Blown Plaster: What Causes It and How to Fix It

What Is Blown Plaster?

Blown plaster, also known as plaster debonding or hollow plaster, describes the condition where plasterwork separates from its backing surface. You’ll recognise it immediately when you tap the wall — instead of a solid sound, you’ll hear a distinctive hollow, drum-like noise.

The plaster itself may appear perfectly sound visually, but the loss of adhesion means it’s only a matter of time before it bulges, cracks, or falls away completely. In severe cases, you can press the surface and feel it flexing beneath your hand.

This isn’t just an aesthetic problem. Blown plaster compromises the structural integrity of your wall finish and creates ideal conditions for further deterioration. Left untreated, small affected areas inevitably expand as moisture and movement exploit the void between plaster and substrate.

Common Causes of Blown Plaster

Understanding what causes plaster to blow is essential for both preventing the problem and ensuring repairs last. After three decades in the trade, I’ve seen the same underlying issues repeatedly:

1. Moisture and Damp Penetration

This is by far the most common cause of blown plaster. When moisture enters the wall cavity — whether from rising damp, penetrating damp, or condensation — it attacks the bond between plaster and substrate.

• Rising damp: Ground moisture travelling up through masonry via capillary action, typically affecting walls within 1.2m of floor level
• Penetrating damp: External water ingress through damaged pointing, cracked render, faulty guttering, or roof leaks
• Condensation: Prolonged surface moisture in poorly ventilated rooms, particularly bathrooms and kitchens
• Plumbing leaks: Hidden pipe failures within walls or beneath floors

Hygroscopic salts deposited by evaporating moisture also contribute to debonding. These salts crystallise behind the plaster surface, physically pushing it away from the substrate — a process visible as white, powdery efflorescence when the plaster fails.

2. Poor Surface Preparation

Inadequate preparation accounts for countless plaster failures. The substrate must be clean, sound, and properly treated before plastering commences.

• Dusty surfaces: Loose particles prevent proper adhesion (should be brushed down and treated with PVA or bonding agent)
• Contaminated substrates: Paint, wallpaper paste, grease, or oil residues create barriers to adhesion
• High suction backgrounds: Very porous surfaces like lightweight blocks or old lime plaster require suction control with bonding agents
• Smooth, dense surfaces: Engineering bricks or concrete need mechanical keying or specialist bonding products

3. Incorrect Plaster Specification or Mixing

Using the wrong plaster type or mixing it incorrectly causes bond failure and premature deterioration.

• Gypsum on lime: Applying modern gypsum plasters directly to historic lime substrates causes incompatibility issues
• Over-watered mixes: Excessive water weakens the set and reduces adhesion strength (multi-finish should be mixed to thick cream consistency)
• Re-tempered plaster: Adding water to partially set material destroys the chemical structure
• Expired materials: Aged plaster loses its setting properties and won’t achieve proper bond strength

4. Building Movement and Settlement

Physical stress from structural movement can fracture the bond between plaster and wall, particularly in newer properties still settling or older buildings with subsidence issues.

• Foundation settlement: Differential movement in new builds during the first 2-5 years
• Thermal expansion: Seasonal expansion and contraction of materials with different coefficients
• Vibration: Properties near railways, busy roads, or construction sites
• Structural alterations: Load redistribution after removing walls or installing openings

5. Inappropriate Base Materials

Some substrates simply won’t accept direct plastering without special preparation or intermediate layers.

• Painted surfaces: Emulsion and gloss create non-porous barriers (must be removed or treated)
• Glazed surfaces: Tiles need physical removal or specialist bonding systems
• Metal lath: Requires proper nailing schedules and appropriate backing coats
• Incompatible blocks: Some lightweight blocks need specific plaster grades or drylining instead

How to Identify Blown Plaster

Early detection prevents extensive damage and reduces repair costs. Here’s how to diagnose blown plaster systematically:

Visual Inspection

• Surface bulging: Raised areas or bubbles in the plaster finish indicating separation
• Crack patterns: Map-pattern cracking often accompanies debonding, particularly around affected areas
• Discolouration: Damp patches, brown/yellow staining, or white salt deposits (efflorescence)
• Loose edges: Plaster lifting away at skirting boards, corners, or where it meets sound areas

Tap Testing

The most reliable diagnostic method. Use your knuckles to tap across the wall surface systematically. Sound plaster produces a solid, dull thud. Blown plaster creates a hollow, resonant sound — like tapping a drum.

