plaster coming away from wall

Quick Answer: Blown plaster occurs when the plaster coating loses adhesion to the substrate beneath, creating hollow-sounding areas that move when pressed. The primary causes are moisture penetration, inadequate preparation, incorrect mixing ratios, or applying plaster to unsuitable surfaces. Repair involves removing all loose material, treating underlying issues (especially damp), applying appropriate bonding agents, and replastering with the correct specification for the substrate. Small patches (under 1m²) cost £80-150 to repair professionally, whilst entire room replastering ranges from £400-800 depending on size. 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 Pro Tip: Always check the bag date on your plaster. British Gypsum products have a typical shelf life of 4-6 months from manufacture when stored correctly. Beyond this, the setting time becomes unpredictable and bond strength diminishes significantly. 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. ⚠️ Warning: Large areas of blown plaster can detach suddenly without warning, particularly on ceilings. If you identify significant ceiling debonding, evacuate the room and call a professional immediately. A 2m² section of plaster and plasterboard can weigh 40kg+ and cause serious injury. 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