Basements and cellars should ideally be waterproofed on the positive side of the walls during the construction phase. However, either this does not happen or at some stage the waterproofing fails.
Under these circumstances it is necessary to waterproof the insides of the walls (negative side) in a process called “tanking”.
Surprisingly, national building regulations have no specific reference to the waterproofing of basements, cellars and other sub-terranean structures. There is, however, in SANS 10021:2006 a reference to “dry” and “wet” sites inferring that there needs to be additional measures undertaken in “wet” sites or wherever water tables are likely to be affected by seasonal rains.
For “dry” sites it is usual to apply one layer of 4mm APP torch-on membrane to the walls or a suitable liquid-based polyurethane waterproofing. The success of torch-on membrane is based on full adhesion of the membrane to the underlying substrate and the careful sealing of all overlaps. Due to the weight of the rolls, it is common for lengths of membrane to be cut from the roll, increasing the number of overlaps that have to be sealed.
Polyurethane waterproofing products are more than capable of handling the additional pressure of “wet” sites, but when using torch-on membrane it is advisable to use 2 layers, with the second layer positioned in such a way as to protect the underlying laps.
Once this initial waterproofing fails, it is almost taken for granted that there can be no remedial action on the positive side. In many cases there are adjacent roads or walkways or simply backfill that has been applied all around the building. The only way of treating moisture ingress is, therefore, from the negative side of the walls.
There are few waterproofing products that can handle negative water pressure. Torch-on membrane, cold bitumen and polyurethanes are all prone to separation from the substrate under such conditions. The most common alternative is the use of a cementitious product in a process called “tanking”.
Depending upon the depth underground and the degree of water pressure, this can normally be achieved using a cementitious slurry such as our SWS Slurry. 2-3 coats of slurry will need to be applied to the walls once plaster has been removed. NEVER apply slurry over plaster especially if this plaster is already contaminated with salts. This is inviting failure. Always pay particular attention to the wall/floor joint which is the weakest point in the structure, and preferably re-inforce this using Fillet Seal. Cut a “V” joint into the wall at the base and pack this area with the Fillet Seal mixed with water or diluted Bondaid Plus. You may also need to apply several coats of slurry to the floor if this too is leaking. It is absolutely vital that each layer of product fully cures before application of an additional coat.
There will be occasions, however, when normal waterproofing products are either inappropriate or inadequate to deal with the presence of subterranean moisture.
In these circumstances it is necessary to seek an alternate solution which accept the presence of moisture and merely channels it to a means of extraction.
One such product is Cavity Drainage Membrane (CDM).
The underground vaults in the Roman Baths in Bath, England were the subject of a major project run by Safeguard Europe. These vaults were some considerable distance underground and were constantly under water as a result of the high water table and lack of waterproofing.
Since there was considerable water pressure on the existing walls, it was decided to address the problem by installing a Cavity Drainage Membrane system. The CDM allowed water to be channelled to a self-actuating pump which then evacuated the moisture as long as it was present. By fitting the CDM system, it was possible to provide new walls and floor which gave a totally dry appearance, even though beneath the surface water penetrating the original walls and floor was being effectively drained.
Rolls of CDM are joined together with a special waterproof double-sided tape. Plugs attaching the CDM to the wall are waterproofed using a further waterproof sealing rope around the head of the plug.
When joining two sections of CDM, apply the lower section over the upper section to ensure a watertight seal.
The finishing touch is the fitting of a pump in a sump dug into the floor and to which all moisture drains. From this self-actuating pump water is evacuated to the outside by means of piping. Remember that such pumps always require battery back-up or some other power source, particularly in South Africa!