Work in a grid pattern across the entire wall to map affected areas. Mark the perimeter of hollow sections with a pencil to establish the full extent before starting repairs.

Press Testing

Apply firm pressure with your palm against suspected areas. Blown plaster will flex noticeably beneath your hand, sometimes even cracking under moderate pressure. Sound plaster remains completely rigid.

Step-by-Step Guide to Repairing Blown Plaster

Proper repair requires addressing both the symptom (loose plaster) and the cause (usually moisture). Rushing this process guarantees failure within months.

Step 1: Identify and Resolve the Underlying Cause

Never replaster until you’ve eliminated the root cause. If moisture is present, you must treat the damp problem first and allow the wall to dry completely.

• Commission a CSRT-qualified damp surveyor for accurate diagnosis
• Install or repair damp-proof courses where rising damp is confirmed
• Fix external defects: repoint masonry, repair render, clear gutters, fix roof leaks
• Improve ventilation in condensation-prone rooms (extract fans, air bricks)
• Allow walls to dry naturally (typically 4-6 weeks minimum for damp-affected areas)

Use a moisture meter to verify walls have returned to acceptable levels (below 20% for masonry, below 15% for plasterboard) before proceeding.

Step 2: Remove All Defective Plaster

Remove all blown plaster plus a 100-150mm margin around it. Attempting to save compromised edges inevitably leads to further failure.

• Score the perimeter with a Stanley knife to create a clean edge
• Use a bolster chisel and club hammer to remove material systematically
• Work from the centre outward to prevent damaging sound plaster
• Remove right back to the substrate — any remaining loose material will compromise repairs
• Dust down thoroughly with a brush and vacuum debris

For extensive blown areas (whole walls), complete removal may prove more economical than patch repairs. Beyond 40% wall coverage, full replastering typically makes more financial sense.

Step 3: Prepare the Substrate

Preparation determines bond strength more than any other factor. Cut corners here and you’re wasting your time.

For damp-affected walls, apply a specialist salt inhibitor and replastering system. Products like Renderproof Damp Resisting Plaster or Limelite Renovating Plaster contain water-repellent additives and resist salt contamination. Standard gypsum plasters will fail on salt-laden substrates.

Step 4: Apply Bonding Coat (If Required)

Deep patches or damaged substrates need an undercoat before final skimming. Thistle Bonding Coat remains the industry standard for most applications.

• Mix to a thick, creamy consistency (approximately 0.6 litres water per 25kg bag)
• Apply in 10-12mm layers maximum (deeper areas require multiple coats)
• Key the surface with a devil float or scratching tool whilst still wet
• Allow 2-3 hours set time before applying finish coat (check manufacturer guidance)

For very shallow repairs (under 5mm depth), you can skip the bonding coat and apply finish plaster directly to properly prepared substrates.

Step 5: Apply Finish Coat

The final skim brings everything flush with surrounding plaster. Thistle Multi-Finish suits most repair situations, providing a 2-3mm finishing layer.

• Mix to thick cream consistency (approximately 0.57 litres water per 25kg bag)
• Apply in two thin layers rather than one thick coat
• First pass fills and levels; second pass achieves final finish
• Trowel up when surface becomes firm (typically 20-40 minutes after application)
• Polish with light water spray and steel trowel for smooth finish

Feather edges carefully to blend seamlessly with existing plaster. Poor blending creates visible repair edges that show through any paint finish.

Step 6: Drying and Decoration

Patience during drying prevents decoration failures. Gypsum plaster needs 3-4 weeks to dry completely in normal conditions, longer in cold or humid weather.

The plaster will change from dark brown/pink when wet to pale pink when superficially dry, then pure white when fully cured. Only decorate once completely white throughout.

Apply a proper mist coat before final paint: 80% water to 20% emulsion, rolled on evenly. This seals the surface without creating impermeable layers that trap residual moisture.

