Soil or subsoil is the source of rising damp as these are always wet. Soil water is not pure water as it contains the soluble materials from its composite parts; from decaying plant materials and bacteria, moulds and soil-living animals.
Of the soluble materials in water, nitrogen containing salts (which eventually become nitrates) are the most prolific, and chlorides are also generally universal. Rising damp is therefore a rising, dilute solution of various materials including nitrates and chlorides. When water evaporates these are left behind because they can’t evaporate.
Although rising damp is a slow process and the solution is dilute, if it continues for a number of years the result can be high concentrations of certain nitrates and chlorides at the surfaces from which evaporation occurs.
This is the basis for diagnosis of rising damp by analysis of wallpaper or surface scrapings of plaster, for the presence of nitrates and chlorides. Typically the highest concentration will be in wallpaper, a the highest point to which the dampness has been rising, however this is not always the case as the paper may not have been in position for many years. Typically, though, wallpaper will contain the greatest weight of salts, followed far behind by the finishing coat plaster and then the bonding plaster. Often the salt content deep in brick walls is relatively low.
Soil salts are hygroscopic, meaning they absorb water from the atmosphere (unless it is very dry) and form a solution. This means they may keep wall surfaces damp even if the rising damp which deposited them may have been cured. The evidence of dampness from hygroscopic salts is that occupants of the building will vary with the weather, becoming more pronounced when the atmosphere is humid.
These salts are electrically conductive and will give high readings on an electrical type of moisture meter. Therefore if a high meter reading is given, this is a condition that should be investigated even if the wall appears dry. It should be noted that their presence is a sign of continuing trouble if the source is not stopped. Therefore removal of and replacement of contaminated plaster is usually required.
Non-hygroscopic salts can sometimes be contained within the inner surfaces of walls particularly in very old buildings, but this concentration of salts is not due to rising damp. These can cause high readings on a conductance type meter, and are considered to be due to occasional rain penetration under exceptional circumstances over the years, which has leached these salts from the fabric of the building.
If you require any further information on the above, require help with a damp problem in your property or think you may need to hire a dehumidifier to dry your building, then please contact us on 0800 078 6999.