Damp proofing is a protective measure in the form of a layer of impervious material applied to building foundation walls to prevent moisture from passing upwards or sideways through the walls into interior spaces. A damp-proof course (DPC) is a horizontal barrier in a wall designed to prevent moisture rising through the structure by capillary action - a phenomenon known as rising damp. A damp-proof membrane (DPM) performs a similar function for a solid floor.
Rising damp can arise for various reasons - the failure of an existing damp proof course, bridging due to the raising of external ground or internal floor levels, or in older buildings, the complete absence of a damp proof course.
Brick, stone and mortar are porous allowing damp from the ground to rise by capillary action, carrying with it ground salts including chlorides and nitrates. These salts from the ground can absorb moisture from the atmosphere leading to wall dampness in conditions of high relative humidity. Also they can ruin decorations and break down internal plaster.