RISING DAMP
Rising damp is the upward movement of ground water through a permeable masonry wall.  The water rises through the pores in the masonry via a process called capillarity.  Capillarity is a process whereby water molecules are electrochemically attracted to mineral surfaces, enabling water to move vertically through pores of a certain size despite the counteractive force of gravity.
The same process is responsible for the movement of moisture from tree roots to tree tops via intricately stacked hollow cells.   Fortunately masonry includes pores with different forms and dimensions, none of which are as fine as those in trees and moisture therefore rarely ascends more than about one metre in height.

Rising damp only becomes a problem if it affects vulnerable materials or finishes that are in contact with the wall.  For example if it affects timbers such as joist ends, skirting boards or the bottom of door frames such as to provide wood rotting  potential or if it causes wall plaster or wall coverings to deteriorate or become stained.

DAMP PROOF COURSE
There are two main types of damp-proof course (often abbreviated to DPC).
The first is an impervious physical barrier that located at the base of a wall during construction to prevent moisture rising through the structure by capillary action.  Damp-proof courses were not common in domestic construction until required by the Public Health Act 1875 and early damp proof courses comprised a layer of dense engineering brick, a layer of slate or a continuous layer of tar.  Modern built-in damp courses comprise a continuous synthetic membrane that is laid in the mortar bed some 150mm above ground level.

Rising damp typically occurs in buildings having no built-in damp-proof course and in structures where the built-in DPC has been bridged, for example by external renders, plaster or pointing that cover over the DPC or by a change in ground level.

With cavity wall structures, a build up of unseen debris at the base of the perimeter walls, between inner and outer skins, can reach a level that breaches the damp proof course.  This may give a potentially false indication that the physical DPC has broken down.

In fact very few physical damp proof courses are liable to break down failure unless they has become damaged by structural movements or alterations that have resulted in the DPC becoming discontinuous

For free assistance in matters relating to rising damp in domestic housing phone the Twistfix technical help and advice line on 0845 123 6006 or visit www.twistfix.co.uk