Rising damp is found naturally in most masonry structures as the moisture in soil and masonry tries to achieve equilibrium. This equilibrium occurs in nature and is evident in many forms such as, temperature - where heat spreads from hot to cold, electricity - where a positive charge flows to the negative and in water - where dampness invades a dry area.
As moisture rises from the earth by capillary action through the walls pore structure it creates an electrical potential, (Zeta Potential), between the wall and the moisture. This "potential" causes more and more water molecules carrying damaging ground salts to travel from the positive, (the earth), towards the negative, (up the walls).
Rising damp can be caused by the absence of an effective damp-course or by bridgi of the damp-course. During bricklaying excess mortar often falls down behind the brickwork, into cavities where it sticks to brick ties, and lands in the gaps between the floor joists, bearers and brickwork. These mortar droppings can create a moisture bridge - a direct path for moisture to creep from damp masonry to your floor bearers and joists, perimeter timber framing, plaster and timber trims.
As the moisture content varies within the masonry it induces a wetting/drying cycle causing the salts to expand and contract accordingly, hence the breakdown and deterioration of the plaster, brick/stonework and mortar. As excess moisture evaporates a surface salt residue is left behind.
How common is it?
Most masonry walls can exhibit rising damp symptoms up to the height of the dampcourse. Hence the Building Code of Australia requirement that a damp-course be installed in a manner to prevent:
"undue dampness or deterioration of building elements".
That is why builders install a damp-course BELOW the timber floor bearers. Although a damp-course is intended to stop the upward movement of moisture into drier materials, rising damp may often be seen to rise higher than a metre on a wall in older houses where there is an ineffective or non-existent damp-course. A "rendered" finish applied over a damp-course creating a moisture "bridge" is a common cause often found in new or renovated structures. Factors contributing to rising damp may include the water bearing capacity of the soil, length and depth of masonry below ground level, wall thickness and surface evaporation area.
Rising damp may also appear to fluctuate as weather conditions change. Seasonal effects must be taken into account during diagnosis, as the rise and fall of the water table can cause rising damp to apparently disappear in summer but return in winter. Drier conditions with improved ventilation may temporarily increase evaporation rates, but will not stop "rising damp".