When homeowners tackle wall crack repairs themselves, most end up with decent results. Some, however, make mistakes that actually make the problem worse. Understanding what goes wrong helps you avoid these pitfalls and save yourself time, money, and frustration.
Mistake 1: Repairing Without Diagnosing
This is the most common and costly mistake.
What happens: You spot a crack, assume it’s just cosmetic, and immediately fill it without understanding what caused it. If the underlying issue is still present, the crack will simply reopen.
Why it matters: A crack caused by ongoing subsidence, damp, or structural movement will keep opening no matter how many times you fill it. You’re treating the symptom, not the disease.
The wrong approach: “I’ll just fill this crack and redecorate. Problem solved.”
The right approach: Before filling anything, ask yourself: “What caused this crack? Is it still happening?” If the answer is “I don’t know” or “it might still be happening,” get professional advice first.
How to avoid it:
- Monitor the crack for at least two to three months before repairing
- Check for associated signs (damp, sticking doors, other cracks)
- If you’re unsure about the cause, ask a professional before you spend money on repairs
Mistake 2: Not Cleaning Out the Crack Properly
A surprisingly common error that leads to failed repairs.
What happens: You apply filler directly into a crack that’s full of dust, loose plaster, and debris. The filler doesn’t bond properly because there’s no clean surface for it to grip.
Why it matters: Within weeks, the filler falls out or cracks again because it’s not properly anchored. You’ve wasted time and materials.
Signs this has happened:
- The filler falls out of the crack within a few weeks
- The repair cracks again at the same point
- You can see gaps between the filler and the crack edges
How to avoid it:
- Use a putty knife or old screwdriver to scrape out all loose material
- Use a vacuum or brush to remove dust thoroughly
- For deep cracks, use compressed air to blow out dust
- Wipe the crack with a damp cloth if needed (let it dry before filling)
- Some professionals use a primer or bonding agent in very dusty cracks to help adhesion
Mistake 3: Filling Over Damp
This is a serious error that can trap moisture and create bigger problems.
What happens: You notice a crack with a damp patch nearby. Rather than investigating, you simply fill the crack and paint over it. The moisture now has no way to escape and can spread behind the plaster, causing more damage.
Why it matters: You’re sealing in the problem. This leads to:
- Mould growth behind the plaster
- The damp spreading to wider areas
- Eventual structural damage if it affects brickwork
- Significantly more expensive repairs down the line
How to tell if there’s damp:
- The area around the crack feels damp or cold
- There’s discolouration or staining on the wall
- There’s a musty smell
- The plaster feels soft or spongy
- Paint is bubbling or peeling in the area
How to avoid it:
- Never fill a crack if there’s any sign of moisture
- Address the damp issue first
- Get professional advice on the cause (rising damp, penetrating damp, condensation)
- Only fill the crack once the moisture problem is resolved
Mistake 4: Using the Wrong Filler Material
Different cracks need different products.
What happens: You use whatever filler or caulk is available rather than choosing the right product for the job. The filler doesn’t adhere properly, shrinks excessively, or cracks again within weeks.
Common errors:
- Using rigid filler in a crack that moves seasonally (it will crack again)
- Using flexible sealant in a crack that needs structural repair (it’s cosmetic only)
- Using interior filler on an exterior wall (it won’t weather properly)
- Using silicone caulk on plaster (it doesn’t take paint well)
The right products for different situations:
| Crack Type | Best Product | Why | Application |
|---|---|---|---|
| Hairline in plaster | Flexible filler or fine surface filler | Moves with the wall, won’t crack again | Interior walls |
| Small vertical crack (under 3mm) | Plaster filler or spackling paste | Designed for plaster, sands smoothly | Interior plaster |
| Larger crack (3-5mm) | Flexible caulk or decorator’s caulk | Handles movement, paintable | Interior plaster |
| Fine cracks in render | Flexible exterior filler | Weatherproof, flexible | Exterior walls |
| Mortar joint cracks | Repointing mortar | Matches original material | Brickwork (professional job) |
How to avoid it:
- Read the product label to understand what it’s designed for
- Ask at the hardware store what’s suitable for your crack type
- Check online reviews or YouTube tutorials for your specific crack type
- When in doubt, ask a professional what they’d recommend
Mistake 5: Overfilling and Poor Finishing
Taking shortcuts on the finishing work creates poor results.
What happens: You fill the crack quickly without smoothing it off properly, then paint over the rough surface. The repair looks obvious, amateurish, and the imperfections actually make the crack more noticeable.
Why it matters: A visible repair draws attention rather than blending in. You’ve achieved the opposite of what you intended.
Common errors:
- Leaving the filler proud (sticking up above the wall surface)
- Not sanding the repair smooth before painting
- Painting without filling small air pockets that create bumps
- Using the wrong colour paint, making the repair stand out
- Not feathering the edges to blend with the surrounding wall
How to avoid it:
- Apply filler in thin layers rather than one thick layer
- Smooth the surface with your putty knife while it’s still slightly damp
- Sand thoroughly with 120 grit paper, then 240 grit for a smooth finish
- Apply paint in thin coats to match the surrounding wall texture
- Step back and view from a distance before declaring it finished
- Match your paint colour carefully (get the existing paint colour code from the wall)
Mistake 6: Ignoring Multiple Cracks
Thinking each crack is independent when they’re part of a pattern.
