NHBC Standard 5.4 Basement Waterproofing
As I understand it NHBC have approximately 70/80% of the structural warranty insurance market, and so they have felt the effects of basement waterproofing failures more than any other. More specifically, between 2005/6 - 2010/11 they published the detail of a spend within the region of £21m on failed basement waterproofing claims. As a result chapter 5.4 calls for design by qualified persons. Trace can assist with this.
NHBC started a basement campaign in 2013 to highlight the issues, with this including the publication of a new basement standard – ‘5.4 Waterproofing of Basements and Other Below Ground Structures’, released in December 2014.
NHBC are promoting an approach to waterproofing design and installation which falls broadly into line with the guidance detailed within British Standard 8102 (2022) Code of practice for the protection of below ground structures against water ingress, which advises design by specialist waterproofing designers, and specifications driven by an assessment of risk, primarily relating to site conditions and structure – could not agree more.
To NHBC’s credit however, they have pushed even further with specific requirements which will greatly assist in reducing problem basement waterproofing systems.
Firstly they state that installation should be by trained, qualified operatives. This will address the historic situation where tanking products were sold through building merchants, with general contractors/ground workers picking these up and then using untrained labour to install.
Another example relates to the use of ‘combined protection’, which is a section within BS8102, advising that where site conditions are aggressive (very wet) or the consequences of failure are dire – (generally the case within habitable basements representing the majority of NHBC’s insured property) – that two forms of waterproofing system be employed to provide suitably low risk levels.
Within the new standard ‘combined protection’ is stipulated for habitable space, wherever the retained earth is greater than 600mm depth, unless there is long term monitoring of site via bore holes, i.e. for more than one year, suitable number of holes and checks at min. 3 month intervals, indicating that water table is permanently below basement slab level. In which case they will accept a Type B integrally waterproof structure.
In addition, they state that for grade 2 environment (no water penetration acceptable, same as grade 3) and anything with retained earth greater than 600mm, tanking membranes which aren’t fully bonded, cannot be used in isolation. This is again significant because it prevents the use of external adhesive sheet membrane tanking systems in isolation, and based on statistics from the basement campaign presentation, tanking generally has provided NHBC with a greater number of claims (66%) versus the other forms of waterproofing. I personally see more issues with external tanking membranes than any other form of waterproofing.
In any case, the main factor is that beyond 600mm, they will not generally accept anything other than combined protection, unless the site is essentially assumed to be dry based on long term site investigation.
We would wholeheartedly agree with this approach. We recommend combined systems and design these in an efficient manner (manage risk while managing spend).
In any case, credit to NHBC for raising standards and protecting homeowners from disruptive claims.