Cracked and blown plaster is one of the most common problems in UK homes, particularly in Victorian and Edwardian properties where the original lime plaster has been patched over the decades with incompatible gypsum-based products. Understanding the difference between a cosmetic crack and genuinely failed plaster — and choosing the right repair — saves money and prevents the problem from recurring within months.
Diagnosing the Problem: Cracks vs Blown Plaster
Blown plaster has detached from the substrate behind it — the bond between the plaster and the masonry (or lath) has failed. It sounds hollow when tapped with a knuckle and may flex or move slightly under pressure. In extreme cases it bulges visibly or falls away in sheets.
Cracked plaster is still bonded — the surface has fractured but remains attached. Cracks range from hairline fractures (less than 0.2 mm wide) to structural cracks several millimetres wide.
The diagnostic process:
- Tap the wall systematically across the affected area. A dull, hollow sound indicates a void behind — the plaster is blown. A solid sound means the bond is intact.
- Check the crack width. A fingernail catches in any crack wider than about 0.5 mm. Cracks wider than 3 mm warrant investigation of the cause before repair.
- Check for pattern. Stepped cracks following mortar joints suggest differential settlement or thermal movement. Diagonal cracks from window/door corners often indicate lintel movement. Horizontal cracks near the top of walls in a terraced house can indicate lateral thrust from roof spread.
Causes of Blown and Cracked Plaster
| Cause | Typical pattern | Action before repair |
|---|---|---|
| Age / general bond failure | Widespread hollow areas, older properties | None — simply re-plaster |
| Damp penetration | Staining, soft or powdery plaster | Fix the source of damp first |
| Incompatible repairs (hard gypsum over lime) | Cracks at patch edges | Remove incompatible material |
| Thermal movement | Fine cracks around window frames | Flexible filler, not hard plaster |
| Structural movement (active) | Wide / progressive cracks | Structural engineer assessment first |
| Impact damage | Localised hollow section | Patch repair |
| Frost damage (external walls) | Delaminating layers, especially solid walls | Investigate insulation / damp |
Never repair over damp plaster. If the plaster smells musty, shows tide-marks, or tests positive with a moisture meter (above 15–18% in gypsum, above 20% in lime), the source of moisture must be resolved before any plastering work.
Repair Options
Hairline Cracks (Under 0.5 mm)
Fill with a flexible decorator’s filler (Polyfilla Fine Surface or similar), sand when dry, and paint. These cracks often reappear seasonally with thermal movement — apply fine joint tape (paper or fibreglass mesh tape embedded in filler) to bridge the crack and prevent re-cracking.
Materials cost: £3–£8 for filler and tape.
Wider Cracks (0.5–5 mm)
- Rake out the crack with a screwdriver or crack tool to create a V-profile and remove any loose material.
- Dampen the crack with clean water.
- Fill with a hard setting filler (Thistle Multi-Finish thinned to a thick paste, or a proprietary patching plaster such as EasiFill 60).
- Allow to shrink and dry, then apply a second coat.
- Sand flush, apply fine filler if needed, and redecorate.
Materials cost: £5–£15.
Blown Plaster (Small Areas, Under 0.5 m²)
Small blown areas can be repaired by injection grouting — a specialist technique where a low-viscosity lime or acrylic adhesive is injected through small drilled holes to re-bond the plaster to the substrate. This is particularly appropriate in listed buildings or rooms with ornate plasterwork where hacking off and re-plastering would destroy irreplaceable historic fabric.
DIY injection repair kit (adhesive + applicator): £35–£60.
More commonly, small blown areas in non-historic properties are hacked off and re-plastered:
- Cut around the blown area with a bolster chisel — extend the cut to sound plaster.
- Hack off the blown section back to the masonry or lath.
- Treat with a bonding agent (PVA diluted 1:3, or purpose-made bonding agent such as Unibond).
- Apply a scratch coat of Thistle BondCoat (or equivalent bonding plaster) and allow to firm up.
- Skim with 2 mm Thistle Multi-Finish and feather into the surrounding surface.
Materials cost (per m²): £8–£18.
Large Areas of Blown Plaster (Over 1 m²)
At this scale, professional plastering becomes more cost-effective than DIY repairs, and the result will be significantly better.
| Scope | Typical plasterer cost (2026) |
|---|---|
| Single wall patch (0.5–2 m²) | £150–£350 |
| Full room re-plaster (25–40 m²) | £800–£1,800 |
| Re-plaster with hacking off and prep | £1,200–£2,500 |
| Per m² (day rate basis) | £25–£50/m² |
Day rates for plasterers in 2026 run £180–£280 per day outside London, and £250–£350 per day in London. A two-coat full-room plaster job (scratch coat plus finish skim) typically takes one day for a skilled plasterer in an average-sized bedroom.
Lime vs Gypsum Plaster: An Important Decision
In pre-1920 properties, the original plaster is almost certainly lime-based. Lime plaster is breathable — it allows moisture to migrate in and out of the wall mass, preventing damp build-up. Patching lime plaster with modern gypsum plaster creates a hard, impermeable patch in a flexible, breathable substrate. This is the primary cause of recurring cracks at patch edges.
If you are patching a significant area in a pre-1920 property, consider using a NHL (natural hydraulic lime) or Jesmonite-based patching material, or ask a plasterer experienced in traditional lime work. For a straightforward modern property, gypsum is the correct choice.
When to Re-Plaster Instead of Patching
Patching is appropriate when:
- Less than 20–25% of the room’s plaster area is blown.
- The surrounding plaster is sound and well-bonded.
- The property is post-1960 and gypsum throughout.
Re-plastering is the better option when:
- More than a quarter of the wall area is blown.
- There are multiple generations of incompatible patches.
- You are about to redecorate and want a flat, uniform surface.
- Damp has caused widespread plaster failure across a wall.
A new plaster skim coat (on existing sound browning coat) costs less than full re-plastering and is often the right compromise — the plasterer removes loose areas, patches, re-skims the whole wall, and you get an even, flat surface that takes paint perfectly.
