When a wall is built in contact with the ground, the soil, which normally has a high moisture content, will cause it to become wet. The drying action of the air on the upper parts of the wall then causes the wall to act as a wick, pulling water up through the mortar and bricks by capillarity. The height to which dampness rises in any particular wall depends on wall construction, thickness, surface coatings, temperature and the height of the water table. It rarely exceeds one metre above ground level, as the force of gravity then starts to exceed capillarity.
Signs of rising damp include a "tide-mark" on the wall above skirting boards, peeling wallpaper, mould growth and white, fluffy efflorescent salts which have been drawn out of the brickwork by the passage of water.