How to fix a crack in a wall (2026)
Quick Answer
To fix a crack in a wall, first work out whether it is a harmless surface crack or a sign of movement, damp or structural trouble. Small, stable cracks in plaster or drywall can usually be opened slightly, cleaned, reinforced if needed, filled, sanded and redecorated. If the crack is widening, keeps coming back, follows a stair-step pattern in masonry, or comes with sticking doors, bulging walls or water damage, stop cosmetic repairs and get a qualified professional to assess the cause first.
Safety first
- Wear eye protection and a suitable dust mask when scraping or sanding filler, plaster or paint.
- If the property is older and has textured coatings or other suspect materials, do not sand or scrape aggressively until you have considered whether asbestos may be present and obtained appropriate advice.
- Use a stable step stool or ladder correctly if the crack is high on the wall; do not overreach.
- Turn off power at the consumer unit before opening a crack near sockets, switches or known cable routes, and do not cut into the wall if you are unsure what is behind it.
- Treat damp walls carefully: mould and contaminated water from leaks can be hazardous, and electrical fittings in damp areas may be unsafe.
- Do not attempt DIY structural repairs to load-bearing walls, significant masonry cracks or bulging surfaces.
When to call a professional: Stop DIY work and consult a qualified builder, plasterer, surveyor or structural engineer if the crack is widening, keeps returning, is diagonal or stepped through masonry, runs through a load-bearing wall, appears with bulging or sagging, follows water damage or persistent damp, affects an external wall, is near structural openings, or is accompanied by sticking doors/windows or sloping floors. Also get professional advice before disturbing suspect asbestos-containing materials or if electrical services may be affected.
Overview
Wall cracks are common, but the right fix depends on why the crack appeared. Many are cosmetic: plaster shrinks as it dries, taped plasterboard joints can move slightly, and buildings expand and contract with seasonal temperature and moisture changes. Those cracks can often be repaired successfully with the right filler or joint compound, and reinforcement tape where movement is likely. Other cracks point to a deeper problem, such as damp, leaks, poor plaster adhesion, foundation movement or structural stress. In those cases, filling the crack without fixing the cause will usually lead to the crack returning. A practical approach is: inspect the pattern, width and location; check for signs of active movement or moisture; repair only once the wall is dry and stable; then choose a repair method that matches the wall type and crack type. Hairline plaster cracks need a different treatment from a loose patch of blown plaster or a crack running through brickwork. This guide focuses on diagnosing common causes and making safe repairs to interior finished walls. It also explains when a crack may need a builder, plasterer, structural engineer or surveyor rather than a DIY fix.
Who this is for
Homeowners, tenants with permission to make minor repairs, and DIYers dealing with small non-structural cracks in internal plaster or plasterboard walls.
What you’ll need
- Torch or good lighting
- Dust sheet
- Safety glasses
- Dust mask suitable for fine dust
- Filling knife or scraper
- Utility knife
- Stiff brush or vacuum with brush attachment
- Suitable wall filler or joint compound
- Joint tape or mesh tape for recurring joint cracks
- Sandpaper or sanding sponge
- Primer or mist/sealer coat if needed
- Matching paint
- Optional: straightedge, pencil, phone camera for monitoring, stain-blocking primer if there has been water staining
Before you start
Check whether the crack is dry, stable and only in the finish layer. Look for staining, mould, bulging, crumbling material, gaps around skirting or frames, sticking doors or windows, and any change in the crack over time. If there is any sign of damp, a leak, active movement, loose masonry or possible asbestos-containing textured coatings in an older property, do not sand or disturb the area until you have the right advice.
Step-by-step
- 1
Inspect the crack and look for warning signs
Examine the crack in good light. Note its location, direction and whether it is only in paint/plaster or appears to run through the wall itself. Check nearby ceilings, door frames, windows and floors for related signs such as movement, distortion, staining or softness. Take clear photos and mark the ends lightly in pencil so you can tell if it changes.
Why: You need to separate cosmetic cracking from movement, moisture or structural problems before choosing a repair.
- 2
Identify the likely cause
Treat hairline cracks in paint or plaster as likely surface or shrinkage cracks if the wall is otherwise sound and dry. Suspect plasterboard joint movement if the crack is straight and follows a board seam. Suspect blown or loose plaster if tapping sounds hollow or the surface moves. Suspect damp or leakage if there is staining, bubbling paint, mould or crumbly material. Suspect structural movement if cracks are wider, stepped through masonry, diagonal from openings, recurring, or accompanied by sticking doors and windows.
