'Rising Damp'

Although first patented in 1841 a damp-proof course (D.P.C) is not thought to have been routinely incorporated into ‘new builds’ in the London area until circa 1877, following the model byelaws, although this should still be regarded as a rule-of-thumb, with variances depending on location and type of construction etc. Contrary to popular belief, there is no mention of a damp-proof course in the Public Health Act.

A physical damp proof course is traditionally a layer of impervious material incorporated at the base of a wall during construction, in order to prevent ground water from rising up the wall by capillary action. 

True rising dampness can only be defined as moisture rising up a wall by capillary action due to the lack of a damp-proof course or the failure of the damp-proof course to perform its design function, which in our experience is a rare occurrence. However, bridging of the D.P.C is common.

A failure of the D.P.C may occur due to structural movement, and on one or two occasions I have seen slate turned to a paste-like consistency, due to attack by a particular combination of salts present in the surrounding sub-soil. I have also seen a Bitumen based D.P.C turned to a powdery consistency on a couple of occasions. Modern Polythene D.P.C's are unlikely to fail, assuming they were installed defect free. 

True rising dampness most commonly occurs in walls where the foundations extend down to below the groundwater table, the level of which fluctuates throughout the course of the year. Water rises through the bricks and/or mortar joints via the fine capillaries and pores present in all building materials. The four main factors that determine the quantity of moisture in a wall and the height to which it rises are:

The permeability of the material that the wall has been constructed from.

The thickness of the wall.

How wet the surrounding ground is.

How quickly the moisture can evaporate.