fix cracked walls

Quick Answer: Most cracked plaster walls can be repaired DIY using flexible filler for hairline cracks (under 2mm) or proper filling and replastering techniques for larger structural cracks (over 5mm). Small cosmetic cracks take 1-2 hours and cost £10-£30 in materials, whilst major crack repairs requiring replastering typically cost £200-£600 professionally. The key is identifying why the crack formed — settlement, moisture, or structural movement — before attempting repairs. 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: Crack Type Appearance Likely Cause Severity Hairline cracks Under 2mm wide, surface level Natural settlement, thermal movement, plaster shrinkage Low — cosmetic only Vertical cracks 2-5mm, running floor to ceiling Wall movement, drying out of new buildings, seasonal changes Low to medium Diagonal cracks Running from corners of doors/windows Lintel failure, settlement, structural movement Medium to high Horizontal cracks Running along walls, often mid-height Structural issues, ceiling joist movement, subsidence Medium to high Crazing/map cracking Multiple fine cracks in random pattern Plaster applied too quickly, poor suction control, overworking Low — surface issue Stepped cracks Following mortar joints in brickwork Serious subsidence or structural movement High — professional assessment needed ⚠️ Warning: Any crack wider than 5mm, cracks that are visibly getting wider, or multiple cracks appearing in a short timeframe should be assessed by a structural engineer before attempting cosmetic repairs. These may indicate serious foundation or structural issues that need professional attention. According to RICS guidance, cracks over 15mm can indicate severe structural damage requiring urgent professional intervention. 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 Pro Tip: Always buy slightly more material than you think you’ll need. A 25kg bag of finishing plaster covers roughly 10-12 square metres at 3mm thickness, but having extra on hand prevents mid-job trips to the builders’ merchants and allows for inevitable wastage. Unopened bags of multi-finish can last 3-4 months if stored properly in a dry location. 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