Sources Of Ingress
This commonly occurs to walls which are exposed to the prevailing wind. Such dampness is generally most visible during or after periods of heavy rain.
Faulty rainwater goods, such as gutters or downpipes, as well as defects to roof coverings, masonry work, pointing and external renders are also a common source of moisture ingress. Therefore an external inspection should be included during any investigation of dampness affecting external walls.
This is where the internal floor level is below external ground level, i.e. the wall is wholly or partially earth retaining. Consequently, moisture from the ground will penetrate laterally into the walls unless isolated by a vertical damp proof membrane. In areas where the water table is high, lateral penetrating dampness may be aggravated by hydrostatic pressure.