Rising damp occurs when moisture from the ground travels up through the walls by capillary action. This means that ground water is effectively sucked up through tiny tubes in the bricks, like a series of straws. This water contains salts that also travel up through the wall.
Around the affected wall, you get other porous building materials such as plasterwork and the timber found in the floor boards, joints and skirtings. These materials will also absorb the ground water and you will find evidence of wet rot in the timber.
Generally, rising damp is first noticed by the damage it causes to the internal walls of a building. Plaster and paint can deteriorate and any wallpaper tends to loosen. A visible stain often appears on the wall in the form of a tide mark at the point where the ground water has reached. You may also see salts blooming on the internal surface. This is something often associated with rising damp and will lead to the debonding of paints and even plaster work. Externally, mortar may crumble and white salt stains may appear on the walls.
Most buildings have some form of barrier installed at the lower level of the wall to prevent water rising up in this way. It is called a damp proof course (DPC). This can be made of non-absorbent, water-resistant materials such as slate, bitumen and plastic, depending on the period in which the property was built. Sometimes these physical DPCs may fail over time; in older houses they may not exist at all. If you don’t have a DPC or there is evidence that it has failed, then there is nothing to prevent the water from traveling up your wall.