Professional Repair vs DIY: What to Consider

Small repairs suit competent DIYers, but extensive blown plaster demands professional intervention. Here’s how to decide:

When to DIY

• Small affected areas (under 0.5m²)
• Accessible locations at normal working height
• Non-structural issues with clear, simple causes
• You have basic plastering experience and proper tools
• The underlying damp issue is definitively resolved

When to Call a Professional

• Large affected areas (over 1m² or multiple patches)
• Ceiling repairs (safety risk from falling material)
• Uncertainty about the cause (particularly damp-related)
• Historic buildings requiring lime plastering techniques
• Pattern cracking suggesting structural movement
• Commercial or rental properties requiring proper documentation

Cost to Repair Blown Plaster in 2026

Professional repair costs vary significantly based on extent, access, and underlying causes. Here’s what to expect across Kent and Southeast England:

These prices assume straightforward repairs with resolved damp issues. Additional costs apply for:

• Damp-proof systems: £500-£2,000+ depending on extent
• Structural repairs: Variable, requires surveyor and engineer input
• Specialist plasters: Lime or renovating plasters cost 50-100% more than standard gypsum
• Access equipment: Scaffolding or tower hire adds £150-£400+
• Decoration: Mist coating and finishing adds £150-£300 per room

For comprehensive pricing information on various plastering projects, see our complete cost guide.

Preventing Blown Plaster

Prevention vastly outweighs cure. Follow these practices to minimise debonding risk:

During New Plastering

• Use appropriate plaster specifications for each substrate type
• Follow manufacturer mixing ratios precisely (never estimate)
• Apply adequate bonding agents where required
• Don’t plaster in freezing conditions (below 2°C) or excessive heat (above 35°C)
• Allow proper drying between coats (never force-dry with heaters)
• Avoid plastering onto damp substrates (moisture content should be below 15%)

Long-Term Maintenance

• Maintain gutters and downpipes to prevent water penetration
• Repoint masonry before erosion allows moisture ingress
• Install adequate ventilation in bathrooms and kitchens
• Address condensation issues promptly (often requires multiple interventions)
• Check for plumbing leaks regularly, particularly in concealed pipe runs
• Monitor known damp areas with periodic checks during wet weather

When Renovating

• Commission proper surveys before plastering walls in older properties
• Install or upgrade damp-proof courses where necessary
• Use breathable lime-based systems in pre-1919 solid-wall buildings
• Consider tanking systems in below-ground rooms (but ensure adequate ventilation)
• Never trap moisture behind impermeable layers

Common Mistakes When Repairing Blown Plaster

I’ve been called to rectify countless failed repairs over the years. Avoid these frequent errors:

1. Plastering Over Damp

The number one mistake. Moisture will destroy any repair, regardless of technique. Never plaster until walls are provably dry and the source eliminated.

2. Insufficient Removal

Leaving compromised edges or partial debonding guarantees progressive failure. Remove everything that sounds hollow plus a safety margin.

3. Wrong Plaster Type

Using standard multi-finish on salt-contaminated or perpetually damp walls fails within months. These situations require specialist renovating plasters formulated for challenging conditions.

4. Inadequate Bonding

Skipping bonding agents on low-suction surfaces or painted substrates causes immediate debonding. When in doubt, use a bonding coat — it’s insurance worth paying for.

5. Rushing Decoration

Painting onto incompletely dried plaster traps moisture, damages paint film, and can cause further plaster failure. Wait the full 3-4 weeks regardless of deadlines.

6. Patching Too Small

Tiny patch repairs rarely blend successfully. If you’re repairing multiple small areas on one wall, complete wall replastering often looks better and costs little more.

Alternative Solutions to Traditional Replastering

When blown plaster affects large areas or substrates remain problematic, consider these alternatives:

Dry Lining

Fixing plasterboard over the existing surface avoids removing compromised plaster entirely. Particularly suitable when:

• Substrate condition is poor but structurally sound
• Adding insulation simultaneously (insulated plasterboard saves energy and space)
• Working with uneven masonry that would require extensive dubbing out
• Time constraints prevent proper drying periods

Methods include dot-and-dab adhesive on reasonably flat surfaces, or battening for very uneven walls. The finish requires only jointing compound rather than full skimming, though most professionals skim the entire surface for superior results.

Skim-Over Systems

Purpose-designed systems like British Gypsum’s Thistle Bond-It allow direct skimming over difficult surfaces including painted walls, providing the existing plaster remains firmly attached.

These products work excellently for cosmetic refreshes where old plaster is sound but painted, but won’t solve underlying bond or damp issues.

Lime Plastering

For period properties, reverting to traditional lime plaster systems often proves the only permanent solution. Lime’s breathability prevents moisture accumulation that causes gypsum failure in solid-wall buildings.

Lime plastering requires different techniques and longer working times, but provides lasting repairs in challenging damp-prone situations where modern materials repeatedly fail.

