The term Rising Dampness’ is used to describe the vertical movement of moisture through masonry that has no physical barrier to dampness and that is standing in water or damp spoil.
Physical damp proof courses have been used in construction for a great many decades and become compulsory in 1875. Early types of physical damp proof course include a double layer of slate, bituminous products, plastic and engineering bricks that all provide an impervious layer.
Over a period of years some damp courses start to ‘break down’ and become porous allowing moisture to rise upwards by capillarity. Where external plinths, paths and driveways have been added, these often ‘bridge’ the dampcourse adding further problems.
For rising dampness to occur, there must be a supply of moisture at the base of the wall. When rising dampness occurs, the ground moisture rises up through the masonry bringing ground salts i.e Chloride and Nitrates that migrate up through the damp masonry and migrate out onto the surface of the wall. As evaporation occurs the low concentration salts become concentrated near the surface and will crystalise out. Chloride and Nitrate salts are ‘hygroscopic’ and will attract moisture from the atmosphere, keeping the surface of the wall damp in conditions of high relative humidity.
If rising dampness is diagnosed and remedial works are undertaken it is an integral part of the remedy that the renewal of the internal wall plaster is carried out. Timbers that are in contact with damp masonry should be considered as ‘suspect’ and should be exposed and inspected for signs of decay.
During the ‘drying out’ process ‘efflorescence’ some times occurs. Efflorescence is a white powder like substance on the surface of the wall and is a sign of drying out.
Under optimum drying conditions it will take approximately 1 month per 25mm thickness of wall to dry out.