NCC 2022 vs. NCC 2025: The Critical Shift in External Slab Compliance

As we transition from NCC 2022 to NCC 2025, we need to understand the shift in the design and documentation of external slabs, balconies, and roofs. While weatherproofing has always been a core focus, the new code moves from general performance to highly prescriptive structural mandates.

If you detail balconies or podiums, here's a breakdown of how requirements have evolved.

1. The Mandatory 70mm Structural Step-Down

The most significant addition in NCC 2025 Part F1D4 is a prescriptive vertical requirement for the concrete setdown itself.

• NCC 2022: The set-down was often dictated by the chosen door threshold detail and the waterproofing termination height required by AS 4654.2. There was no specific "hard-coded" minimum for the structural slab-to-slab drop.

• NCC 2025: Mandates a minimum 70mm step-down from the surface level of the internal structural substrate to the surface level of the external structural substrate.
  Crucial Distinction: This 70mm is a structural requirement, not a waterproofing one. It is independent of the waterproofing termination height specified in AS 4654.2. If AS 4654.2 requires a termination height greater than 70mm, then you need to comply with AS 4654.2 in addition to the minimum structural setdown requirement using a suitable compliant method.

2. Substrate Falls:

• NCC 2022: Requires a minimum substrate fall of 1:100. (This fall could be achieved either through the slab or screed)

• NCC 2025: Tightens the requirement to a steeper 1:80 structural fall. (The fall needs to be on the structural slab)

Impact: The steeper fall demands a greater vertical depth in your set-down. For a 3-meter balcony run, you now need nearly 40mm of fall in the slab alone, before factoring in finishes.

3. Floor Finish Integration

NCC 2025 places greater emphasis on preventing water stagnation within the floor build-up, a common cause of long-term structural damage and efflorescence.

• NCC 2022 focused on compliance with AS 4654.2.

• NCC 2025 requires the floor finish to be either:

  • Directly fixed to the waterproofing membrane (which is applied to the structural slab). Or

  • By using a Self-Draining system. (The system must be truly self-draining to ensure water reaches the membrane and flows to the outlets without obstruction. The most common solutions used for a self-draining system are a pedestal system or decking. )

4. The 70mm Monolithic Hob that is integral with the structural substrate

The most significant structural change involves how we treat the perimeter of external slabs.

• NCC 2022: Allowed various hob constructions, often leading to retrofitted hobs that were prone to leaking at the junction.

• NCC 2025: The external perimeter of the slab needs to have a 70mm high monolithic hob that is integral with the structural substrate. This requirement applies to all roofs, balconies, and podiums (unless the perimeter drains directly into a gutter).

5. Balcony Overflows & Invert Levels

To protect internal spaces, the relationship between the overflow and the internal floor level is now strictly regulated.

• NCC 2022: Was not prescriptive about overflows and their invert levels.

• NCC 2025: Specifies the requirement of overflows and that the invert level(the bottom) of the overflow must be installed at least 20mm below the adjacent internal floor's structural slab substrate level.

Summary Table: NCC 2022 vs. NCC 2025

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