Failed Tanking and Broken Business Models

At present we are completing a remedial waterproofing scheme within a residential basement, where we are installing cavity drainage waterproofing to address issues of penetration past the internal cementitious slurry tanking system.

Nothing particularly unusual about that, but it is significant to me at least, in that this takes our turnover remedying failures in that specific slurry tanking product, to over £250k excluding VAT.

Must be a terrible product??

No not at all, it is a polymer modified cementitious tanking slurry and we use these regularly, albeit typically as a detailing product or as part of a combined waterproofing system, rather than as a standalone waterproofing system in its own right.

While we may use a polymer modified slurry system from a different supplier, there is no technical reason why I would not use the product referred to above.

This is because the technology is essentially the same, slurry systems are thin (1-3mm) barrier coatings comprising graded aggregates, cements and chemicals. The polymer additive grants a degree of flexibility and enhanced bonding characteristics.

Thankfully, products will generally perform to specification (this product will resist a hydrostatic head of X metres etc. etc.), just as the manufacturers say they will, because of quality control typical in the manufacturing process.

Why therefore has so much been spent on remedial work to address failed installations of this particular cementitious slurry?

This is because in most cases it is not product failure; it is associated with the fact that waterproofing systems must be designed and installed correctly in order to function correctly.

This might sound simple, but if we take internal cementitious tanking as a relevant example, this form of waterproofing seeks to provide protection by forming a physical barrier to exclude / block water out of a basement space, so that a dry internal environment is created.

In order for this to be achieved, that barrier must be 'perfect', i.e. free of holes/defects, because like with any liquid holding 'tank', it will not hold or exclude water if it has holes in it.

In addition, where such barriers are applied to the internal face of a structure, water in the ground externally moving through that structure (from the outside in) will essentially bear upon the tanking at the internal face, applying a pressure which seeks to push the tanking away from the substrate upon which it is applied.

Because of this, the method is entirely reliant on the bond that is formed between the tanking product and the substrate upon which it is applied.

This means that the substrate must be suitably prepared (to provide a 'key') in order to receive the tanking. This takes a great deal of care, attention and understanding in the installation.

The structure itself must also be capable of absorbing the loads applied to it by hydrostatic pressure, because once you tank that structure, it is the structure (to which the barrier tanking has been bonded) that has to support the weight of water bearing against it.

In short there is a lot to consider and hence the requirement for care and attention.

Lets look at some examples where this was lacking:

In this example a render base-coat was applied over masonry, to provide a level substrate to receive the polymer slurry tanking system.  Not unusual, however the surface preparation, render specification and installation was totally incorrect with this resulting in water ingress, remedied by full strip out and replacement with internal cavity drainage waterproofing (click the first photo to browse through).  Homeowner had a guarantee certificate signed by installer (no longer trading) and product manufacturer, who said they don't guarantee workmanship (this without having even inspected), leaving the homeowner to remedy.  

A different property now, this one had screed laid over a cementitious multi-coat render system.  Water came past the render and caused the screed to heave and crack:

Render applied over weak brick substrate, the face of the brick de-laminates / comes away with the render:

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This (below) is an example of not considering the nature of the structure, the cellar flooded severely and the installer decided that cementitious tanking was the answer.  As a result tanking was applied over the existing floor construction (thin layer of concrete over the original historic floor) and when the ground filled with water resulting in hydrostatic pressure, the floor went into floatation resulting in differential movement (cracking) at the wall floor junction.  Once the tanking cracks, water can obviously penetrate.

So, a repair was attempted by installing a flexible product at the wall floor junction (white putty-like material), but this did not have the desired affect and the space flooded again.  We remedied by adding drainage channels and a reliable sump pump system.

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Not cementitious tanking, but a good example of the forces involved where hydrostatic pressure comes to bear.  In this case a bitumen sheet membrane has been applied to the internal face of a retaining wall within a cavity.  Bitumen membranes remain malleable and so when pressure came to bear, the membrane 'bowed' in, and pushed over the inner block leaf which was not tied back (so as not to perforate the membrane)