What happens: You fill one crack, and within weeks another appears nearby. You fill that one too, and more appear. You end up in an endless cycle of filling cracks rather than addressing the root cause.
Why it matters: Multiple cracks are usually a sign that something more serious is happening in your building. It might be subsidence, structural movement, or damp issues. Ignoring this pattern means you’re missing the actual problem.
Red flag patterns:
- New cracks appearing regularly in the same area
- Cracks appearing in multiple rooms at roughly the same time
- Cracks forming in a stair-step pattern following mortar joints
- Multiple horizontal cracks at different heights
- Cracks appearing both inside and outside the building
How to avoid it:
- When you spot a crack, look around carefully for others
- If you find multiple cracks, stop filling them and investigate
- Take photos of all the cracks and their locations
- Note when they appeared and any pattern you notice
- Contact a professional before filling anything
- It’s far cheaper to diagnose a problem once than to fill countless cracks
Mistake 7: Not Using Appropriate Tools and Safety Measures
Taking shortcuts with equipment leads to poor results and safety issues.
What happens: You use whatever tools are lying around rather than proper filling tools. The work is messy, uneven, and takes twice as long as it should.
Common errors:
- Using an old kitchen knife instead of a proper putty knife (causes gouges in the plaster)
- Sanding without a dust mask (health risk and mess everywhere)
- Not protecting furniture or flooring (dust and splatters get everywhere)
- Using ladders that aren’t stable (safety hazard for high cracks)
- Not wearing eye protection when sanding (dust in your eyes)
How to avoid it:
- Use proper filling knives (50mm and 100mm sizes are most useful)
- Invest in a dust mask if you’re sanding
- Use drop sheets or old newspapers to protect floors and furniture
- Use stable ladders or step stools for high cracks
- Wear safety glasses when sanding
- Use a vacuum with a dust collection attachment if sanding large areas
Mistake 8: Repairing Without Addressing the Cause
Similar to Mistake 1, but worth emphasising because it’s so common.
What happens: You identify that the crack is caused by something specific (like a pipe leak causing damp, or a missing gutter causing water damage) but you only fill the crack. The underlying cause continues, and the damage keeps getting worse.
Why it matters: You’re applying a plaster to a wound that needs surgery. The crack is just a symptom of a deeper problem.
Examples of underlying causes you might spot:
- Water damage from a leaking pipe or gutter
- Damp from rising moisture or condensation
- Cracks following structural stress patterns
- Deterioration from age or weathering
- Foundation movement or subsidence
How to avoid it:
- Before you fill a crack, identify and fix what caused it
- If there’s water damage, find and repair the leak first
- If there’s damp, address the moisture problem before filling
- If there’s structural movement, get professional assessment before cosmetic repairs
- Don’t put a band-aid on a problem that needs real medical attention
Mistake 9: Starting Repairs When the Weather Is Wrong
Timing matters more than you might think.
What happens: You fill a crack on a cold, damp day. The filler doesn’t dry properly, adhesion is poor, and the repair fails within weeks.
Why it matters: Most fillers and caulks need specific conditions to cure properly. Using them in the wrong conditions wastes your time and materials.
Temperature and humidity issues:
- Filling in temperatures below 10 degrees (most fillers won’t set properly)
- Filling in very humid conditions (moisture prevents curing)
- Filling on a damp wall (the filler can’t bond properly)
- Filling just before rain (water can wash out uncured filler)
How to avoid it:
- Check the manufacturer’s instructions for temperature and humidity requirements
- Choose a dry day with moderate temperature (15-25 degrees is ideal)
- Make sure the wall is completely dry before applying filler
- If possible, avoid filling in winter or during rainy seasons
- Allow extra drying time in cool or damp conditions
Mistake 10: Not Getting Professional Advice When Unsure
The most costly mistake overall.
What happens: You’re not sure if a crack is serious or cosmetic, so you guess. You spend money on materials and time on repairs only to discover later that professional help was needed all along. Now you’ve also got failed repairs to deal with.
Why it matters: A professional assessment might cost £100-200 upfront, but saves you from wasting money on inappropriate repairs and potentially making things worse.
When you should definitely get professional advice:
- If you’re unsure whether the crack is cosmetic or structural
- If you don’t know what caused the crack
- If there are multiple cracks
- If the crack is large or growing
- If there’s any damp or moisture involved
- If you’re anxious about whether to DIY or call someone
How to avoid it:
- Don’t be embarrassed to ask for help
- Get a professional assessment before you start repairs
- Use the money saved from avoiding failed DIY repairs to pay for professional advice
- Think of it as insurance against wasting time and money
A Quick Reference Guide
| Mistake | Impact | Prevention |
|---|---|---|
| Repairing without diagnosing | Cracks return repeatedly | Monitor first, understand the cause |
| Not cleaning the crack | Filler falls out quickly | Scrape and brush out all debris |
| Filling over damp | Moisture problems worsen | Always address damp first |
| Using wrong filler | Repair fails within weeks | Choose appropriate product |
| Poor finishing work | Repair looks obvious and bad | Sand and blend carefully |
| Ignoring multiple cracks | Missing a bigger problem | Look for patterns, investigate |
| Wrong tools/safety | Poor results and health risk | Use proper tools and protection |
| Not fixing the cause | Problem keeps recurring | Address underlying issues first |
| Wrong weather conditions | Filler won’t cure properly | Work in suitable conditions |
| Not getting professional help | Wasting money on wrong repairs | Get assessment before starting |