Cellar Tanking is a term used to describe the waterproofing of a cellar, it is often applied as part of a cellar conversion project or may be installed on its own to create a dry storage space.
By definition, cellar tanking systems are bonded to the walls and floor of an otherwise wet cellar, with the aim of holding back water or dampness, to produce a ‘waterproof tank’.
Ground water contains dissolved salts, particularly Nitrates, Chlorides and sometimes Sulphates. They can affect cement based tanking systems in two different ways:
* Sulphate Attack of Cement
* Growth of Salt Crystals (Chlorides and Nitrates)
1. Sulphate Attack
This is a reaction between the Sulphate and the Calcium Hydroxide and Calcium Silicate hydrate in the cement paste, it results in cracking, expansion and loss of bond between the cement paste and the aggregate with ultimate failure of the tanking system.
Sources of Sulphate include:
* Seawater (so be wary in coastal areas)
* Oxidation of Sulphide in adjacent clay
* Bacterial action producing Sulphur dioxide, which dissolves in water and then oxidises to produce Sulphuric acid.
2. Growth of Salt Crystals.
These can grow within or behind the render of the tanking system itself causing the render to crumble or delaminate. Why do salt crystals grow and what do they look like? The answer to this is simply that ground water contains dissolved salts and as the water evaporates the salt gets left behind, just like it does on a kettle element. So any ‘breathable’ render will allow salt crystals to be deposited at the point of evaporation, which may be within or behind the render. They are normally white in color but sometimes have a yellowy tint. If water is evaporating from the surface of the wall they will appear as wispy filaments that often get mistaken for mold.
In order to avoid the risk Sulfate attack the ground water should be tested for Sulfates, and if found to be present, a tanking system should be selected which is non-cement based. If cement based tanking material must be used, for whatever reason, then a salt inhibitor based on Zinc hexafluorosilicate (also known as Zinc hexafluoride – ZnSiF6) can be used, it is available under various brand names and is easily obtained from most specialist material suppliers. This should be applied to the masonry before the cellar tanking slurry or render. Particular attention should be paid to the health and safety data which will normally be supplied with the the product.
In order to avoid damage by the growth of salt crystals a tanking system can be selected, which is non-breathable, a bonded sheet membrane would fit this category but be aware of limited ability to hold back water pressure. A non-bonded plastic or foil lined cavity drainage membrane is the best, as these allow water pressure to dissipate – see my other article on ‘cavity drainage membranes’.
Of course anything that does not breathe is at risk of condensation in certain circumstances and I have written another article on this very issue.
