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Design Philosophy

It is not always possible to design or produce the perfect system, no matter how much money is available. Certainly, the more we can spend, the easier it is to reduce risk but, because perfection is not possible to achieve, it would take an infinite amount of money to achieve a totally risk free system.

When water tables are low, or where external drains exist, moisture is held in the structure by capillary action alone, and defects in the system will tend to manifest themselves only where the defect occurs. Furthermore, because there is no pressure behind the moisture, flooding will not occur as a result of a defect with the usual signs being point of isolated dampness or salting issues. Localised remedial works are usually effective in this instance, provided the full extent of the defective area is discovered.

However, when water tables are high, it is a very different story. With undrained systems such as Type A form of waterproofing, even the tiniest defects will result in water being pushed through by the hydrostatic pressure. This may manifest itself in the area where the defect has occurred but, when the membrane is applied externally, the water can travel within the structure, and may manifest itself at a point some distance from where the defect occurs. If repairs are attempted locally, this can, and usually does, result in the leak reappearing at a point adjacent to the repair which can be frustrating.

Where water tables are high, water will penetrate even the minutest defect so, in order to be completely watertight, a waterproofing system is going to have to be defect free. Defect free means perfect, and perfection is not something easy to achieve so the choice of waterproofing form must be a reflection of site conditions now and anticipated in the future.

BS8102 says we must always assume a high water table, even if there is none present at the time of survey. This implies that we are going to have to achieve perfection in our waterproofing systems.

It is important to accept, at design stage, that defects may occur. To negate the affect of these defects, we need to incorporate two important concepts into the design:

  1. Firstly, we must choose a design which will have a low risk of failure.
  2. We have to accept that some defects are likely to occur, so we MUST design the system in such a way that either the defects have no effect possibly by the inclusion of a maintainable correctly positioned external land drain, or we are able to access those defects and undertake remedial works as with internally applied systems.