The usual causes
- Rain water getting in through the roof or the outside wall.
- A leaking pipe or water tank connection inside the wall.
- Water rising from the ground into the lower part of the wall.
- Water from a bathroom or kitchen going into the wall next to it.
- Steam settling on cold walls in rooms with little fresh air.
How to tell which one you have
- The patch grows after rain: water is coming from the roof or the outside wall.
- The patch stays even in dry weather: a pipe may be leaking.
- Damp at the bottom of the wall, with white salt marks: water from the ground.
- Damp next to a bathroom or kitchen: look at the floor and pipes there.
- Take photos every few days. They help the mistri a lot.
What not to do
Do not paint or put putty straight over a damp patch. It hides the problem for a short time and then peels. Do not touch switches or sockets on a damp wall with wet hands: switch off that room at the main board and ask an electrician to check the wiring. Find the cause first, fix it, let the wall dry, and only then repaint.