Cracks Across A Ceiling

What's happening?

Ceiling cracks are read by their shape, not their length. A fine crack that wanders and branches is shrinkage in the plaster or the paint film, and it is a decorating fault. A straight crack that runs the width of the room, especially in a line, is a plasterboard joint that has opened, which is a finishing fault. A crack that follows the line of a wall below, or opens and closes with the seasons, is movement in the structure being shown by a brittle finish that cannot follow it. And a crack with sag, a bounce underfoot in the room above, or brown staining is a different order of problem, because that is the ceiling itself or water, not the plaster on it. The pattern tells you which trade you need.

What usually causes it?

• Shrinkage in a finish coat that dried too fast, giving fine map or crazing cracks over a whole area
• A plasterboard joint that was not taped, or was taped over dust, so the scrim has let go in a straight line
• Seasonal movement of timber joists as the house heats and dries, opening the same crack every winter
• Deflection from load above: a new bath, boxed storage, or someone boarding the loft and walking on the joists
• New-build or new-extension drying shrinkage as timber and plaster lose their construction moisture
• Water damage from a leak above, which weakens the plaster and stains as well as cracking it
• An older lath and plaster ceiling starting to lose its key, which cracks before it sags

Is it dangerous?

Most ceiling cracks are cosmetic, and a hairline crack you can barely fit a fingernail into is not a structural warning. Four things change that. Sag, even slight, means the ceiling is no longer fully supported and is a genuine collapse risk. A crack that is growing measurably, rather than seasonally opening and closing, points at movement that needs diagnosing. Brown or tide-marked staining means water is arriving from above and the plaster is wet. And a springy floor above a cracked ceiling suggests the joists are deflecting, which Approved Document A treats as a structural matter rather than a finish one. Any of those four is a call, not a filler.

Can I fix it myself?

Fine crazing and a single settled hairline crack are ordinary decorating: rake the crack out slightly, fill with a flexible filler, and paint. That is worth doing. What is worth doing first is recording it: pencil a dated line across each end of the crack and measure it again after a month, because a crack that has stopped moving is a very different finding from one that has not. Do not fill a crack that has sag beside it, do not paper over a stained crack, and do not stand on a bathroom ceiling's joists in the loft to investigate. Anything above head height on steps is where DIY ceiling repairs go wrong.

When should you call a plasterer?

Call a plasterer where cracks keep reopening after filling, where a plasterboard joint has opened along a line, or where the surface needs re-taping or a fresh skim to come right. Call a builder or a structural engineer instead if the ceiling sags, if the crack is widening, or if the floor above bounces. Call a roofer or plumber first if there is staining, because the plaster cannot be repaired until the water stops.

What will the plasterer actually do?

• Read the crack pattern to separate shrinkage, joint failure, movement and water damage before repairing anything
• Cut out and re-tape opened plasterboard joints with scrim or jointing tape rather than filling the line
• Refix loose boards to the joists so the surface is solid before any finish goes on
• Skim the affected ceiling in full where crazing or repeated cracking means patch repairs would still read through
• Use a flexible detail or a shadow gap where a junction is known to move, so the repair is not brittle again
• Report sag, deflection or staining to the owning trade rather than concealing it under a new finish

Who fixes this

• Wall and ceiling repairs

Questions people ask

Sources & References

1. GOV.UK: Approved Document A (Structure)
2. LABC: Guidance for homeowners
3. NHBC: Standards