When Blown Plaster Indicates Serious Problems

Occasionally, blown plaster signals more significant structural issues requiring immediate professional assessment:

Subsidence or Settlement

Progressive cracking with diagonal patterns, particularly wider at the top, accompanied by blown plaster may indicate foundation movement. This requires a structural engineer’s assessment, not just cosmetic repair.

Concealed Structural Damage

Hidden timber decay, corroded wall ties, or masonry deterioration sometimes manifests as plaster debonding. If tapping reveals extensive hollow areas without obvious damp, investigate further before replastering.

Building Defects

Repeated plaster failure in the same location despite apparently successful repairs often indicates building envelope failures: missing damp courses, failed cavity trays, or chronic condensation from inadequate ventilation.

In these situations, consult specialists for proper diagnosis rather than repeatedly replastering. The Royal Institution of Chartered Surveyors maintains registers of qualified building surveyors who can provide independent assessments.

Frequently Asked Questions

Can I Paint Over Blown Plaster?

No. Painting over blown plaster is purely cosmetic and temporary. The debonding continues beneath the paint, eventually causing visible bulging, cracking, and detachment. The affected plaster must be removed and the wall properly replastered for a lasting repair.

Painting may temporarily disguise minor debonding, but it accelerates deterioration by trapping moisture and preventing natural drying. Address the problem properly rather than masking symptoms.

How Long Does Repaired Blown Plaster Last?

Properly executed repairs using appropriate materials should last 50+ years — effectively the remaining life of the building. However, this assumes the underlying cause (particularly damp) has been permanently resolved.

If moisture issues persist or weren’t properly addressed, repairs may fail within 6-18 months. This is why identifying and treating root causes before replastering is absolutely critical.

Is Blown Plaster Covered by Home Insurance?

Generally no. Most home insurance policies exclude wear and tear, gradual deterioration, and damage from damp or condensation. Blown plaster typically falls into these categories.

However, if the plaster damage resulted from a sudden, insured event (like a burst pipe or storm damage), the repair costs might be covered. Check your specific policy wording and speak with your insurer about the cause of the damage.

Can Blown Plaster Cause Health Problems?

Indirectly, yes. The damp conditions that cause blown plaster also promote mould growth, which can trigger respiratory problems, allergies, and asthma, particularly in vulnerable individuals. The UK’s government guidance on damp and mould provides comprehensive information on associated health risks.

Additionally, deteriorating plaster releases dust and particulates. Large sections detaching from ceilings pose direct injury risks. Address blown plaster promptly, especially in homes with children, elderly residents, or those with existing health conditions.

Does Blown Plaster Affect House Value?

Yes, significantly. Visible plaster damage raises immediate red flags for buyers and surveyors, suggesting potential damp problems or poor maintenance. Even if the actual issue is minor, the cosmetic impact and uncertainty about underlying causes can reduce offers by £5,000-£15,000 or more.

Mortgage surveyors often require satisfactory damp reports and completed repairs before approving lending on properties with blown plaster. If you’re planning to sell, repairing plaster issues beforehand typically returns substantially more than the repair cost in achieved sale price.

What’s the Difference Between Blown Plaster and Cracked Plaster?

While both are defects, they’re fundamentally different problems:

Blown plaster involves loss of adhesion between the plaster and its substrate. The plaster coating separates but may remain visually intact until advanced stages. Diagnosed primarily by hollow sound when tapped.

Cracked plaster maintains adhesion to the substrate but develops fissures through the plaster itself due to movement, shrinkage, or stress. The plaster remains firmly attached. More information about cracking causes and repairs is available in our comprehensive guide.

However, the two often occur together — blown areas frequently develop cracks as the unsupported plaster flexes, and extensive cracking can eventually lead to debonding at crack edges.

Final Thoughts

Blown plaster is symptomatic rather than causative — it’s the visible result of underlying problems, most commonly moisture-related. Successful repair requires patience, proper diagnosis, and thorough preparation. Rush the process or skip essential steps, and you’ll be repeating the work within months.

For extensive damage, undiagnosed damp issues, or if you’re uncertain about any aspect of the repair, professional assessment and remediation provides peace of mind and guaranteed results. The relatively modest cost of proper repairs far outweighs the expense, disruption, and potential health implications of repeated failures.

At Kent Plasterers, we’ve dealt with every conceivable plaster problem across thousands of properties. Whether you need advice on a small patch repair or complete room replastering, we’re here to help. Contact us for an honest assessment and detailed quotation — no job too small, no problem too challenging.