How do I fix cracks in plaster?

Most cracks in plaster are cosmetic: seasonal movement, drying shrinkage, or a joint between two materials that expand at different rates. A few are not. The repair below handles the cosmetic majority, and the FAQ explains which ones to stop and think about.

What you'll need

• Flexible or ready-mixed filler
• Filling knife
• Sandpaper (120 and 180)
• Old screwdriver or crack-raking tool
• Self-adhesive scrim tape for wider cracks (optional)

Step by step

1. Look at the crack before filling it: note the width, direction, and whether it's uniform.Photograph it with a date. If it reappears, you'll want the comparison.
2. Rake the crack open with an old screwdriver to remove loose material.Slightly wider is better than hairline — filler needs somewhere to sit and something to grip.
3. Brush out all dust and wipe with a barely damp cloth.Dust in the crack is the main reason repairs fail.
4. For hairline cracks, work flexible filler in firmly with the knife, leaving it slightly proud.Flexible filler tolerates the small seasonal movement that caused the crack in the first place.
5. For cracks over about 2mm, apply a first coat, bed scrim tape into it, then fill over.The tape bridges the crack so future movement doesn't reopen the same line.
6. Let it dry fully, sand flush and spot-prime before painting.Filler is more absorbent than plaster and will flash through paint if not primed.

Tips

• Cracks along the ceiling-to-wall junction and around door frames are usually normal movement between materials. Flexible filler or a decorator's caulk handles them best.
• Decorator's caulk is better than filler for a crack in a corner or where two surfaces meet, because it stays slightly flexible.
• If a crack reopens each winter, stop using rigid filler. It's telling you there's ongoing movement.
• Fill in two thin passes rather than one thick one — deep filler shrinks and cracks as it dries.

Common mistakes

• Filling a hairline crack without opening it up — There's nothing for the filler to key into and it falls out.
• Using rigid filler on a moving crack — It cracks again in the same place, usually within a year.
• Filling a structural crack and forgetting about it — You've hidden a symptom that needed investigating.

Questions people ask