Waterproofing is an essential consideration for anyone building a basement that they intend to use as storage or habitable accommodation.
Too often basement waterproofing is not given high enough priority and as a result a cheap and inappropriate basement waterproofing option is used to try and get an unrealistic budget. This often leads to failure and the resulting losses can be disastrous for the owner of the building or property.
New-build basements are often designed and built from well-classified and even “waterproof” concrete blocks, concrete blocks, or concrete-filled blocks. A well designed and constructed structure will generally form the main resistance to water ingress, as concrete is too dense for water to pass through. However, we must always keep in mind that where there are construction joints there is a risk of failure even if the water bars, hydrophilic / hydrophobic strips are well installed.
With this in mind, it is always a recommendation that, in addition to the structure itself, a secondary form of basement waterproofing is adopted, even if the structure is new and / or built with ‘waterproof concrete’.
When the concrete construction itself is considered integrally waterproof, it is described as a type B form of basement waterproofing.
When the structure is not considered integral waterproofing, we have 2 options to deal with the potential for water ingress. One is Tank and the other is Waterproofing it with a Cavity Drainage Membrane.
Tanking a basement means that a product is applied internally or externally and is designed to physically stop and retain water. This is known as Type A basement waterproofing.
A cavity drainage membrane system is applied to the inner face of the earth containment structure where it can accept the ingress of water, depressurize it and manage it to a safe evacuation point. This is known as Type C basement waterproofing.
The tank as a form of waterproofing the basement for new structures has inherent risks. Whether applied internally or externally to the structure, tank systems must be applied 100% flawless to be effective, if water is supplied under pressure against the structure. The dispute and the High Court case between two companies, Outwing Construction and Thomas Weatherald, has set a precedent in the basement waterproofing industry and UK law. The judgment of the Superior Court of Justice was that it is not reasonable for a watertightness or waterproofing system to be applied 100% free of defects. What this means is that if you design or apply a basement waterproofing system to the structure that must be 100% defect free to function and fail, you may be liable for the consequences of that failure. By their very nature, all tank systems must be 100% defect free to function when tested with pressurized water from the ground.
Most new-build basement waterproofing designs that incorporate an external tank system also include a ground drain, the purpose of which is to help dehydrate the surrounding ground areas and reduce the level of water that is discharged against the structure. There are often many problems associated with dirt drains used in basement waterproofing. An earth drain should be placed on the exterior of the structure below the level of the internal slab to help relieve the volume of water to the full depth of the retained soil. Many times the position of the earth drain is shown on a drawing or installed above the level of the internal slab and therefore could only be partially effective.
“The form and feasibility of corrective treatment” is a buzzword in the basement waterproofing industry. It means that if there is a problem, having the ability to go back to something to identify where and what the problem is and then rectify it. This is a problem with both external tanks and onshore drains, as they are often buried under tons of soil. The size of the land’s drainage is another problem, since who can accurately predict the volume of water that it could be expected to hold at any time in the future? Also, as stated, the tank system can be highly dependent on the drainage of the land and how someone can guarantee it against defects or blockages in the future; again, the form and feasibility of corrective treatment are in question.