The ceiling sounds hollow and bulges like a bladder: plaster sag before it becomes a hole

A ceiling that drums hollow when you tap it, or shows a soft dome where the light's shadow draws a curve, is a ceiling whose skin has stopped being attached to its structure. It will fall - not all at once, probably not tonight, and the sooner the decision is made the smaller the repair bill, because a sagging plaster is fixable and a fallen one is a room.

Know which ceiling you have, because the repairs differ

• Cement/sand plaster on concrete slab - the Nigerian workhorse: the "hollow" you hear is a delamination of a hard shell from the slab, often after a water event. The tell: the crack around a light fitting, a corner that has detached visibly, or a dome that moves slightly when you press a broom handle to it - gently, once, as a test, not as a hobby.
• Gypsum board on metal or timber battens - the drywall ceiling: its failures are screw-pop, taped-joint crack, and the soft black stain of a slow leak eaten into the board. The sheet that "moves like a membrane" when pushed is a sheet that has lost its perimeter of fixings, and it can be re-screwed while intact; a board whose face paper has gone mushy cannot - gypsum is a chalk sandwich and water is its only enemy that wins at room temperature.

The decision tree after the tap map

Tap a grid of the whole ceiling with a knuckle and mark the drumy zones with pencil - the map is the diagnosis and the quote, and it is the one page the tradesman will thank you for taping to the door. Small zones under a fastening point (a screw can re-clamp the skin to the slab or the batten behind it): re-fix with the correct anchors - masonry screws with washers for cement skins, board screws into the joist for gypsum - and re-tape, re-fill, re-prime the patched line with the stain-blocker step the ceiling-stain article carries. Larger zones, or a skin that moves under the broom handle, or any ceiling sitting in water: the affected area comes down - cut out, safely, with the room emptied and the eyes and hands protected, because old plaster dust and its black mould tenants are a health page of their own - and the new sheet is either skimmed back over a sound parent or boarded fresh, which is the cheaper half of the comparison everyone wants and this desk will not fake: a replaster of a whole ceiling is priced by the metre and the moisture's damage, and the honest quote is always three numbers - the plaster, the cause of the damp, and the painting. If a quote offers only the first, it is a promise about a repeat.

The call that cannot wait

A ceiling that has visibly dropped below its line, or a crack that runs across it with a step at the edges (one side lower than the other), is not a plaster problem any more - it is the slab or the frame moving, and that is the structural conversation with the engineer: this desk has said all it is licensed to say, and the line from a sagging skin to a building's argument is the one page that should end with "call today," not "schedule." The rest is the painter's calendar.