A leak caused plaster to fall and left a hole in the ceiling. When and how should it be repaired so it doesn't fall again?

When water from the roof terraceFlat roof (roof terrace)The flat concrete roof on top of a building. It is the most common kind of roof in Greece and the part that takes the most sun and rain., a balcony or an upstairs plumbing leak soaks through the concrete slab, the ceiling plasterRender (plaster)A layer of mortar (sand, cement or lime, and water) spread over brick or concrete to give a wall a straight, sound surface ready for paint or tiles. loses its grip and falls away, leaving a hole. Repairing the ceiling starts once the source of the water above has been fixed.

Rather than simply filling the hole as it is, any surrounding plaster that has also come loose is tapped and removed back to a firm edge. If steel reinforcement bars are exposed inside the patch, they are cleaned and coated with anti-corrosionAnti-corrosion coatingA protective coat brushed onto steel reinforcement, once the rust has been cleaned off, so the bars don't keep rusting before they are covered with new material. primerPrimerThe thin first coat that goes on before the main material. It binds dust, seals the pores and helps paint or waterproofing stick firmly..

The patch is then built back up with repair mortarRepair mortarA special cement-based mortar for rebuilding concrete that has cracked or fallen away, for example around rusty steel. It bonds well to old concrete and seals the reinforcement in. and plaster, skimmedFillerA fine paste or powder that fills holes, fine cracks and dents so a wall, ceiling or piece of wood is smooth before it is painted. and sandedSandingSmoothing a surface with sandpaper or a power sander to remove bumps, loose old paint or gloss, so the new paint or varnish grips well and comes out smooth. flush with the rest of the ceiling, and painted along with the room.