By Franz Phan, Registered Design Practitioner (NSW) — 15+ years in multi-residential design and building compliance
Introduction
Waterproofing is the single most prevalent serious defect in NSW apartment buildings. According to the 2025 Strata Defects Research Report published by Building Commission NSW, 22% of surveyed Class 2 strata buildings had serious waterproofing defects — more than any other category, including fire safety systems at 16%. For developers, builders, and strata managers, this statistic is not abstract. Each percentage point represents buildings where water has penetrated membranes, degraded structural elements, and triggered remediation costs that routinely exceed $200,000 per building.
Over fifteen years of designing and documenting Class 2 residential projects across Sydney and Greater NSW, our team at giantA has seen waterproofing failures trace back to the same root cause: design coordination breakdown. The membrane product is rarely the problem. The problem is that the waterproofing design is treated as a contractor responsibility rather than a regulated building element requiring a declared design from a registered practitioner. Since 1 July 2021, the Design and Building Practitioners Act 2020 has changed that paradigm. Waterproofing in Class 2 buildings is now a regulated building element, and the compliance obligations are enforceable.
This article explains exactly what waterproofing compliance means for Class 2 buildings in NSW, how the regulatory framework under the DBP Act intersects with BCA Section F, what the NCC 2025 changes bring, and what developers, builders, and strata managers must do differently to avoid becoming another defect statistic.
Why Waterproofing Is a Regulated Building Element Under the DBP Act
The Design and Building Practitioners Act 2020 (NSW) defines a "building element" to include waterproofing alongside fire safety systems, building structure, building enclosure, and building services required by the Building Code of Australia. This classification has a specific legal consequence: any building work on a Class 2 building that involves a building element requires a regulated design prepared and declared by a registered design practitioner before construction can commence.
The practical implication is significant. A generic specification note referencing AS 3740 is no longer sufficient compliance documentation for a Class 2 project. The waterproofing design must be a discrete, detailed document — drawn, specified, and declared by a registered design practitioner who holds the appropriate class of registration. The declared design must then be lodged on the NSW Planning Portal before the relevant building work begins. A certifier cannot issue a Construction Certificate or Complying Development Certificate for regulated work without these declared designs.
For existing Class 2 buildings, the obligation is equally clear. Remedial waterproofing work — whether addressing balcony leaks, podium membrane failure, or internal wet area defects — triggers the same regulated design requirements. Strata managers and owners corporations who engage a waterproofing contractor directly, without first obtaining a declared regulated design from a registered design practitioner, are non-compliant under the Act.
BCA Section F: The Technical Framework for Waterproofing Compliance
The National Construction Code addresses waterproofing through Section F (Health and Amenity) of Volume One, which applies to Class 2 through Class 9 buildings. The key provisions are Part F1 (Water management) and Part F2 (Wet areas and overflow protection). These parts set out the Performance Requirements that waterproofing systems must meet, with Deemed-to-Satisfy provisions that reference Australian Standards for specific design and installation requirements.
Two Australian Standards are central to Class 2 waterproofing compliance:
| Standard | Scope | Application in Class 2 Buildings |
|---|---|---|
| AS 3740 | Waterproofing of domestic wet areas | Internal bathrooms, laundries, toilets, and shower areas within sole-occupancy units |
| AS 4654.1 | Waterproofing of above-ground external areas — Design | Balconies, podiums, terraces, planter boxes, and external above-ground surfaces |
| AS 4654.2 | Waterproofing of above-ground external areas — Installation | Installation requirements for membrane systems on external surfaces |
The distinction between internal and external waterproofing is critical because they involve different standards, different membrane types, different substrates, and different failure modes. A common design error is treating balcony waterproofing as an extension of AS 3740 wet area requirements when it actually falls under AS 4654, which has fundamentally different performance criteria and detailing requirements.
Below-grade waterproofing — basements, podium slabs at ground level, and retaining walls — is not covered by a specific Australian Standard in the same prescriptive manner. This gap makes below-grade waterproofing one of the highest-risk areas in Class 2 development, requiring performance-based design solutions that must be carefully documented as regulated designs.
What NCC 2025 Changes for Waterproofing in Class 2 Buildings
The NCC 2025 edition, which NSW will mandate from 1 May 2027 (with voluntary early adoption from 1 May 2026), introduces substantial changes to Section F that directly affect Class 2 waterproofing design. These changes represent the most significant overhaul of water management provisions in over a decade.
The key NCC 2025 changes include:
1. Restructured water management provisions — Part F1 has been reorganised to address both rainwater and groundwater risks holistically, including basements and external waterproofed areas that were previously under-specified.
