Why does decorators' caulk crack along my ceiling line?

Bead depth, movement and paint film — the three reasons a caulked line splits, and how to lay a bead that survives a winter.

Difficulty: beginner
Time: 30 min
Read: 4 min

Short answer

Caulk cracks because the bead is too thick, because it was painted before it had cured, or because the joint moves more than a thin bead can accommodate. Lay a shallow bead that bonds to two faces, tool it, let it cure fully before painting, and rake out a joint that is too deep rather than filling it in one go.

The line between a wall and a ceiling, or between a wall and skirting, moves. Timber shrinks, plasterboard flexes, and the building itself expands and contracts. Caulk exists to absorb that, and it can only do so if it is applied in a shape that can stretch.

What you'll need

decorators' acrylic caulk
caulking gun
damp cloth or a caulk tool
filling knife for raking out

Step by step

Rake out the old failed caulk completely.New caulk over cracked old caulk cracks in the same place within weeks. A filling knife or a caulk removal tool takes it out cleanly.
Check the size of the gap.Caulk suits gaps up to about 5mm. Beyond that it sags, skins over and cracks as the deeper material shrinks.
Fill a deep gap in stages or with something else.For a wide gap, use a backing material or fill the bulk with a rigid filler first, leaving only the last few millimetres for caulk. That way the caulk is a thin flexible skin, not a solid block.
Cut the nozzle small.A small opening cut at 45 degrees gives a modest bead. Most cracking traces back to a nozzle cut far too large and a bead far too fat.
Apply in one continuous pass.Steady gun pressure and steady movement. Stopping and starting leaves lumps and thin spots, and the thin spots split first.
Tool the bead immediately.A wet finger, a damp cloth over a fingertip, or a caulk tool. Tooling presses the caulk into both surfaces and forms the shallow concave profile that can stretch.
Do not let it bond to a third surface.Caulk should span between two faces. If it also grips the bottom of a deep gap it cannot stretch and it splits down the middle.
Let it cure fully before painting.Touch-dry is not cured. Painting an uncured bead traps solvent and water, and the paint film then cracks as the caulk continues to shrink underneath.
Use a flexible paint over it if you can.A rigid emulsion film over a flexible caulk can crack even when the caulk has not. Some ranges offer more flexible finishes for exactly this.
Accept that some lines will always move.A new-build ceiling line, or a timber-frame wall, will move seasonally. Recaulking every few years is normal rather than a failure of technique.

Tips

Cut the nozzle much smaller than feels right. A thin bead survives movement that a fat one cannot.
Tool the bead within a minute or two of applying it — once it skins, tooling drags rather than smooths.
Painting over caulk before it has fully cured is the most common single cause of a hairline crack appearing in the paint.

Common mistakes

Applying a thick bead to hide a big gap — A deep bead skins over while the inside stays wet, and as it eventually shrinks it splits. The gap needs backing or filling first so the caulk stays thin.
Painting the same day — Caulk continues to shrink for some time after it feels dry. The paint film cures rigid over a moving surface and cracks in a fine line along the joint.

If it doesn't work

Hairline crack along the painted caulk line

Cause: Painted before the caulk cured, or normal seasonal movement — Fix: Rake out, recaulk thinly, allow a full cure, then repaint. Expect to repeat it occasionally in a moving building.

Caulk split down the centre of the bead

Cause: Bead too thick or bonded to three surfaces — Fix: Remove, fill the depth with backing or rigid filler, and apply a thin caulk bead spanning only two faces.

Caulk peeled away from one surface

Cause: Dusty or greasy surface, or no key — Fix: Remove, clean and dry both faces, and apply fresh. Caulk needs a clean sound surface like any adhesive.

Caulk sagged before it set

Cause: Bead too heavy on a vertical joint — Fix: Apply in a thinner bead, and build up in two applications if necessary.

Questions people ask

Caulk or filler at the ceiling line?

Caulk, because the joint moves. Rigid filler there cracks every time, and it cracks in a ragged line that is harder to make good.

Can I use silicone instead?

Not where it will be painted — most silicone rejects paint completely. Use acrylic decorators' caulk indoors and keep silicone for wet areas that stay unpainted.

How long should I leave it before painting?

At least the tube's stated overpainting time, and longer in cold or humid conditions. Overnight is a reasonable minimum for a normal bead.