For years, much of the public discussion about waterproofing defects has focused on the parts of buildings people can see: bathrooms, balconies, roofs, podiums and facades.

Below-ground waterproofing has received less attention, but anyone who works in the field knows it is often where the consequences are most difficult to unwind.

Basements, lift pits, retaining walls, plant rooms, storage areas and below-ground car parks are exposed to water in its most persistent forms. Water moves through soil, builds behind walls, rises through substrates, tracks through construction joints and changes behaviour with weather, drainage conditions and adjacent development. Once the structure is complete, the waterproofing is usually buried, concealed or built over.

That is why below-ground failure is rarely a simple maintenance issue. It can affect structural durability, services, finishes, air quality, occupant amenity, storage use and long-term asset value. It can also be extremely expensive to investigate and remediate, because the failure point is often inaccessible.

The industry has not been blind to this. Experienced consultants, engineers, builders and specialist contractors have been managing below-ground risk for decades through project specific judgement, international guidance, manufacturer testing, drainage design, inspection regimes and practical site experience.

The weakness has been the absence of a clear Australian compliance pathway.

Australia still does not have a dedicated Australian Standard for below-ground waterproofing.

The commonly referenced AS 4654 framework deals with external waterproofing above ground level, not basements and other below-ground structures. International guidance such as BS 8102 has been useful, but it is not a referenced Australian Deemed-to-Satisfy solution.

The result has been inconsistency. On some projects, below-ground waterproofing has been treated as a specialist compliance question from the outset. On others, it has been left too late, reduced to a product selection, pushed into subcontractor scope or treated as a construction detail rather than a performance requirement.

NCC 2025 changes the posture of the conversation.

The important shift is not that the code suddenly discovers below-ground water. The risk has always been there. The shift is that the water management requirements in Volume One are consolidated around a revised Performance Requirement, F1P1, and the term “water” is defined more broadly for Section F. That definition includes surface water, sub-surface water, rainwater, stormwater, rising damp, water services overflow and surface water seepage.

For below-ground construction, that matters. The language now better reflects the way water behaves in real buildings. A basement wall is not only dealing with rainwater or drainage in neat isolation. It may be exposed to groundwater, perched water, surface water seepage, rising damp and changing hydrostatic conditions across the life of the building.

The practical implication is that project teams will need to be clearer about how below-ground waterproofing satisfies the relevant Performance Requirements. There is no simple Deemed-to-Satisfy pathway that covers basement walls, below-ground slabs and substructures exposed to sub-surface water. In most cases, the answer will need to be a Performance Solution supported by evidence, design reasoning and verification.

That will change expectations across the project chain.

Certifiers are likely to ask earlier and sharper questions about the compliance pathway.

Designers will need to document the intended water exposure, design strategy, system limitations and basis of suitability. Builders will need to maintain the quality assurance record that supports the design. Subcontractors will need installation conditions, sequencing and protection measures to be treated as compliance-critical, not optional.

Suppliers will need stronger evidence behind claims relating to hydrostatic pressure, durability, compatibility and below-ground use.

The bigger change is cultural. Below-ground waterproofing cannot keep being pushed to the end of the design process. The right answer depends on site conditions, excavation, structure, drainage, access, sequencing, intended use, maintenance expectations and consequence of failure. Those decisions are made early, even when the waterproofing discussion happens late.

NCC 2025 should therefore be read as a prompt for earlier coordination. It gives certifiers a clearer basis to ask for evidence. It gives consultants a stronger reason to insist on project specific assessment. It gives builders a reason to protect installation quality and documentation. It gives owners and developers a better question to ask before construction begins: if water enters this part of the building, what was the compliance strategy supposed to be?