2. New balcony and podium provisions for Class 2 — Specific Deemed-to-Satisfy provisions for concrete balconies in Class 2 apartment buildings have been added, addressing a long-standing gap where standard details that passed under NCC 2022 may now require review.
3. Stronger carpark waterproofing — Provisions for carparks in apartment and commercial developments have been strengthened, addressing the common failure mode of water ingress through podium slabs supporting landscaped areas.
4. NSW-specific variation for remedial works — NSW has introduced a waterproofing flexibility variation for apartment remedial works, recognising that retrofitting full compliance in existing buildings can be impractical. Developers and strata managers undertaking remediation should discuss this variation with their certifier early in the scoping process.
5. Hierarchy clarification — Where conflicts exist between NCC provisions and AS 3740 or AS 4654.2, the NCC provisions now take precedence. This resolves ambiguity that previously allowed inconsistent interpretations.
For projects with a Development Application formally lodged before 1 November 2026, building approval may proceed under either NCC 2022 or NCC 2025 until the DA expires. After that, NCC 2025 is mandatory. This transition window is critical for developers with projects in the pipeline.
The Compliance Workflow: From Design to Construction Certificate
Understanding the waterproofing compliance workflow is essential for avoiding the documentation gaps that lead to both regulatory non-compliance and construction defects. The process involves multiple practitioners and strict sequencing.
| Stage | Responsible Party | Deliverable | lodgement |
|---|---|---|---|
| Design development | Registered design practitioner (waterproofing) | Regulated waterproofing design with details, specifications, and performance criteria | — |
| Design compliance declaration | Registered design practitioner | Design compliance declaration confirming BCA compliance | NSW Planning Portal |
| Principal design practitioner coordination | Principal design practitioner | Integration check across all building element designs | — |
| Construction Certificate | Principal certifier | CC issued with declared designs verified | NSW Planning Portal |
| Construction stage | Builder + building practitioner | Installation per declared design, with as-built documentation | — |
| Variation (if required) | Registered design practitioner | New regulated design for varied work before construction continues | NSW Planning Portal |
A critical point that developers often miss: if a variation occurs during construction that involves a waterproofing building element — for example, changing the balcony membrane system from a sheet membrane to a liquid-applied membrane — a new regulated design must be prepared and declared before the varied work can proceed. This is not optional. Continuing construction without an updated declared design is a breach of the DBP Act.
Real Project Examples: What Goes Wrong and What Goes Right
Example 1: Balcony Membrane Failure — 24-Unit Development, Mascot
A Class 2 residential development in Mascot, completed in 2019 under the pre-DBP Act framework, experienced balcony water ingress affecting 18 of 24 units within two years of occupation. The root cause was a design coordination failure: the balcony waterproofing was specified generically in the architectural documentation as "membrane to AS 4654" without detailed junction designs at the sliding door threshold, balustrade fixings, or drainage outlets. The membrane contractor interpreted the generic specification using standard details that did not account for the specific slab edge geometry.
Remediation required removal and replacement of balcony tiles and membranes across all 18 affected units, costing approximately $480,000 and taking 14 weeks with residents in occupancy. Under the current DBP Act framework, this failure mode would be preventable: the generic specification would not satisfy the regulated design requirement, and the threshold junction detail would need to be drawn, specified, and declared by a registered design practitioner before construction.
Example 2: Below-Grade Basement Waterproofing — 36-Unit Development, Parramatta
A Parramatta development with two basement levels encountered groundwater ingress during construction when the below-grade waterproofing system — designed as a performance solution — was not coordinated with the structural slab design. The structural engineer had specified a 400mm thick basement wall, but the waterproofing consultant's membrane system required a 50mm rebate at the wall-to-slab junction that was not shown in the structural drawings. The result was a cold joint where water entered the basement carpark during heavy rainfall.
The project was delayed by 9 weeks while the junction was redesigned, the membrane system re-specified, and a new regulated design declared and lodged on the Planning Portal. The direct cost was $95,000 in remedial works, plus $40,000 in delay-related costs. This example illustrates why the Principal Design Practitioner role exists under the DBP Act: to ensure that designs from different practitioners are integrated and compatible before construction begins.
Example 3: Successful Compliance — 12-Unit Townhouse Development, Merrylands
A recent Class 2 development in Merrylands, designed and documented by giantA under the full DBP Act framework, demonstrates the compliance workflow working as intended. The waterproofing design was prepared as a discrete regulated design document with 14 detailed junction drawings covering balcony thresholds, internal wet area transitions, planter box integration, and the below-grade basement wall connection. The design was declared by our registered design practitioner and lodged on the Planning Portal before the Construction Certificate was issued.
