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

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