EXACTLY WHAT IS SALT DAMP?
"Salt damp" is an expression that describes damage in buildings caused by the destructive action of moisture and salts. The problems range from tell-tale signs, such as fretting mortar, bubbling paint and plaster through to serious decomposition of the wall materials.
The main cause of salt damp in walls is rising damp, which develops because the damp-proof-course (DPC) in the wall is ineffective and allows moisture from the ground to soak upwards into the porous masonry wall materials by capillary action, just like oil into a wick.
The soil moisture in South Australia is usually high in salts, consequently rising damp will aggravate the problem by progressively increasing the salt content in the walls. If the walls have been sealed, the damp will climb higher and higher up the wall, up to the level that it can evaporate.
Salt damp damage occurs in two principal ways. Firstly, the presence of damp causes some materials to deteriorate. For example, plaster will soften, paint peels, steel reinforcing and nails will rust and timber will rot. There will be unhealthy mould and a general slow decomposition and smells. Damp conditions also encourage insect pests such as termites.
Secondly, the presence of salts in the damp causes a microscopic attack on the masonry materials themselves. The salts crystallise as the moisture evaporates, usually just beneath the surface, up to the "tide-mark" of the damp. The enormous pressure created by individual salt crystals growing within the pores will physically break down the matrix of masonry surrounding each salt crystal. In due course the process repeats itself, perhaps several cycles each year. Eventually, the masonry breaks down in layers and spalls off. Some of the salts cause a chemical reaction that also destroys the internal structure of the masonry.
We recommend that you take all of the preventive or corrective options possible and necessary for your building, as the first step.
However, these measures will not eliminate well-established rising damp that has resulted from an ineffective damp-course. We recommend positive action to reinstate this, either a silicone damp-course or by undersetting, to ensure that the cause of the problem is fixed.
For more comprehensive information, refer to the booklets "Rising Damp and Salt Attack" and "Salt Attack and Rising Damp" by David Young for the respective Heritage authorities of NSW, Vic, SA and the City of Adelaide.
TREATMENT OF RISING DAMP
If you have rising damp, the only positive way to stop it is to install a new damp-course. There are two effective options to do this, as follows:
UNDERSETTING requires demolition of the salt damp affected masonry and replacement with new, physically building in a new plastic sheet membrane damp-course at the base of each wall. We do the work in small sections, to minimise the extent of demolition at any one time, to avoid dangerous structural conditions. The advantage of undersetting is that the worst of the damaged and salt-saturated wall materials can be removed and replaced with sound materials.
SILICONE DAMP-COURSE is a cheaper and less disruptive method to install the damp-course. We drill holes into the existing wall materials and gravity feed siloxane damp proofing liquid into the holes. The liquid soaks in and cures to create a layer of water-repellent silicone plastic within the masonry itself. There is little mess and we complete the job quickly, without demolition. We have formulated Ab-Tech Damp Proofing Liquid for this purpose.
Silicone treatment has been used in Europe since the 1960s. Many historic buildings are affected in Europe and a benign silicone damp-course is preferred for restoration and preservation projects.
SALTS, DRYING OUT & SALT REMOVAL
A new damp-course will stop the source of rising damp, however, the affected walls will still retain the moisture and salts that have accumulated over the years. Some of these salts will slowly move to the surface as the walls dry out, over a period of several months. The residual salt in the wall will concentrate near the surface, consequently we recommend that the old, porous, lime-based plaster be left in place to absorb some of these excessive salts.
During humid weather salt near the surface may attract moisture from the air and beads of very salty condensation may form. If you see such beading, rinse it off with a clean moist sponge, to help remove the salt. If the paint bubbles and plastered surfaces appear worse by becoming loose and flaky in the months after the damp-course treatment, it indicates that the salt content in the wall is too high and replastering is needed.
If the accumulated salt content in the walls is high and they are visibly affected by salt attack or have been covered with hard plaster, we recommend desalination treatment, such as a poultice plaster immediately after the silicone treatment. This poultice is very porous to help draw excessive salts and moisture from the walls during the drying-out period. Eventually the poultice plaster would be removed and replaced with the final plaster system. We can do the work, however, there is additional cost and inconvenience.
When repairing old buildings, our preference is to use soft, lime-based mortar in the final plaster system. This produces a porous and flexible plaster, similar to the original. Sometimes, if appropriate, we use a salt resistant waterproof plastering system that is non-porous and provides a barrier to keep the salt locked within the wall.
Before replastering, we would remove the damp-affected, crumbly and salt-saturated render and plaster to a level about 300mm above the "high-tide-mark" of the salt damp. If any other restoration is needed, we would remove fretting bricks, stones and mortar and replace these with matching materials, to blend in with the existing walls.
Any maintenance deficiencies such as leaking water and sewer pipes or faulty roof plumbing must be corrected first, to eliminate these causes of damp in walls. You should consult an architect, building professional, plumber or roofing specialist to help advise and correct such problems.
The other main cause of damp is from the rain soaking directly into porous external walls. If needed, we would apply Ab-Tech Damp Proofing Liquid to weatherproof the external surfaces. It makes the surfaces water-repellent, but allows the walls to breathe naturally after treatment. We do not use nor recommend impervious "surface seal" waterproofing treatments, varnish and paint, which trap moisture within the walls and do not allow the walls to breathe, possibly leading to problems later.
Damp in cellar walls and retaining walls is a much more difficult problem to treat. However, we can offer advice, if requested.