Australia’s approach to defining and achieving durable structures could do with a better grounding in the fundamentals. By Craig Kay, national product engineer, Tilling

“How long is a piece of string?” is often a colloquial or humorous response to a question such as “How long will it take?” or “How big is it?” when the length or size is unknown, infinite, variable or relative. In the domestic building construction arena, this answer could often be used when someone asks the question “What Design Life of a structure does the performance-based NCC mandate?”

Unlike our friends across the ditch in the land of the long white cloud, the NCC refrains from setting firm numerical values on the required Design Life of the structure. If one does a search of the words ‘Design Life’ in the combined NCC 2022 Volume Two & Housing Provisions, the only references that appear are within Queensland-specific clauses dealing with termite management.

The basis of and/or assumptions about Design Life used in the NCC instead have been outlined in a ‘Durability in Buildings’ Guideline Document released by the ABCB in 2007 in response to comments and concerns expressed by government, industry and the community that relate to the built environment.

It is not my purpose in this article to debate whether the use of the Durability in Buildings Guideline document is a good way to approach this topic or otherwise, but I do want to explore how we can use its contents to better understand the background fundamentals that underpin the Design Life methodology in the NCC in the absence of defined numerical targets.

This guidance document introduces a number of terms and concepts, and unless we understand exactly what they mean, wrong conclusions may be drawn. Some of the fundamental definitions are included below.


This means the period for which a building, or a building element or subsystem is expected to fulfil its intended function.

Table 1 (above right) outlines the philosophy inherit in the NCC concerning the Design Life of buildings and components.

The Design Life of buildings should be taken as Normal for all building importance categories unless otherwise specified.

Although there is a general correlation between the category of building Design Life and the importance level of a building, the concepts should not be confused. An important building may have a short Design Life and conversely an unimportant building may have a long Design Life.

Examples of a building with a short Design Life are a temporary building on a mining lease or a pavilion at an expo. Examples of buildings with a long Design Life are monumental building structures or buildings of high importance such as town halls and other municipal centres.

Examples of sub-systems ‘not accessible or not economical to replace or repair’ are structural frames. Examples of sub-systems with ‘moderate ease of access but difficult or costly to replace or repair’ are roof cladding systems or gutter and downpipe systems.

I could be very wrong, but I don’t think that it is clearly understood by most users that while the target Design Life of a structure may be 50 years, there could be components or subsystems within that structure that may have had maintenance carried out several times or may have even been replaced multiple times during those 50 years.

This brings us to the next definition that is vitally important for us to understand the underpinnings of the NCC.


This means the capability of a building or its parts to perform a function over a specified period of time. Durability is not simply an inherent property of a material or component. It is the outcome of a complex interactions among all the factors contained in this list:

a: the service conditions;

b: material characteristic;

c: design and detailing;

d: workmanship and

e: maintenance.

I must confess that for many years until I specifically studied this issue, I saw durability solely as individual component issue, not as a holistic approach for the entire building,

The addition of maintenance and inspection as an integral part of achieving the Design Life of the building was also new to me, despite working for 15 years as a local government engineer. It was not something that I see written into design briefs, but according to the NCC documentation, the maintenance and inspection required for buildings or their components to achieve their Design Life should be clearly specified.

The guidance document suggests that the identification of essential maintenance, as distinct from maintenance for appearance only, should be considered for critical elements affecting health and safety. Examples of durability problems that can be overcome by appropriate maintenance are regular removal of corrosion deposits by cleaning, renewal of elements of short Design Life, testing of components for proper functioning, etc.

It is interesting that the New Zealand Acceptable Solutions and Verification Methods for New Zealand Building Code clause B2 Durability, goes further and breaks down the need for maintenance to achieve the expected durability of a building element into two quite distinct categories:

  1. Normal maintenance: the document lists eight (8) normal maintenance tasks but is not limited to only those listed.
  2. Scheduled maintenance: this comprises the inspection, maintenance and reporting procedures for building elements required to have a compliance schedule.

Durability design should consider the natural durability of the materials as well as maintenance and inspection, if used. In those cases where maintenance cannot be or is not expected to be carried out, design should be managed carefully so that deterioration will not lead to failure.

In Australia, I don’t think our building designers or regulators, at least in the domestic building space, clearly define what maintenance needs to be undertaken or what subsystems may need to be replaced in the assumed Design Life of the structure, but I’m happy to be corrected if I’m wrong.

Craig Kay is the national product engineer for Tilling. For more information on this topic, contact Craig Kay and the Tilling engineers via email at