Common causes
- Failed waterproofing on the upper floor or shower tray.
- Concealed supply/waste pipe leaks above or in your flat.
- External wall or window leaks travelling through concrete.
- Condensation near AC pipes with poor insulation.
What owners can do first
- Photograph and track the stain.
- Inform management; ask the upper flat to check if needed.
- For cross-unit disputes, review Joint Office / 1823 guidance.
- Keep electrics safe if water is near fittings.