How to Repair Cracked Plaster Walls: Step-by-Step Guide

Understanding Why Plaster Cracks Matter

Cracked plaster walls are one of the most common issues we encounter as professional plasterers in Kent, and they’re rarely just a cosmetic problem. Whether you’re dealing with fine hairline cracks around door frames or worrying splits that seem to be getting wider, understanding what’s causing the damage is essential before you reach for any filler.

The good news? Most plaster cracks are perfectly repairable, and many smaller ones are well within the capabilities of a competent DIY enthusiast. The challenge lies in distinguishing between harmless surface crazing and cracks that signal something more serious like subsidence or structural movement.

This comprehensive guide will walk you through everything you need to know about repairing cracked plaster walls, from diagnosis through to professional finishing techniques. We’ll cover the tools you’ll need, the materials that actually work, and the step-by-step processes that deliver lasting results.

Types of Plaster Cracks and What They Mean

Not all cracks are created equal. Before you start any repair work, you need to identify what type of crack you’re dealing with and what’s causing it. Here’s what different crack patterns typically indicate:

When to DIY vs When to Call a Professional

Knowing your limits is crucial when it comes to plaster crack repair. Here’s an honest assessment of what you can tackle yourself and when you need to bring in the professionals:

Safe for DIY Repair:

• Hairline cracks under 2mm caused by natural settlement or minor thermal movement
• Cosmetic surface cracks in plasterwork that’s otherwise sound and stable
• Small areas of damaged plaster (under 1 square metre) where the backing is solid
• Cracks around door frames and windows that haven’t changed in 6+ months
• Map crazing or spider web patterns that are purely surface level

Requires Professional Assessment:

• Any crack over 5mm wide or showing signs of active movement
• Stepped cracks following mortar joints in the brickwork beneath
• Cracks accompanied by damp patches, which may indicate water ingress issues (see our damp proofing guide for more information)
• Bulging or bowing walls where plaster has separated from the substrate
• Multiple new cracks appearing over a short period (3-6 months)
• Cracks in newly built properties under warranty — document and report to your builder

If you’re unsure about the severity of cracking, it’s worth monitoring it for 3-6 months using tell-tales — small strips of glass or plaster fixed across the crack with reference marks and dates. If the tell-tale breaks, your crack is still moving and needs professional investigation before repair.

Essential Tools and Materials for Crack Repair

Having the right tools and materials makes all the difference between a repair that lasts and one that fails within months. Here’s what you’ll need based on the type of repair you’re undertaking:

For Hairline Crack Repair (Under 2mm):

• Flexible crack filler — Polyfilla Fine Surface Filler or Toupret Interior Filler (£6-£10 per tub)
• Filling knife — 75mm and 150mm widths (£8-£15 from Screwfix)
• Fine sandpaper — 120 grit and 180 grit (£3-£5)
• Paintbrush for dusting and applying mist coat
• Decorator’s sponge for smoothing and blending
• Mist coat paint — white emulsion diluted 70/30 with water

For Larger Cracks (2-5mm):

• Crack rake or scraper to widen and clean the crack (£5-£8)
• Scrim tape or fibreglass mesh (£4-£7 per roll from Wickes)
• Bonding plaster or filler — Gyproc EasiFill or similar (£8-£15 per bag)
• Filling knives — multiple sizes from 75mm to 200mm
• PVA solution for sealing (diluted 3:1 with water)
• Finishing plaster — Thistle Multi-Finish for final skim (£9-£12 per 25kg bag)
• Plasterer’s hawk and trowel for final finishing (£15-£35)

For Major Replastering Work:

• Everything from the above lists, plus:
• Float and devil float for keying backing coats (£12-£20)
• Mixing paddle and drill for consistent plaster mixing (£15-£40)
• Bucket and water supply — clean water is essential
• Dust sheets and masking tape for protection
• Spirit level and straight edge to check flatness

Step-by-Step: Repairing Hairline Cracks

Hairline cracks are the most common type we see, and fortunately they’re also the easiest to repair effectively. Here’s the professional method that delivers invisible, long-lasting results:

Step 1: Clean and Prepare the Crack

Use a dry paintbrush to remove all loose dust and debris from the crack. Don’t skip this step — any contamination will prevent the filler from bonding properly. For particularly dusty walls, lightly dampen a cloth and wipe the area, then allow to dry completely.

Check the plaster either side of the crack by pressing firmly with your thumb. If it feels spongy or moves, you’ve got a bigger problem than a simple surface crack and should read the section on larger repairs below.

