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26 July 2021

Is Durability in the Building Code Up to Scratch?

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New Zealand is unique among jurisdictions that have performance-based building codes in having an explicit code clause covering durability for all building elements. In most other codes, durability is implicit and generally applies only to structural components.

The framework is relatively clear in having two basic requirements. The first (in B2.3.1) is deciding which of the three defined durability lives of building elements applies (5, 15 and the life of the building but not less than 50 years). Calculating the life are based on three criteria;

  • Whether they provide structural stability,
  • The degree of difficulty in accessing and replacing the building element and;
  • The degree of detectability of failure of those elements

The second (in B2.3.2) is ensuring that where building elements which are components of a building system and are difficult to access or replace must either:

  • All have the same durability or
  • Be installed in a manner that permits the replacement of building elements of lesser durability without removing those elements of greater durability and are not specifically designed for removal and replacement

The relevant lives are all bound by being subject to ‘normal’ maintenance.

Like all Code clauses there are acceptable solutions and verification methods to provide pathways for compliance. The verification method B2/VM1 outlines test methods and other means of evidence to establish compliance of a particular building element. The acceptable solution B2/AS1 cites various Standards (with some modifications) to establish durability lives of some generic materials (e.g. timber, concrete and steel). It also has some good guidance covering maintenance and guidance (decision tree and examples) on the application of B2.3.1 and B2.3.2 and the tests within it to establish specific lives of building elements.

However, there are some interesting and potentially problematic aspects to applying the durability provisions. They are:

  • The limits on application on the Code clauses set the start of the life of a building element as being the issue of the Code Compliance Certificate (CCC), so what is the durability life of an element in building work done under Schedule 1 or done without a consent where one was required? There is no CCC under these circumstances and if a certificate of acceptable is issued, should it cover durability?
  • What is ‘normal maintenance’? Is it what a building owner should do or what they actually do? For residential buildings in particular, the gap between these two is growing as our free time is swallowed up by the numerous other activities on offer as well as the busyness of modern life
  • Care needs to be taken in applying the examples given in B2 as being the Code. For instance, B2.3.1 (b) (15 years) includes as one of its examples for 15 years durability the ‘building envelope’. Some people interpret that as being all (non- structural) claddings and the things that go behind it (e.g. underlays) only need 15 years durability. B2/AS1 introduces the concept of ‘very durable’ building elements (e.g. brick veneer) and quite correctly concludes that underlays behind brick veneers (and masonry tiles) since their failure would go undetected during both normal use and maintenance of the building, need to have a durability life of B2.3.1 (a) (‘the life of the building but not less than 50 years‘).

Beyond these practical challenges with the durability provisions of the Code there are some strategic issues that need addressing, they include:

  • If the Code is the “public expectation” of the performance of buildings, do they really expect some elements such as weatherboard cladding and guttering to only have durability lives of 15 and 5 years respectively? I don’t think so!
  • If we are serious about building for climate change we should be incentivising the building sector to maximise the design lives of building elements (in general amortising any carbon used in manufacturing and logistics over a longer life as well as well as reducing demolition waste)
  • The meaning of the term ‘the life of the building’ and the implications. The average economic life of NZ buildings is 70 years. Are we designing buildings to spend their later years in a degraded state?

Despite its wrinkles, the durability provisions of the Code has served us well for 30 years, but maybe it is at the end of its durability life and a major refurbishment is needed!

Through Building Confidence Ltd, I provide building regulatory advice on matters relating to the requirements of the NZ Building Act including its regulations like The Building Code and associated documents — such as acceptable solutions and verification methods.

Visit or contact me on [email protected] to learn more.

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