Think damp and the instant thing that springs to mind is water. However, contaminant salts can also cause damp problems, and lead to unsightly surfaces and crumbling walls. Soluble salts known to cause damage to building masonry include chlorides, carbonates, nitrates and sulphates of calcium, magnesium, potassium, sodium sulphate and magnesium chloride (City of Adelaide, Department of Environment and Natural Resources, 1997)
Hygroscopic (water attracting) salts include chloride and nitrate salts (which often contaminate ground water), carbonates and ammonium (which can be caused through the combustion of fossil fuels and wood). Former farm buildings can be particularly susceptible to nitrates as a result of animal waste and farming activities (e.g. using fertilizers) Buildings can also be contaminated with sulphate salts, which can cause efflorescence on wall surfaces. Technically speaking, sulphate salts are soluble but not hygroscopic. That said, as they take on water, they behave in a similar way to hygroscopic salts.
Common causes of salt-contamination include:
- Windbourne salt spray if the building is near a sea or river.
- Pollution from nearby factories
- Biological factors such as bird droppings, leaves in guttering or sewer leakage
- Brick clay puddling (salts used in the process leach into the soil)
- Unsuitable chemicals used for cleaning
Testing can indicate which salts, if any, are present in your building. Some salts will be visible (for example, showing on painted surfaces – a process known as efflorescence). Others may be harder to detect, but can still cause damage by penetrating from below the surface (sub-fluorescence), which is why it’s important to use a PCA approved contractor/surveyor when checking for salt contamination.
Treating Salt-Contaminated Buildings
If hygroscopic salt is present in your plasterwork, it will attract water even if a damp course is fitted. This is why it’s essential to remove any contaminated plaster before starting any repair work: otherwise, you’re just delaying the problem. The wall should be re-plastered using a plastering system that does not allow salts to migrate to the surface. An additive such as Renderpel can be added to your render to help keep salts at bay.
However, don’t rush to re-plaster immediately after removing salt-contaminated plaster. The Department of the Environment advisory leaflet No 58 recommends that re-plastering should be delayed for a long as possible to allow soluble salts to move from the brickwork. Delaying re-plastering also allows more residual moisture to dry out so you can more easily establish the level of dampness caused by the hygroscopic salts.
If you are short on time, re-plastering can occur shortly after the completion of damp proofing. However, if this is the case, the plaster work must be able to hold back hygroscopic salts during the drying out period. Any waterproofing additives must allow the passage of water vapour.
Wall plaster should be allowed to dry out before redecoration. Temporary redecoration such as a coat of emulsion may be carried out but more extensive redecoration shouldn’t take place until the walls have fully dried out.
If you have a problem with damp, it makes sense to get the work done by a PCA-approved contractor unless you are dedicated to DIY. That way, you can ensure work is done to the required standard and avoid making potentially costly mistakes.
For products to deal with salt issues please see our Salt Neutraliser and Renderpel Render additive.