Cracks in plaster walls are one of those things that look alarming and are usually harmless — but “usually” isn’t “always.” Knowing how to fix cracks in plaster walls starts with knowing which cracks matter. Here’s how to tell, and how to repair each type properly.

First: is this crack dangerous?

Most plaster cracks are cosmetic. But check for these warning signs that mean call a professional (a structural engineer, not just a plasterer):

• Cracks wider than 5mm
• Cracks that run diagonally across the wall
• Cracks that appear suddenly and are growing
• Cracks around door and window frames that are widening
• Doors or windows that stick where they didn’t before
• Cracks visible on both sides of the wall (inside and outside)
• Sloping floors alongside the cracks

If none of these apply — and for most hairline cracks, none will — you’re dealing with normal settlement, thermal movement or old plaster. Carry on.

Types of plaster cracks and their causes

• Hairline cracks (under 1mm): the most common. Caused by the building settling, temperature changes, or plaster drying out. Purely cosmetic.
• Crazing (fine network of tiny cracks): looks like a spider’s web. Usually the paint or top coat shrinking, not the plaster itself. Often fixed by repainting with flexible paint.
• Cracks along ceiling/wall joints: thermal movement between different materials. Very common, very fixable.
• Stepped cracks in old plaster: often follow the line of underlying brickwork or lath. May indicate the plaster is blowing (coming away from the wall) — tap it; a hollow sound confirms it.
• Cracks around new extensions or alterations: differential settlement between old and new. Usually stabilises within a year or two.

How to fix hairline cracks: step by step

What you’ll need

• Flexible decorators’ filler or caulk (£5–£8)
• Filling knife
• Fine sandpaper (120-grit)
• Damp sponge
• Primer/sealer and paint for touching up

The method

1. Widen the crack slightly. Counterintuitive, but run a filling knife or old screwdriver along the crack to open it into a V-shape. Filler needs something to grip — it won’t bond inside a hairline slit.
2. Clean it out. Brush away dust and loose material. A vacuum with a brush attachment is ideal.
3. Apply flexible filler. Press decorators’ caulk or flexible filler firmly into the crack with a filling knife, slightly overfilling. For cracks at wall/ceiling joints, caulk is better than rigid filler — it moves with the building.
4. Smooth it. Run a damp finger or sponge along caulk for a perfect finish; use the filling knife for rigid filler.
5. Let it dry fully. Follow the product’s drying time — usually 1–2 hours for caulk, longer for deep fills.
6. Sand lightly if you used rigid filler (caulk shouldn’t need sanding).
7. Prime and paint. Filler absorbs paint differently from plaster — spot-prime first or you’ll see the repair through the topcoat. Then paint as normal (see our professional painting guide).

Fixing larger cracks (2–5mm)

For wider cracks, filler alone may shrink and re-crack. The professional approach:

1. Open the crack into a V-shape as above, going a little deeper.
2. For deep cracks, use a two-stage fill: first a layer of filler, let it dry, then a second layer. Deep single fills shrink and crack.
3. Consider scrim tape (self-adhesive mesh) over the crack before filling — it bridges the gap and prevents the crack telegraphing through. Essential for cracks that have reappeared before.
4. Skim the repair flush, sand, prime, paint.

When plaster is blowing (coming off the wall)

If tapping the wall around the crack gives a hollow sound, the plaster has separated from the background. Small areas of blown plaster can be repaired with plaster repair adhesive (injected behind the plaster to re-bond it). Large areas — anything bigger than a dinner plate — usually need hacking off and re-plastering, which is a job for a plasterer (£150–£300 per wall for a skim).

Preventing cracks returning

• Use flexible filler at junctions between different materials (wall/ceiling, wall/skirting) — rigid filler will just crack again as the building moves.
• Don’t fill cracks in the first year of a new build or extension — the building is still settling. Wait, then fix once.
• Control moisture. Leaks and damp cause plaster to crack and blow. Fix the water source before the plaster.
• Don’t hang heavy things on cracked plaster until it’s repaired — use proper fixings for plasterboard walls to avoid creating new cracks.

Repainting after repair

Fresh filler needs sealing — paint it with a mist coat or primer first, or the repair will flash through the finish. If the whole wall needs repainting anyway, check our guide to paint drying times so you plan the job properly. And if you’re choosing a new colour for the room, our guide to paint colours for small rooms might help.

Plaster repair products compared

• Ready-mixed filler (£5–£10/tub): for small holes and hairline cracks. Convenient, no mixing, but shrinks in deep fills — apply in layers.
• Powder filler (£8–£15/bag): mix to the consistency you need. Stronger, less shrinkage, better for larger repairs. Sets fast — mix small batches.
• Flexible decorators’ caulk (£4–£8/tube): for cracks at junctions (wall/ceiling, skirting/wall) where movement happens. Paintable, permanently flexible.
• One-strike filler (£8–£12): fills deep holes in one application without shrinking. The time-saver for DIYers — genuinely works as claimed for holes up to a few cm deep.
• Plaster repair patches (£5–£10): self-adhesive mesh for larger holes in plasterboard. Quicker than cutting and patching.
• Finishing plaster (multi-finish) (£10–£15/bag): for skim repairs over larger areas. Needs skill to get flat — practice on an unseen area first.

For 90% of crack repairs, ready-mixed filler or caulk is all you need. Buy the powder version once you’re comfortable — it’s cheaper per repair and stronger.

If cracks are accompanied by other signs of movement, the RICS guide to subsidence explains when to get a professional structural assessment — most cracks never need one, but it’s worth knowing the thresholds.

The takeaway

To fix cracks in plaster walls: check they’re not structural (wider than 5mm, diagonal, growing = get professional advice), open hairline cracks into a V, fill with flexible filler, sand, prime and paint. Use scrim tape for repeat offenders. Most cracks are the building breathing — fix them properly once, and they’ll stay fixed.