Why: The repair must match the cause; a filler-only repair will fail if the substrate is loose, wet or still moving.
- 3
Fix the underlying problem first
Dry out and repair leaks, improve ventilation for condensation issues, and remove any loose or failed plaster before filling. If the wall is still moving or the cause is uncertain, pause here and get professional advice. Only repair once the area is dry, sound and no longer changing.
Why: Moisture, poor adhesion and ongoing movement are the main reasons crack repairs reappear.
- 4
Prepare the crack properly
Protect the area, then scrape away any flaking paint or loose filler. Use a knife or scraper to open a fine crack slightly into a shallow V so filler can key into the sides, then brush or vacuum out dust. For a cracked plasterboard joint, remove any loose tape or compound. For loose plaster, cut back to sound edges before patching.
Why: Filler sticks better to clean, solid edges than to a thin skin of paint or dusty, polished plaster.
- 5
Reinforce where movement is likely
If the crack is along a plasterboard joint or has returned before, apply suitable joint tape or reinforcing mesh as recommended by the compound manufacturer. Bed the tape into compound smoothly and avoid leaving ridges. For isolated hairline plaster cracks that are stable, reinforcement may not be necessary.
Why: Tape helps bridge slight movement and reduces the chance of a repaired crack printing through again.
- 6
Fill in thin layers and let each layer dry fully
Apply the appropriate filler or joint compound with a filling knife, pressing it firmly into the crack or patch. Build up deeper repairs in layers rather than one thick application, following the product instructions for drying and recoating. Feather the edges wider than the crack so the repair blends into the wall.
Why: Layering reduces shrinkage, cracking and weak spots in the repair.
- 7
Sand, prime and redecorate
Once fully dry, sand lightly until smooth and level with the surrounding wall. Wipe away dust. Prime if the filler manufacturer or paint system requires it, and use a stain-blocking primer first if the area had previous water staining. Then repaint, often extending paint beyond the repair for an even finish.
Why: Priming evens out porosity and helps prevent flashing, patchiness or stains bleeding back through the finish.
- 8
Monitor the repair
Keep an eye on the area over the following weeks and through seasonal changes. If the crack reopens, widens or new related cracks appear, stop re-filling and arrange a professional inspection.
Why: A recurring crack usually means the wall is still moving or another root cause was missed.
Why this works
Crack repair lasts when the wall behind the finish is sound, dry and stable. Opening and cleaning the crack gives filler a solid surface to bond to, reinforcement helps where slight movement is expected, and building up filler in layers limits shrinkage. If movement or moisture is still present, cosmetic materials alone cannot hold the repair.
Common mistakes to avoid
- Filling a crack without checking for damp, leaks or movement first
- Painting over a crack instead of opening and cleaning it
- Using a brittle filler on a joint or crack that moves slightly
- Applying filler too thickly in one go so it shrinks or cracks
- Sanding old textured coatings without checking whether they may contain asbestos
- Ignoring cracks that keep returning or are accompanied by distortion elsewhere
Troubleshooting
The crack reappears after a few weeks
Reassess the cause. If it is on a plasterboard joint, remove failed material and redo the repair with tape and suitable compound. If there are signs of movement or damp, stop cosmetic repairs and get the cause investigated.
Filler falls out or will not bond
Remove loose material, open the crack slightly, clean out dust thoroughly and refill on a sound, dry surface. If the plaster is hollow or detached, cut back and patch or have it replastered.
The patch shows through the paint
Sand the edges flatter, remove dust, then prime the repair before repainting. Extend the topcoat further across the wall if needed for an even sheen.
Brown marks or paint bubbling return
Treat this as a moisture issue, not just a crack. Find and fix the leak or condensation source, allow the area to dry, then seal stains with an appropriate primer before redecorating.
The wall sounds hollow around the crack
This suggests blown or detached plaster. Remove all unsound material back to firm edges and patch properly, or ask a plasterer to assess if the loose area is extensive.
