How do I fix the crack where the wall meets the ceiling?

The line where a ceiling meets a wall is a junction between two structures that move independently — a floor above deflecting, a ceiling shrinking, a wall settling. It is the most commonly refilled and most commonly re-cracked line in any house.

What you'll need

• Decorator's caulk and a skeleton gun
• A scraper or a filling knife
• Self-adhesive scrim tape, for the taped option
• Fine filler or finishing plaster
• A damp cloth
• A dust brush

Step by step

1. Check it is a movement crack, not a structural one.A hairline to 3mm line following the junction, opening and closing with the seasons, is movement. A crack that steps, widens along its length, or is accompanied by others at door heads deserves a proper look.
2. Rake it out.The corner of a scraper or a filling knife run along the crack, removing loose material and old filler. Anything sitting loose will simply crack again.
3. Dust and dampen.Brush the debris out, then a damp cloth or brush. Caulk goes off far better on a clean, slightly damp surface than on dust.
4. Choose caulk for anything up to about 5mm.Flexible decorator's caulk is designed for exactly this joint. It stretches, it takes paint, and it accommodates the movement instead of resisting it.
5. Cut the nozzle small.3–4mm at 45 degrees. A big bead is harder to tool and shrinks unevenly.
6. Run the bead in one pass.Steady speed, steady pressure, moving away from yourself along the whole wall. Stopping and starting shows.
7. Tool it with a wet finger.One pass, wiping the finger between runs, water in the bowl. Going back over a tooled joint is what makes it look uneven.
8. Expect to need a second pass on a deep crack.Caulk shrinks a little as it dries. Fill the bulk, let it skin overnight, then run a finishing bead.
9. Tape and skim a recurring wide crack.Where caulk has failed repeatedly, rake back further, apply self-adhesive scrim tape across the junction, and skim over it with finishing plaster or fine filler, feathered out onto both surfaces.
10. Consider coving as the permanent answer.Coving bridges the joint entirely and hides all future movement. On a house where the crack has come back three times, it is the honest solution.
11. Investigate if it is getting worse.A junction crack that widens year on year, or appears at the same time as cracks elsewhere, is worth having looked at rather than filled again.
12. Paint promptly.Caulk is not a finish. Prime and paint within a few days or it discolours and picks up dust.

Tips

• The crack usually appears on one or two walls only — typically those running parallel to the joists above, because that is the direction the ceiling deflects.
• A drop of washing-up liquid in the tooling water gives a much smoother finish and stops the caulk dragging.
• If you are coving anyway, do not bother filling first. The coving covers it.

Common mistakes

• Filling it with powder or ready-mixed filler — Rigid filler has essentially no elasticity. The joint moves by up to a millimetre or two through the year, which is far more than the filler can accommodate, so the crack reappears within a season — often in exactly the same line.
• Painting caulk before it has cured — A thick bead can be touch-dry on the surface and soft underneath. Paint over it wrinkles, and the caulk continues to shrink beneath the paint, opening a hairline you can see but cannot fix without redoing it.

If it doesn't work

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