The pattern normally begins at the base
Moisture supplied from the ground rises into porous materials and is progressively lost by evaporation from the wall surfaces. Damage is therefore normally concentrated low down rather than appearing as an isolated patch high on one wall.
The DPC can fail without being physically broken
A traditional DPC may still be present but bypassed by raised paving, render carried below DPC level, internal plaster bridging or debris in a cavity. Correcting the bridge can be more relevant than adding another chemical barrier.
Salts complicate what happens after treatment
Ground moisture can carry salts into plaster. Some remain hygroscopic after the masonry moisture source has been reduced, leaving a surface that still feels damp or records high on an electrical meter.
Diagnosis is a collection of evidence
Wall construction, ground level, DPC position, the moisture profile, salt distribution and other potential sources all need to agree. One meter reading cannot establish that chain on its own.
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