Mould needs moisture; painting over a damp surface does not remove the conditions that let it grow. Visible liquid water is not required for a surface to remain damp enough to support growth.

Possible causes include condensation on cold surfaces, leaks, rain penetration and combinations of building and ventilation problems. Their patterns can guide an investigation, but appearance alone cannot establish the cause.

If you rent, report damp and mould promptly. You do not need to diagnose the building before reporting it, or complete a cleaning and window-opening experiment first. The useful record is where it appears, when it returns and how the room is affected.