At this point in time, it is relatively common knowledge in our construction industry that a waterproofing system should be applied to a substrate with falls to drains in order to achieve suitable performance and lifespan of the system. This close connection between waterproofing, water-shedding and performance outcomes is appropriately explained in the TAFE NSW online course ‘Waterproofing Design Principles’ released this year. However, the question “but what is the minimum we can do and still achieve compliance with the NCC” is still voiced daily on construction sites when structural falls are identified to be insufficient.
This review is intended to address that question directly and in order to do so we have to start at the top of the NCC compliance hierarchy with ‘performance requirements’. Performance requirements typically define an outcome such as ‘water shall not enter the building’ and there are effectively two pathways for demonstrating that a design and/or construction has satisfied this requirement. The first being the use of a deemed-to-satisfy (DTS) solution and the second being the development of a performance solution. It is critical to note that where a design does not comply with DTS provisions adopted by the NCC, it will be considered a performance solution by default. This will be discussed further in the context of waterproofing external areas of a Class 2 residential project.
Typically considered the simplest option is a solution following DTS provisions, in this case that is AS4654.2:2012 is a standard provided by Standards Australia that has been adopted into the NCC. The provision for falls is covered with Part 2.5.2. Falls, that states a fall must exist to ensure water drains to the drainage outlet and the fall gradient should be 1:100. This wording of this provision is not overly detailed on the fall being located at the waterproofing level or the floor finish level and often leads to contention.
This wording of this provision is not overly detailed on the fall being located at the waterproofing level or the floor finish level and often leads to contention. Statements such “the standard does not say falls are needed at the waterproofing level so we will just put a fall in the floor finish screed” are often made to justify unsuitable construction works.
This reasoning is a fallacy that does not align with the NCC functional structure we established earlier and will result in non-compliant design/ construction.
This is due to the standards referenced by the NCC being deemed-to-satisfy provisions, meaning that if something is shown/stated in the standard it can be done and will be compliant. However if something is not clearly shown/stated then we must effectively fall back to consideration of performance and fit for purpose. So the next question is, is it possible to verify that a design without 1:100 falls at the membrane level provides sufficient performance to be considered a Performance Solution and compliant with the relevant NCC performance requirements? There are a few things to consider here.
The NCC provides the following requirement for performance solutions to be successful ‘the solution is at least equivalent to the Deemed-to-Satisfy Provisions’. Since AS4654.2:2012 states that there must be a fall in the substrate we can say that any performance solution must also include a fall to be considered suitable.
Many waterproofing materials currently available in the marketplace are not suitable for scenarios without falls that would result in ponded water. This is often clearly stated in the product data sheet to ensure the intended performance is achieved and where this is ignored will result in a voided product warranty and accelerated deterioration of the waterproofing system. A product that does not provide a warranty period for a specific scenario is typically not considered fit for purpose and would not be a valid performance solution.
Where fall does not exist to shed water to drains at the waterproofing level ponding will occur, often resulting in accelerated or other unwanted consequences such as efflorescence and debonding of finishes build-up.
In summary, where the need for fall at the membrane level is not clearly defined in the DTS provisions we must instead consider the design on a performance basis.
As there are numerous issues with a design that does not include water shedding it would often be difficult to justify that the design provides performance suitable to be considered a performance solution. Because of this we would say that most often a design without suitable falls at the membrane level is noncompliant with NCC performance requirements.
Any optimistic interpretation of requirements and provisions contrary to this is easily refuted will carry significant risk to the designer and builder.
David Previte - Managing Director
This article was originally published in the Australian Institute of Waterproofing Newsletter, Issue 21, January 2023