Photograph 'House on Water' by baldeaglebluff
Rising damp is associated with capillarity and limited accordingly by the characteristics of the materials in which it is rising, by gravity and by atmospheric pressure. Consequently, damp which is more than about 1,500 mm above adjoining ground level is unlikely to be due to rising groundwater.
Because soluble salts are often present in groundwater and in concretes and masonry, rising damp is often revealed by the deposition of salts.
Aqueous salt solutions will rise until the water is lost by evaporation. The salts which are left behind crystallize out (effloresce). When this occurs at the surface, the crystal will form a fragile feathery crystalline growth usually white in colour and often forming ‘tide’ marks. When it occurs below the surface, e.g. behind a paint film or within plaster, the crystals will grow and can lift the paint film or disrupt the plaster.
The presence of efflorescence strongly indicates that water has reached the surface from within the wall and is not condensate. The absence of efflorescence is not proof that the dampness is not due to rising damp, as rising damp may not contain sufficient dissolved salt to cause efflorescence.