) in thin coats, allowing each coat to dry fully.
- Sand smooth, prime with a quality sealer, and repaint. Resene and Dulux both offer suitable interior primers and paints matched to existing wall sheens.
Safety note: If you're working on ceilings, use a proper scaffold or platform — never a chair or unstable ladder. Falls from height are a leading cause of serious injury at home.
What a Professional Plasterer Should Handle
Call a qualified plasterer for:
- Cracks wider than 3 mm or that are still actively moving
- Cracked or lifting tape over large areas
- Ceiling sag or plasterboard that sounds hollow when tapped (fixing failure)
- Damage following a water leak (once the leak is fixed)
- Cracks at structural elements like load-bearing walls or around large openings
- Any repair where you want a smooth, invisible finish — re-stopping and feathering is a skilled trade
A licensed plasterer will assess whether the issue is purely a finishing problem or whether the framing or fixings need attention first. For structural concerns, they may recommend an LBP (Licensed Building Practitioner) assessment.
How to Prevent Cracks Recurring
- Control indoor humidity: NZ homes — particularly older ones — can suffer from high moisture levels. Good ventilation reduces the seasonal swelling and shrinking of timber framing. See Energywise for ventilation tips.
- Use flexible stopping compounds at internal corners and around door and window frames, where movement is greatest.
- Don't skip the tape: Properly embedded paper or fibreglass mesh tape over all joints is essential — paint alone will always crack at a seam eventually.
- Allow new builds to settle: In a new home, wait at least 12 months before doing final paint touch-ups. Most builders will include a maintenance visit to address settlement cracking.
- Fix leaks promptly: Water is the enemy of plasterboard. Even a slow drip will eventually cause stopping compound to fail.
When to Call a Professional
If you're unsure about the cause of cracking, or if cracks are wide, stepped, growing, or accompanied by moisture, don't just fill and paint over them. Get a professional opinion. A qualified plasterer can assess the damage and advise whether a builder, structural engineer, or plumber also needs to be involved.
You can find experienced, vetted local plasterers on tradedirectory.co.nz — search by your region to get quotes from tradespeople who know NZ conditions and the NZ Building Code.
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