During construction, one variation was required: the builder proposed substituting the specified sheet membrane with a liquid-applied alternative to accelerate the programme. A new regulated design was prepared, declared, and lodged within 5 working days, and the variation was implemented without delay. The project achieved practical completion with zero waterproofing defects identified at handover inspection — a result directly attributable to the detailed, coordinated, and declared design documentation.
Stakeholder Implications
For Developers
Developers bear the ultimate cost of waterproofing failures, both directly through remediation and indirectly through reputational damage and delayed settlement. The DBP Act compliance framework, while adding documentation cost upfront, functions as a risk management mechanism. Engaging a registered design practitioner with specific waterproofing expertise — not just a generic building designer — is essential. The cost of a properly prepared waterproofing regulated design typically ranges from $8,000 to $25,000 depending on project complexity, which is negligible compared to the $200,000 to $500,000 typical remediation cost for a multi-unit waterproofing failure.
For Builders
Builders must understand that under the DBP Act, they cannot deviate from the declared waterproofing design without triggering a new regulated design and declaration. The old practice of "we've always done it this way" substitutions is now non-compliant. Builders should review the declared waterproofing design at tender stage, flag any constructability concerns before contract execution, and ensure that the waterproofing subcontractor is engaged with the declared design as the basis of their scope, not a separate interpretation.
For Strata Managers
Strata managers are often the first to receive reports of water ingress from residents. The critical compliance point is that remedial waterproofing work on existing Class 2 buildings requires the same regulated design process as new construction. Engaging a waterproofing contractor directly, without first obtaining a declared regulated design, creates non-compliant building work that may not be covered by insurance and may not satisfy the Building Commission's requirements. Strata managers should ensure that any building work commissioned by the owners corporation is scoped with DBP Act compliance from the outset.
Frequently Asked Questions
Is waterproofing a regulated building element under the DBP Act?
Yes. Waterproofing is explicitly listed as a "building element" under the Design and Building Practitioners Regulation 2021. This means any waterproofing work on a Class 2 building requires a regulated design prepared and declared by a registered design practitioner, lodged on the NSW Planning Portal before construction begins.
Does the DBP Act apply to waterproofing remediation on existing apartments?
Yes. The DBP Act applies to both new and existing Class 2 building work. Remedial waterproofing projects — including balcony membrane replacement, wet area repairs, and basement water ingress treatment — require regulated designs prepared and declared by registered design practitioners before work commences.
What Australian Standards govern waterproofing in Class 2 buildings?
AS 3740 governs waterproofing of internal domestic wet areas (bathrooms, laundries, showers). AS 4654.1 and AS 4654.2 govern external above-ground waterproofing design and installation respectively (balconies, podiums, terraces). Below-grade waterproofing lacks a specific prescriptive standard and typically requires performance-based design solutions.
When does NCC 2025 become mandatory for NSW Class 2 projects?
NCC 2025 becomes mandatory in NSW from 1 May 2027. Voluntary early adoption is available from 1 May 2026. Projects with a Development Application formally lodged before 1 November 2026 may continue under either NCC 2022 or NCC 2025 until that DA expires. After 1 May 2027, NCC 2025 applies to all new building approvals.
What happens if a waterproofing variation is needed during construction?
A new regulated design must be prepared by a registered design practitioner, declared, and lodged on the NSW Planning Portal before the varied waterproofing work can proceed. Continuing construction without an updated declared design is a breach of the DBP Act and may invalidate insurance coverage and building certificate validity.
How much does a waterproofing regulated design cost for a Class 2 project?
For a typical 12 to 30 unit Class 2 development in Sydney, a properly prepared waterproofing regulated design costs between $8,000 and $25,000 depending on complexity, number of junctions, and whether below-grade areas are involved. This is a fraction of typical remediation costs, which range from $200,000 to $500,000 for multi-unit waterproofing failures.
Can a licensed waterproofing contractor prepare the regulated design?
No. A licensed waterproofing contractor can install the membrane system, but the regulated design must be prepared and declared by a registered design practitioner who holds the appropriate class of registration under the DBP Act. The contractor's installation must comply with the declared design, not a separate interpretation of the waterproofing requirements.
What is the most common waterproofing failure in NSW Class 2 buildings?
According to the 2025 Strata Defects Research Report by Building Commission NSW, waterproofing is the most prevalent serious defect category at 22% of surveyed buildings. The most common failure mode is water ingress through balcony membrane junctions and internal wet area transitions, typically caused by design coordination gaps rather than membrane product failure.
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This article is for informational purposes and reflects the regulatory framework as of August 2026. Always consult the current NCC, NSW legislation, and your certifier for project-specific compliance advice. For help with Class 2 building design and DBP Act compliance, contact giantA at he@gianta.com.au or visit gianta.com.au.
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