Compare your options
Fine surface filler
Best for: Small stable hairline cracks in plaster or painted walls
Pros: Easy to apply and sand; good for cosmetic repairs
Cons: Not suitable for ongoing movement, deep voids or loose plaster
Joint compound with tape
Best for: Cracks on plasterboard seams or recurring straight cracks
Pros: Better at bridging slight movement; standard method for board joints
Cons: Needs more careful feathering and finishing
Patch repair after removing loose plaster
Best for: Localised blown, crumbling or detached plaster
Pros: Addresses unsound substrate rather than hiding it
Cons: More work; may need skimming for a seamless finish
Professional structural or damp assessment
Best for: Cracks with signs of movement, masonry cracking, bulging, or moisture problems
Pros: Targets the root cause and helps prevent repeat damage
Cons: Higher upfront cost and may lead to broader repair work
| Option | Best for | Pros | Cons |
|---|---|---|---|
| Fine surface filler | Small stable hairline cracks in plaster or painted walls | Easy to apply and sand; good for cosmetic repairs | Not suitable for ongoing movement, deep voids or loose plaster |
| Joint compound with tape | Cracks on plasterboard seams or recurring straight cracks | Better at bridging slight movement; standard method for board joints | Needs more careful feathering and finishing |
| Patch repair after removing loose plaster | Localised blown, crumbling or detached plaster | Addresses unsound substrate rather than hiding it | More work; may need skimming for a seamless finish |
| Professional structural or damp assessment | Cracks with signs of movement, masonry cracking, bulging, or moisture problems | Targets the root cause and helps prevent repeat damage | Higher upfront cost and may lead to broader repair work |
Alternatives
- Use a paintable flexible decorator's filler where the manufacturer states it is suitable for minor movement cracks
- Skim-coat the whole wall after local repairs if there are many fine cracks and the wall is otherwise sound
- Replace damaged plasterboard section if the board itself is broken or water-damaged
Pro tips
- Photograph the crack before repair and compare later if you are unsure whether it is still moving
- A straight crack often suggests a plasterboard joint; a random hairline crack in plaster is more often shrinkage or minor settlement
- If paint edges are raised, scrape back further than the visible crack so the finished patch does not show
- Match the repair material to the wall type and follow the product label for drying, recoating and sanding guidance
- If only one area is affected after a leak, make sure the wall has fully dried before closing it up with filler and paint
Legal & regulatory notes
Building safety and repair responsibilities vary by country, tenure and building type. In rented homes, listed buildings, flats with shared structure, or properties under warranty or insurance claim, report significant cracks and get permission or formal advice before carrying out repairs. Any work affecting structure, fire separation or regulated materials may need professional input and compliance with local building rules.
What this guide does not cover: This guide covers general diagnosis and repair of small non-structural wall cracks in interior plaster or plasterboard finishes. It does not provide a structural assessment, engineering design, insurance advice, asbestos testing procedure, or detailed instructions for major masonry repair, foundation movement, fire-rated assemblies, or concealed services behind the wall.
Cost considerations
Minor cosmetic crack repairs are usually low-cost if the wall is dry and sound, but costs rise quickly if the cause is damp, failed plaster, water damage or structural movement. Paying for diagnosis early can save money if it prevents repeated patching and repainting.
Prefer to hire a professional?
If this job is beyond a confident DIY — or the work needs to be done by a licensed professional — you can find a verified qualified local tradesperson through Trade Directory (tradedirectory.co.nz), New Zealand's directory of real, verified trade businesses. Compare local tradespeople, read their details and request quotes before you commit.
Find a qualified local tradesperson on Trade DirectoryFrequently asked questions
Can I just paint over a wall crack?+
Usually no. Paint rarely bridges a real crack for long. Proper preparation, and often filler or tape, is needed for a durable repair.
How do I know if a crack is structural?+
Warning signs include cracks that are widening, diagonal cracks from doors or windows, step-like cracks in masonry, bulging, repeated reappearance, or movement symptoms such as sticking doors and windows. Those need professional assessment.
Why does the crack keep coming back?+
The common reasons are ongoing movement, damp, poor preparation, loose plaster, or using the wrong repair material. Repeating the same cosmetic fix without addressing the cause usually will not last.
Should I use filler or joint tape?+
For a small stable plaster crack, filler may be enough. For plasterboard seams or cracks that have previously reopened, a taped joint repair is usually the better option.
Can damp cause wall cracks?+
Yes. Moisture can weaken plaster, cause paint to blister, loosen finishes and lead to cracking or patch failure. Fix the source of the moisture before repairing the surface.
Sources & references
Guidance on this page is traced to documented sources. Last checked 24 September 2026.
- HSE - Asbestos · government
Supports the safety warning not to sand or disturb suspect older wall or textured coating materials without considering asbestos risk.
The repair principles are stable, but product instructions, safety guidance on hazardous materials, and local building requirements can change.