Short answer
Plaster crumbles because moisture contained in the wall evaporates, creating pressure on the plaster from the inside. Possible causes are rising damp by capillary action or water damage.
The symptom
The plaster becomes friable, powdery, detaches, or reveals a damaged substrate. This is not only an aesthetic problem.
Why you should act before refinishing
Redoing the plaster too early may lead to new deterioration if the substrate remains damp or if the cause has not been controlled.
Do the marks keep coming back despite cleaning?
That is often a sign that the cause needs analysing, not just the surface.
What MURSAIN analyzes
The location, the height, the presence of saltpeter, the building's history, and any water damage help guide the diagnosis.
Frequently asked questions
Does friable plaster mean the wall has to be rebuilt?
Not necessarily. First, you need to understand the cause of the moisture and the wall's actual condition.