tanking defect

Defects, Defects and Defects (BS8102).

As intimated in this post, British Standard 8102 Code of practice for the protection of below ground structures against water from the ground, is the primary design guide which we and others rely upon in our work designing and installed structural waterproofing systems. As an ‘approved code of practice’ it has an elevated legal status versus other design guides and hence its importance.

One of the key considerations listed is that of defects and remedial measures. In essence designers should assume the risk that any given waterproofing system might not be installed defect free, with risk of defects being associated either with workmanship or the products installed.

What this means is that when designing, we and others MUST think about what will happen in the event of the completed system not being 100% defect free (perfect), and at the same time to consider how to mitigate those defects, so that required standard of environment (how dry the space is) is created and maintained.

Going one step further, you also MUST consider a strategy for repair so that in the event of the intended environment not being created, you have a practical method by which you can put it right, but this is best covered in a separate post (this covers external tanking repair). Notwithstanding, I mention this here specifically because it helps to illustrates just how careful designers have to be, to comply with the standard.

Assuming that a system might be ‘defective’ sounds counter-intuitive, but if you can design around defects then it does not matter, you will provide a lower risk solution.

Examples would be including internal Type C cavity drainage waterproofing, which can satisfy this in several respects, i.e. The membrane element is not pressured by water and so any minimal defect / hole should not be of any consequence, or when combined with Type A tanking or Type B integrally waterproof structures, if they leak it does not matter because of the redundancy provided by the internal system.

Another example could be the use of effective land drainage in association with barrier tanking / integrally waterproof concrete structures, where that drainage prevents pressure from coming to bear on the structure/waterproofing, and like with cavity membranes, no pressure means no penetration through any defects present.

The importance of land drainage, namely with barrier tanking systems was proven in the High court case Outwing Vs Weatherald, which you can read about here and here. In a nutshell the land drainage with that particular design is considered to be an integral part of the waterproofing system.

Care should be taken as land drainage is not viable in all scenarios, which I mention in my post on assessing sites, but in any case, where ‘designs’ and installations are often lacking, is the assumption that systems will be perfect and free of defects.  This is illustrated by NHBC spend on claims, with the majority of this being on tanking failure, followed by waterproof concrete failure (source NHBC 2014 basement campaign presentation), where systems have been pressured by water and proven not to be perfect.

I think that historically the principle of perfect waterproofing systems has been pushed by manufacturers with a focus on barrier systems.  My experience includes having a representative from one of the major tanking specific manufacturers in the office, who when questioned on this point (consideration of defects) reflected disagreement with the BS8102 advice.

This in part is associated with the fact that systems are constructed which are severely tested by water and successfully exclude this.  No one here is claiming that it is not possible, but that as per the NHBC article, it is not a guaranteed outcome and when we are providing guarantees on the standard of environment provided, we aren’t going to take such risks.

The manufacturers tend to have no come-back when systems do fail because they do not accept design responsibility and do not install, and generally systems fail from a combination of either aspect.  I would think that the advice might differ if the shoe were on the other foot and long term guarantees (against penetration, not just product guarantees) were issued.

So there you have it, BS8102 advises that designers must consider risk of defects and having spent a large proportion of my career dealing with basement failures where it was not duly considered, I would certainly agree.

Because of such issues, movement in the industry is towards qualified waterproofing designers who carry P.I. (design) insurance and will take responsibility for design.  Some manufacturers already recognise the value in this and promote it.  The ideal is that these designers are also contractors who install, meaning no split liabilities and greater guarantee protection.  The up-front cost might be greater but it is lower cost than remedying, and we at least would be following the BS8102 guidance on defects.


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