How to Fill Cracks in Plaster Walls

Cracks in plaster are common, especially in older Devon homes. Houses move, settle, and shift with the seasons. But fill them wrong and they'll be back within months.

Types of Cracks (And What They Mean)

Hairline Cracks

• Thin cracks, often at ceiling/wall junctions or around door frames
• Cause: Normal settling, paint buildup, minor movement
• Concern level: Low - cosmetic issue

Stepped Cracks

• Cracks that follow the mortar lines in a step pattern (if you have exposed brickwork)
• Cause: Structural movement
• Concern level: Medium - worth monitoring

Diagonal Cracks

• Cracks running diagonally from door/window corners
• Cause: Settlement or movement around openings
• Concern level: Low to medium - usually just cosmetic

Large or Wide Cracks

• Over 5mm wide or growing
• Cause: Could be structural movement, subsidence, or tree roots
• Concern level: High - get it checked by a structural engineer

Recurring Cracks

• Same crack keeps reappearing after filling
• Cause: The building is still moving in that spot
• Concern level: Medium - need a flexible filling approach

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What You'll Need

For hairline cracks:

• Flexible filler (Polycell or similar)
• Filling knife (2-3 inch)
• Fine sandpaper (120-180 grit)
• Primer

For larger cracks:

• One-coat plaster or deep-fill filler
• Wide filling knife or plasterer's trowel
• Stanley knife or scraper for preparation
• Jointing tape (for cracks that move)
• PVA for bonding
• Sandpaper and primer

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How to Fill Hairline Cracks

Small cracks in good plaster are the easiest to fix.

Step 1: Prepare the Crack

Using a scraper or corner of a filling knife, open the crack slightly. This sounds counterintuitive, but you want the filler to get into the crack, not just sit on top.

Remove any loose material and dust.

Step 2: Dampen the Area

Use a damp cloth to wet the crack. This helps the filler bond and stops it drying too fast.

Step 3: Apply Filler

Use a flexible filler for hairline cracks - it moves with the wall instead of cracking again.

Load filler onto your knife and press firmly into the crack at 90 degrees. Then smooth over at a shallow angle.

Step 4: Let It Dry

Most fillers need a few hours. Some shrink slightly - that's normal for hairline cracks.

Step 5: Sand and Prime

Once dry, sand smooth with fine sandpaper. Prime with a stain-blocking primer before painting (filler absorbs paint differently than the surrounding wall).

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How to Fill Larger Cracks

For cracks over 3-4mm wide, you need a different approach.

Step 1: Prepare Thoroughly

• Use a Stanley knife to cut along both sides of the crack
• Remove loose plaster and debris
• Undercut the edges slightly (wider at the bottom than top) - this helps the filler key in
• Brush out all dust

Step 2: Seal the Edges

Paint the edges with diluted PVA (3 parts water, 1 part PVA). This helps bonding and stops the old plaster sucking moisture from your filler.

Step 3: Fill Deep Cracks in Layers

For cracks deeper than 5mm, fill in layers:

1. First layer to half depth
2. Let it firm up (not fully dry)
3. Score the surface with a knife (cross-hatch pattern)
4. Apply second layer
5. Build up to surface level

Why layers? Thick filler shrinks as it dries. Thin layers shrink less and crack less.

Step 4: Finish Smooth

Use a wide filling knife or plasterer's trowel to feather the edges into the surrounding wall. The goal is an invisible repair.

Step 5: Sand, Prime, Paint

Sand when fully dry (24 hours for deep fills). Prime before painting.

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Dealing with Recurring Cracks

If the same crack keeps coming back, the building is still moving in that spot. Standard filler will just crack again.

Solution: Flexible filler + reinforcement

1. Prepare the crack as above
2. Apply a thin layer of flexible filler
3. Press jointing tape (or fibreglass mesh tape) into the wet filler
4. Apply more filler over the tape, feathering edges
5. Sand and paint with flexible paint (some brands are designed for cracks)

The tape bridges the crack and distributes movement over a wider area.

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Old Lime Plaster (Common in Exeter)

Many older Exeter homes have traditional lime plaster, not modern gypsum.

How to tell: Lime plaster is often softer, slightly crumbly, and has a more textured finish. The walls might feel uneven.

Important: Don't use modern gypsum filler on lime plaster. The two materials don't bond well, and gypsum is harder - it can cause damage over time.

Use instead:

• Lime-based filler for small repairs
• Lime putty or NHL lime mortar for larger areas
• Or call someone experienced with old buildings

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When to Worry About Cracks

Most cracks are just cosmetic, but watch out for:

• Cracks over 5mm wide - Worth professional assessment
• Cracks that are growing - Monitor with dated photos
• Cracks with one side higher than the other (stepped) - Could indicate movement
• Lots of new cracks appearing suddenly - Get it checked
• Cracks plus sticking doors/windows - Signs of structural movement

When in doubt, get a structural engineer's opinion. It's cheaper than worrying.

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Cost of Professional Filling

DIY filling is cheap - filler costs £5-£15 and tools maybe £10-£20.

Professional crack filling typically costs £60-£100 for a room with multiple cracks, depending on size and condition.

For large plaster repairs, you're looking at plasterer rates - but that's a different job.

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Want cracks filled properly? We repair plasterwork across Exeter. Call 01392 964094 or get a quote.