A crack has opened in my ceiling and I can see rusty steel. Is it dangerous?

Inside a ceiling slab there are steel bars, the reinforcement. When water gets in, from a roof, a balcony or a bathroom above, the bars rust over the years. Rust takes up more room than steel, so it swells and breaks the concrete around it. That is why the crackCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. often follows a straight line, along a bar.

In this bedroom the ceiling had cracks and blistered plasterHollow (blistered) plasterPlaster that has come away from the wall or ceiling behind it. It sounds dull when you tap it lightly, and often blisters or cracks before it falls.. When the loose pieces were removed, rusted steelRebar corrosionRust on the steel bars inside concrete. Rusting steel swells, cracks the concrete around it and makes it fall away in pieces, leaving the bars exposed. showed inside the crack. The crack was opened up that far so it could be repaired properly, rather than just closed with fillerFillerA 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. that would crack again.

On another ceiling the 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. had fallen off, leaving the concrete and its bars exposed. Every exposed bar got a red protective coatAnti-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. so it wouldn't keep rusting, and the surface was 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.. Then it was filled, smoothed and painted, so it doesn't stand out from the rest of the ceiling.

Keep in mind: the repair only lasts if the water that caused the rust is stopped. If the bars have thinned a lot, if the damage covers a large area, or if it is on a beam, a column or a balcony, a structural engineerStructural designAn engineer's study that checks whether a building can take a change, such as an opening in a load-bearing wall, and sets out how it is done and how it is strengthened. needs to see it first. And if pieces are falling, keep the area underneath clear.