Step 2: Apply Flexible Filler

Using a 75mm filling knife, work a small amount of flexible filler into the crack, pushing it firmly into the gap. The key word here is “flexible” — standard rigid fillers will crack again as soon as any minor movement occurs. We recommend:

• Polyfilla Fine Surface Filler — excellent for fine cracks, minimal shrinkage
• Toupret Redlite — slightly flexible, good for areas prone to movement
• Everbuild One Strike — faster drying but less flexible

Apply the filler in one smooth stroke along the length of the crack, then immediately cross-scrape at 90 degrees to press it deep into the gap. Overfill slightly — the filler will shrink as it dries.

Step 3: Smooth and Feather

Within 30 seconds of application, take a clean, slightly damp filling knife and smooth the filler, feathering the edges outward 50-75mm either side of the crack. This creates an invisible transition between the repair and existing plaster.

The damper your knife, the smoother the finish, but too much water weakens the filler. Aim for a knife that’s just damp to the touch.

Step 4: Allow to Dry

Most quality fillers dry in 1-2 hours for hairline repairs, but check the manufacturer’s guidance. Forcing the drying process with heaters can cause cracking. Ensure good ventilation but avoid draughts directly on the repair.

Step 5: Sand Smooth

Once completely dry (the filler will turn from pink or grey to white), sand the repair using 120-grit sandpaper wrapped around a sanding block. Sand in circular motions, working outward from the centre of the repair.

Follow with 180-grit for a perfectly smooth finish. Remove all dust with a dry brush or vacuum before painting.

Step 6: Seal and Paint

Apply a mist coat (emulsion diluted 70% paint to 30% water) to the repair area first. This prevents the filler from sucking moisture out of your top coat and causing patchy finishes. Allow 2-4 hours drying time.

Follow with two coats of your chosen finish paint, feathering well beyond the repair area. For guidance on achieving professional paint finishes, see our complete painting and decorating guide.

Repairing Larger Cracks (2-5mm)

Cracks between 2-5mm wide require a more robust repair approach using proper filler or plaster rather than surface filler. Here’s the professional method:

Step 1: Rake Out the Crack

Use a crack rake or the corner of a paint scraper to widen the crack slightly, creating a shallow V-shaped profile roughly 5-8mm wide and 10-15mm deep. This gives the filler something to key into.

Remove all loose material, dust, and debris. Use a vacuum nozzle to ensure the crack is completely clean — any contamination prevents proper bonding.

Step 2: Apply PVA Solution

Mix PVA with water at a ratio of 1 part PVA to 3 parts water and brush this solution into the crack using a small paintbrush. This seals porous surfaces and provides a bonding bridge for your filler.

Allow the PVA to become tacky (10-20 minutes depending on temperature and humidity) but not fully dry. You want it slightly sticky to the touch.

Step 3: Fill in Layers

For cracks over 5mm deep, build up the repair in multiple layers rather than attempting to fill in one go. Each layer should be no more than 10mm thick to prevent shrinkage cracking.

Use Gyproc EasiFill or similar deep-gap filler for the first layer(s). Mix to a thick, porridge-like consistency and press firmly into the crack using a filling knife, ensuring no air pockets remain.

Allow each layer to dry completely (2-4 hours depending on depth and humidity) before applying the next. The surface should feel dry and firm to the touch.

Step 4: Reinforce with Scrim Tape

For cracks that have shown signs of movement or are in areas prone to stress (corners, door frames), reinforce the repair with self-adhesive fibreglass scrim tape.

Apply the tape along the full length of the crack, centred over the filled gap. Press firmly to ensure good adhesion. The tape should be embedded in your final filler or plaster layer.

Step 5: Apply Final Finish

Mix a small amount of finishing plaster (Thistle Multi-Finish) to a smooth, creamy consistency — think thick double cream. Apply a thin layer (2-3mm) over the crack and scrim tape using a plasterer’s trowel.

Work the plaster outward 150-200mm either side of the crack, feathering the edges to create an imperceptible transition. As the plaster begins to set (30-45 minutes), “trowel up” — lightly spray with water and polish with your trowel to achieve a smooth finish.

Full drying time for finishing plaster is typically 7-14 days depending on temperature and ventilation. For more details on plaster drying times, see our guide on how long plaster takes to dry.

Dealing with Structural and Movement Cracks

Cracks caused by structural movement require a different approach entirely. Simply filling these cracks is pointless — they’ll reappear within weeks or months if the underlying cause isn’t addressed.

Identifying Active Movement

Before attempting any repair, you need to determine whether the crack is still moving. The professional method uses tell-tales:

• Clean the crack thoroughly and allow to dry completely
• Mix a small amount of plaster or filler and apply a 50mm strip across the crack
• Score reference marks with dates on either side of the repair strip
• Photograph the repair with measurements and dates clearly visible
• Monitor for 3-6 months — if the tell-tale cracks, the underlying issue is still active

For seriously concerning cracks, professional structural engineers use digital crack monitors that provide precise movement data over time. This is essential if you’re considering building insurance claims or need to establish whether subsidence is active.

