Rising damp is a form of dampness that occurs, particularly in older buildings, when groundwater rises up through walls, floors and masonry via capillary action. Capillary action is the ability of a liquid to flow along narrow spaces, counter to the forces of gravity. Bricks and mortar can be very porous and contain many fine capillaries, through which water can rise.
Most modern properties are built with a damp-proof course, usually in the form of an impermeable plastic membrane, which is placed along a mortar line near the ground. Some properties, however, were built with no damp-proof course, or the original has failed, meaning that rising damp can occur and it is necessary to make remedial repairs.
Whenever rising damp or salt damage to internal walls is diagnosed, it is important to have the condition correctly treated. Failure to treat rising damp can cause further damage and devaluation to a property. Apart from altering the appearance of the wall, rising damp can deposit salts form the ground that can cause irreversible damage.
It is not sufficient to simply cover up the problem with a special paint or coating, in the hope that the problem will go away. Salts continually deposited by the groundwater can contaminate new coatings and plaster. Only by preventing the dampness from rising up the internal masonry can an effective treatment be achieved.