Can be absent in older buildings, can fail, and can be bridged by external ground levels, a new patio, render carried down over it, or debris in a cavity. Chemically injected replacement DPCs are widely sold as a remedy for damp regardless of its actual cause, which is the single most common misdiagnosis in the field.
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.