Common Causes of Structural Cracking

Repair Strategy for Movement Cracks

Once you’ve confirmed the crack has stabilised (no movement for 6+ months), you can proceed with a reinforced repair:

1. Rake out to sound material — remove all loose plaster to a depth of at least 15-20mm
2. Install mechanical anchors if the crack suggests structural separation (specialist metalwork or resin anchors)
3. Apply bonding coat — Thistle Bonding Coat or similar, keyed for second coat adhesion
4. Embed wide scrim mesh — 300mm wide fibreglass mesh centred over the crack line
5. Apply finishing coat — Thistle Multi-Finish, minimum 3mm thick, trowelled smooth
6. Consider flexible decorating systems — products like Armstead Trade Flexible Crack Cover provide additional movement accommodation

Even with the best repairs, cracks caused by structural movement may eventually reappear. This isn’t a failure of the repair technique — it’s a sign that the underlying structural issue remains. In these cases, addressing the cause (subsidence remediation, drainage improvements, etc.) is the only permanent solution.

Understanding Why Plaster Cracks in the First Place

Prevention is always better than cure, and understanding why plaster cracks helps you avoid future problems. Our detailed guide on why plaster cracks covers this in depth, but here are the most common causes:

Poor Application Technique

• Incorrect suction control — applying plaster to surfaces that are too dry or not sealed properly
• Wrong mix consistency — plaster that’s too wet or too dry won’t perform correctly
• Rushing the process — applying second coats before first coats have set sufficiently
• Over-trowelling — excessive working brings moisture to the surface causing crazing

Environmental Factors

• Rapid drying — heating turned up too high, direct sunlight, excessive draughts
• Temperature extremes — plastering in conditions below 5°C or above 25°C
• Inadequate ventilation — trapped moisture can cause shrinkage cracking as it escapes

Substrate Issues

• Movement in the backing — poorly fixed plasterboard, weak blockwork, structural movement
• Different materials meeting — timber studs meeting masonry, different expansion rates
• Contamination — grease, paint, or dust preventing proper adhesion

Cost Breakdown: DIY vs Professional Repair

Understanding the realistic costs helps you make informed decisions about whether to tackle repairs yourself or bring in professionals. Here’s what you can expect to pay in 2026:

These costs are based on 2026 pricing in the Kent region. London and the South East typically add 15-25% to labour costs, whilst Northern regions may be 10-15% lower. For comprehensive pricing across different plastering jobs, see our complete plastering costs guide.

Common Mistakes That Lead to Failed Repairs

We’re often called out to repair previous repair attempts that have failed. Here are the most common mistakes homeowners make:

Using the Wrong Filler

Not all fillers are created equal. Standard powder fillers like Polyfilla Original are too rigid for cracks subject to any movement. They’ll crack again within weeks. Always use flexible fillers for crack repairs — they’re designed to accommodate the minor movement that all buildings experience.

Failing to Prepare Properly

Slapping filler over dusty, contaminated surfaces is the number one cause of repair failure. The filler simply won’t bond, and you’ll see it pop out or crack along the edges within months. Proper preparation — cleaning, raking out, PVA sealing — takes time but is non-negotiable for lasting repairs.

Ignoring the Cause

Repairing cracks without understanding what caused them is futile. If there’s ongoing movement, the crack will reappear. If there’s a moisture problem causing plaster to break down, repairs will fail quickly. Always diagnose before you repair.

Not Building Up Layers

Attempting to fill deep cracks in one application leads to shrinkage cracking as the filler dries. Deep gaps should always be filled in multiple layers, allowing each to dry completely before applying the next. This applies to any fill over 10mm deep.

Forgetting About Painting

Repaired areas absorb paint differently than surrounding plaster, leading to obvious patches even with multiple coats. Always apply a mist coat first — 70% emulsion, 30% water — to seal the repair before applying your final paint coats.

Advanced Techniques for Invisible Repairs

The difference between an amateur repair and a professional one often comes down to finishing technique. Here are the insider methods that create truly invisible repairs:

Feathering and Blending

The repair area should never have hard edges. Professional plasterers feather the filler or plaster outward 150-300mm beyond the crack line, creating a gradual transition that’s impossible to spot once painted.

Use progressively lighter pressure as you move away from the crack, and keep your trowel or filling knife at a shallow angle (15-20 degrees) for the final passes.

Polishing the Finish

Once your finishing plaster reaches the early set stage (30-45 minutes after application), lightly spray it with clean water using a plant mister. Then polish with a clean, stainless steel trowel using firm, overlapping circular motions.

This technique, called trowelling up, brings a fine layer of plaster cream to the surface and creates a glass-smooth finish that requires minimal sanding. It’s the hallmark of professional plastering work.

