Plaster ceiling repair: cracks, sagging and bowing

Cracks that are cosmetic and cracks that are not

Hairline cracks along board joints, or a fine web of crazing, are usually movement or shrinkage in the finish. They can be raked, filled, taped where needed and painted. Cracks that are widening, run with a step or drop on one side, or appear alongside doors that stick may point to movement in the structure, and the cause should be looked at before the ceiling is repaired.

Sagging and bowing: when a ceiling may come down

A dip in the middle of a room, a belly between joists, or plaster that moves when pressed lightly with a broom head all suggest the ceiling has detached from what holds it up. Keep people out of the room below the worst area and do not push on it. A ceiling in this state can come down in large, heavy pieces, so it needs making safe first.

Lath and plaster or plasterboard

Older houses often have lath and plaster: thin timber strips nailed to the joists, with lime plaster pushed through the gaps to form keys. When the keys break, the plaster sags even though the laths are sound. Later and newer homes use plasterboard screwed or nailed to the joists, which fails differently: usually through water damage, failed fixings or joints opening up.

Overboarding versus taking the ceiling down

Overboarding means fixing new plasterboard through the old ceiling into the joists, then skimming it. It avoids the mess of pulling down an old ceiling and keeps the original in place behind. It suits ceilings that are sound enough to stay but too cracked to keep filling. Where the ceiling is badly bowed, wet, or you want to deal with wiring or insulation above, taking it down and reboarding is the better choice, though it is a dustier job. Older textured ceilings should be treated with caution; our artex ceiling removal guide explains why.

Skim, finish and redecoration

New or overboarded ceilings are taped and skimmed for a smooth surface, then left to dry before a mist coat and paint. Our guide to skimming vs replastering explains the difference in approach, and our plastering service covers both. For a sense of what drives a quote, read how much does a plasterer charge.

The visible finish is only one part of the decision. Filling addresses a local defect in an otherwise sound surface; it does not replace the support behind loose plaster. Likewise, a fresh skim should not be used to hide a ceiling whose condition has not been checked. Ask which existing layers will remain and which will be removed. That explanation should connect the proposed repair to the defect, rather than treating every cracked ceiling as the same decorating job.

When the cause is water

A brown stain, softness or a sudden sag after a leak means the cause has to be fixed and the ceiling dried before any repair. Our guide to water damaged ceiling repair covers that sequence step by step.

Describing the ceiling before a repair visit

From a safe position, note whether the problem is a fine line, an open joint, a stain or a visible change in level. Say whether it appeared after a leak, has returned after filling, or extends across more than one part of the room. Photographs showing the whole ceiling and the affected area help put a close-up into context. Do not climb up or press on suspect plaster to get a better description.

For the quote, distinguish making the ceiling safe, repairing its surface and redecorating it. Ask whether the proposal is a local patch, overboarding or replacement, and how the final paintwork fits after drying. If the cause was water, keep the leak repair separate in that discussion so a smooth new finish is not mistaken for evidence that the source has been solved. This makes the sequence understandable before the room is taken out of use.