Why is there a long straight crack across my ceiling?

Plaster does not crack in straight lines on its own; it cracks where something underneath it makes a line. On a ceiling that means a board joint, a joist line, a junction between different constructions, or a genuine structural movement. Each of those has a different repair and only one of them is fixed with filler, which is why the same ceiling crack gets filled and repainted three times in some houses.

Step by step

1. Note the direction and compare it with the joists.Find the joist direction with a detector or by looking at the fixing dimples in a raking light. A crack running at right angles to the joists is very likely a board end joint. A crack running parallel and at a regular spacing is following a joist.
2. Measure where the crack sits relative to the walls.Plasterboard comes in standard lengths, so a crack at roughly 1.2m, 1.8m or 2.4m from a wall, running straight, is almost certainly a board joint.
3. Run a fingernail along it and note the profile.A fine crack with a slight ridge or a shallow depression along it is a taped joint that has failed — the tape has debonded or the scrim was bridged with too little compound. A clean crack with no profile is more likely movement.
4. Check whether the crack has an edge you can lift.If a sliver of paint or plaster lifts along one side, the tape underneath has let go and the repair has to address the tape rather than the surface.
5. Look for the accompanying signs that change the answer.Sagging, staining, a bulge, a crack that also appears in the wall below the same line, doors that have started sticking, or a crack that has appeared since work was done above. Any of those move this out of decorating.
6. Consider what is above.A loft that has been boarded and loaded, a new water tank, a bathroom installed above, or a loft conversion all put load on the joists and can crack the ceiling below. So can a partition wall built on the floor above without support.
7. Repair a failed joint properly.Cut the failed tape out along the line, back to sound compound. Re-tape with paper tape bedded in compound or with scrim, then two or three coats feathered wide. Filling a failed joint without replacing the tape simply hides it until next winter.
8. Repair a shrinkage or movement crack flexibly.Where the ceiling is sound but the line moves seasonally — often at a junction between an old and a new part of a building — a flexible sealant finished neatly, or a scrim and skim, outlasts rigid filler.
9. Get an opinion where the signs point to structure.A crack with sagging, or one that is opening, or one that matches a crack elsewhere in the building, is a structural question. That is a structural engineer rather than a plasterer.

Tips

• A torch held flat against the ceiling shows the board joints, the screw dimples and the joist lines all at once, and it usually answers the question in ten seconds.
• Ceilings crack most in the first two years of a new build and after a first winter with new heating, and much of that is normal drying shrinkage.
• If the crack is at the junction between the ceiling and the wall rather than across the ceiling, that is a different and much more common fault with its own answer.

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