Texture Matching

If your existing walls have texture (stipple, artex, slight orange peel), you’ll need to replicate this on your repair. For stipple patterns:

• Apply your final coat slightly thicker (4-5mm instead of 3mm)
• Allow to reach initial set (firming but still workable)
• Use a damp sponge, stippling brush, or roller to create texture
• Match the pattern intensity to surrounding areas
• Allow full drying before painting

For more pronounced textured finishes, consider using textured paint products rather than attempting to recreate complex patterns in plaster.

Preventing Future Cracks

Once you’ve successfully repaired your cracks, you’ll want to prevent new ones forming. Here’s how professionals protect their work:

Address Underlying Issues

• Fix moisture problems — blocked gutters, leaking pipes, rising damp all contribute to plaster deterioration
• Improve ventilation — excessive humidity weakens plaster over time
• Monitor structural movement — regular checks can catch developing issues early
• Maintain heating — sudden temperature changes cause expansion and contraction

Use Movement Joints

For areas prone to stress (where different materials meet, long continuous walls, ceiling perimeters), consider installing movement joints. These are deliberate gaps filled with flexible sealant that accommodate movement without cracking the plaster.

Modern construction standards require movement joints at maximum 15-metre intervals on continuous plastered surfaces, according to British Gypsum technical guidance.

Choose the Right Plaster

Different plasters have different properties. For areas prone to movement or moisture, consider:

• Lime plaster — more flexible, breathable, better for old buildings
• Multi-finish gypsum plaster — good all-rounder for modern buildings
• Sand/cement render — more durable for high-impact areas
• Venetian plaster — naturally crack-resistant due to lime content and polished finish

For more information on choosing the right plaster for your project, see our guide on types of plaster explained.

Frequently Asked Questions

How can I tell if a crack is serious or just cosmetic?

The width, pattern, and movement are your main indicators. Hairline cracks under 2mm are typically cosmetic. Cracks over 5mm, especially diagonal or stepped patterns, suggest structural issues and need professional assessment. Use the tell-tale method (monitoring with dated reference marks) for 3-6 months to determine if the crack is still active. Any crack accompanied by doors sticking, windows not closing, or visible wall bowing should be assessed by a structural engineer immediately.

Can I just paint over small cracks instead of filling them?

You can, but it’s rarely a good solution. Paint will initially hide fine hairline cracks, but they’ll quickly become visible again as the paint settles. Flexible crack cover paint products (like Polycell Crack-Free Ceilings or Armstead Trade Flexible Crack Cover) can bridge very fine cracks under 0.5mm, but anything larger needs proper filling first. Painting over unfilled cracks also means the crack continues to develop beneath the paint, potentially causing larger problems later.

How long should I wait before painting repaired cracks?

For filler repairs, wait until completely dry — typically 2-4 hours for fine surface fillers, 4-6 hours for deep-gap fillers. For plaster repairs, you must wait longer: bonding coat needs 24 hours before applying finishing coat, and finishing plaster requires 7-14 days to dry fully before painting. The plaster should be uniformly light pink/white with no dark patches. Painting too soon traps moisture, leading to poor adhesion and potential mould growth. Check out our complete guide on how long to leave plaster before painting for detailed drying times.

Why do cracks keep coming back in the same place?

Recurring cracks indicate an unresolved underlying issue. The most common causes are: active structural movement that hasn’t been addressed, moisture problems causing plaster degradation, thermal expansion at junctions between different materials, or using inappropriate repair materials (rigid filler in a movement joint, for example). If cracks return within 6-12 months of repair, don’t just fill them again — investigate the cause with a structural survey or damp specialist. Sometimes the solution is as simple as installing proper movement joints or using flexible sealants rather than rigid plaster.

Should I use mesh tape for all crack repairs?

Not necessarily. Mesh tape (scrim tape) is essential for repairs over 3mm wide, cracks that have shown movement, or high-stress areas like corners and where walls meet ceilings. For simple hairline cracks in stable plaster, flexible filler alone is sufficient — adding tape is overkill and creates more work. However, if a crack has recurred despite previous repairs, adding mesh tape provides the reinforcement needed to prevent it happening again. Always use self-adhesive fibreglass mesh rather than paper tape for crack repairs — it’s stronger and won’t bubble or separate over time.

Can I repair plaster cracks myself if I’ve never done it before?

Yes, absolutely — hairline crack repairs are well within DIY capabilities and require minimal special skills or tools. The process is straightforward: clean, fill, smooth, sand, paint. However, larger repairs requiring actual plastering skills (floating, trowelling, skim coats) have a steep learning curve. Poor plastering technique is immediately obvious and difficult to correct, so if you’re attempting proper plaster repairs for the first time, practice on an inconspicuous area or a piece of plasterboard first. For extensive damage or structural cracks, the cost savings of DIY rarely justify the risk of poor results — professional plasterers have years of experience achieving smooth, invisible repairs.