Rising Damp will only affect the walls on the ground floor of your property or any subterranean rooms. If you have dampness on the levels above, this will not be rising damp. Please see below for information on other types of damp in properties.
The source of moisture for rising damp is the soil or subsoil that your property is constructed on or near. The biodegraded plant matter found in soil contains nitrates, chloride and sulphate salts which are drawn into the moisture and rise up the masonry wall. When the moisture evaporates, the harmful salts are left behind and evidence of this can typically be found in the form of a 'tide mark' left on the wall. Rising damp over a number of years can cause high levels of salts in masonry. These residual salts are hygroscopic, meaning they absorb further moisture from the air, and tend contribute to the problems evident.
Rising damp results from the capillary action of water from the ground. In the absence of an adequate damp proof course an area rising to around 50cm (or more) above the skirting level along the whole length of the wall will result in damp patches and discolouration. Where the damp proof course is damaged or breached, patches of dampness may occur.