Caused by defective pointing, failed seals, cracked render, blocked or leaking gutters, failed flashings, or external ground levels piled above the damp proof course. It correlates with rainfall, and often with rain driven from one particular direction. The cause is almost always visible on the outside face of the same wall, and usually only while it is actually raining.
Related terms
Seeing this in your building?
The free diagnostic works out the mechanism from what you can see and measure, and tells you what it cannot conclude.