plastering

How to Fill and Repair Cracks in Plaster Walls: A Definitive Guide

Gabriel

22 Dec 2025 — 3 min read

Cracks in plaster walls are one of the most frequent maintenance tasks in UK homes — and one of the most frequently done wrong. Using the wrong filler or skipping surface preparation produces a crack that reappears within months. Done correctly, a plaster crack repair is invisible, permanent, and surprisingly quick.

Identifying the Crack Type

Before filling, understand why the crack appeared:

Hairline cracks in fresh plaster: Perfectly normal — plaster shrinks slightly as it dries. Fill with fine surface filler and paint.
Horizontal or diagonal cracks appearing throughout a room: Often shrinkage in a newly built or renovated house — normal in the first 1–2 years. Monitor for 6 months before filling to confirm they're not still moving.
Staircase-pattern cracks following mortar joints in masonry: May indicate differential movement — potentially subsidence. If new cracks of this type appear suddenly, or if they're wider than 3mm, get a structural survey before filling.
Cracks above or below a window or door: Often lintel-related deflection. If widening, seek structural advice.
Fine network of cracks (crazing) on plaster surface: Usually a plaster compatibility issue — the old plaster and finish coat have separated. The surface layer may need to be removed rather than just filled.

Choosing the Right Filler

Ready-mixed filler (Polyfilla, Ronseal, Everbuild): Good for hairline and small cracks (under 5mm). Easy to use, sands well when dry. Shrinks very slightly — apply a touch proud of the surface and sand back. Not suitable for cracks that are still moving.
Powder filler (mixed with water): Harder set, less shrinkage, better for deeper cracks. Harder to sand than ready-mixed. Polyfilla Professional is a popular choice.
Flexible decorators' caulk: Essential for cracks at junctions between wall and ceiling, or between wall and architrave. Rigid filler in these locations always cracks again — the junction moves slightly. Caulk accommodates movement. Apply with a gun, smooth with a damp finger, and paint over when dry (unlike silicone, decorators' caulk takes paint).
Two-part epoxy filler: For large holes or structural repairs where maximum strength is needed. Very hard when cured, difficult to sand. Not appropriate for surface hairline cracks.

The Filling Process

Hairline Cracks

Widen the crack slightly with a filling knife or screwdriver — counterintuitively, this provides a better mechanical key for the filler. A 1–2mm wide, 3–5mm deep channel is ideal.
Brush out dust and debris with a stiff brush.
Dampen the crack with water (prevents the substrate drawing water from the filler before it can set properly).
Apply filler in one smooth stroke, slightly proud of the surface.
Allow to dry fully (typically 2–4 hours for ready-mixed, faster for powder filler).
Sand with 120-grit, then 180-grit for a smooth finish.

Wider Cracks (5–25mm)

Large cracks may need to be filled in layers — one application of thick filler will crack as it dries. Apply in 5–8mm depth layers, allowing each to dry before applying the next. The final layer should be approximately 1mm proud of the surface for sanding back.

For cracks over 10mm, consider using jointing tape (fibreglass mesh tape) embedded in the filler layer — this bridges the crack and prevents it reopening.

Holes (Plasterboard Damage)

For holes up to 50mm: cut a piece of plasterboard slightly larger than the hole, pass it through at an angle, and hold it in place with a piece of string tied through two holes in the patch. Apply filler around the edges to bond it in position. Once set, cut the string, fill the remaining gap in layers.

For holes over 50mm: fit a proper plasterboard repair patch, cutting back to the nearest studs if necessary and fitting a timber backing piece.

Preparation for Painting

Filled areas are absorbent and will show as flat patches through paint unless sealed. After sanding, apply a mist coat (emulsion diluted 50:50 with water) over the repair and the surrounding area to seal the surface and even out absorption. This prevents the repair showing through the topcoat as a dull or different-sheen area — a very common finishing mistake.

Preventing Cracks from Recurring

At wall/ceiling and wall/architrave junctions: always use flexible caulk, not filler.
Over plasterboard joints: use the correct jointing method (scrim tape + jointing compound in multiple coats) rather than surface filler.
In new plasterwork: allow full drying time (minimum 4–6 weeks for traditional plaster) before applying paint — painting wet plaster traps moisture and causes cracking and flaking.