The Process of Waterproofing a Class 2 Building
Waterproofing a Class 2 building is not a single trade activity. It is a design-led, multi-stage process that spans from initial specification through to final compliance certification — with defined hold points, testing requirements, and regulatory obligations at every step. Getting it wrong is expensive. In NSW alone, waterproofing defects remain the single most common category of building defect reported in strata buildings.
What Is a Class 2 Building?
Under the National Construction Code (NCC 2022), a Class 2 building is a building containing two or more sole-occupancy units (SOUs), each being a separate dwelling. In practical terms, this covers residential apartment buildings — from a two-storey walk-up with four units to a 40-storey tower with 300+ apartments.
Class 2 buildings may also include single-storey attached dwellings above a common basement or car park. The defining characteristic is that people live above and below each other, which means any waterproofing failure on one level has the potential to damage the unit below.
The technical requirements for Class 2 buildings are governed by NCC Volume One. Waterproofing provisions fall primarily under Part F1 (surface water management and external waterproofing) and Part F2 / Specification 26 (wet area waterproofing). The relevant Australian Standards are AS 3740:2021 for internal wet areas and AS 4654.1-2012 / AS 4654.2-2012 for external above-ground areas.
Design Stage
Waterproofing in a Class 2 building begins at the design stage — well before any membrane touches a substrate.
Specification and Material Selection
The waterproofing specification is prepared as part of the architectural and hydraulic documentation. It nominates:
- The membrane system for each application — internal wet areas, balconies, podiums, roofs, planters, and basement tanking
- The membrane type — liquid-applied, sheet membrane, or hybrid systems — selected based on the specific exposure conditions, substrate type, and expected movement
- Product-specific requirements — manufacturer, product name, and any CodeMark or WaterMark certifications
- Detailing requirements — junction treatments, penetration details, turn-up heights, drainage provisions, and overflow design
Design Declarations Under the DBPA
In NSW, the Design and Building Practitioners Act 2020 (DBPA) requires that waterproofing designs for regulated buildings (which includes all Class 2 buildings) be prepared and declared by a registered design practitioner. The design compliance declaration confirms that the design complies with the Building Code of Australia and that it has been coordinated with other relevant regulated designs.
These declarations are lodged on the NSW Planning Portal before building work commences. Waterproofing is specifically called out in the DBPA — unlike many other trade elements, it is not exempt from declaration requirements, even for minor alterations to bathrooms and kitchens (unless the work is both exempt development and relates to a single dwelling only).
Construction Stages
Once design is complete and declared, construction proceeds through a defined sequence. Each stage must be completed and verified before the next begins.
1. Substrate Preparation
The substrate is the surface that receives the membrane. For a Class 2 building, this is typically a concrete slab (for floors and balconies) or compressed fibre cement sheet (for wall linings in wet areas). Preparation involves:
- Grinding or shotblasting to remove laitance and achieve a suitable surface profile
- Repairing cracks, honeycombing, or surface defects with compatible patching compounds
- Forming hobs at shower entries (minimum 25 mm step-down) and wet area thresholds
- Confirming falls — minimum 1:100 for external areas per AS 4654.2, and graded to floor waste puddle flanges for internal areas per AS 3740
- Allowing adequate drying time — concrete substrates must have a moisture content below the membrane manufacturer's specified limit before application
2. Priming
A compatible primer is applied to the prepared substrate. Primer selection must match both the substrate type and the membrane system — using the wrong primer is a common cause of adhesion failure. The primer must be applied at the manufacturer's specified coverage rate and allowed to cure before membrane application.
3. Membrane Application
The membrane is applied in accordance with the manufacturer's instructions and the relevant Australian Standard:
- Internal wet areas (AS 3740:2021) — liquid membranes are applied in two coats at minimum, with each coat applied in opposing directions. Reinforcing tape or fabric is embedded at all internal corners, external corners, and change-of-plane junctions
- External areas (AS 4654.2-2012) — liquid membranes must achieve a minimum dry film thickness (DFT) of 1.5 mm (typically 2.0 mm is specified for durability). Sheet membranes are lapped and sealed according to the manufacturer's requirements, with minimum overlap widths observed
4. Flashings and Penetrations
Every penetration through the membrane is a potential failure point. Standard details include:
- Floor waste puddle flanges — the membrane is dressed into the flange and clamped with the grate assembly, creating a watertight connection
- Pipe penetrations — sealed with purpose-made collars or formed membrane flashings
- Wall-to-floor junctions — membrane turn-ups to a minimum of 150 mm above finished floor level at wet area perimeters, and to 1800 mm or 50 mm above the shower rose (whichever is higher) on shower walls
- Door thresholds and sills — external membrane dressed to the underside of door sills with appropriate drip edges and back flashings per AS 4654.2
5. Drainage
Drainage design is integral to the waterproofing system. For external areas, AS 4654.2 requires:
- A minimum surface fall of 1:100 to drainage outlets
- Overflow provisions designed for a 1-in-100-year storm event, increased by 50% for potential debris blockage, with 100% redundancy (twice the number of overflows calculated by the hydraulic engineer)
- Overflows positioned at a level below the membrane termination height at door thresholds
Common Wet Areas in a Class 2 Building
A typical Class 2 building contains numerous waterproofed areas, each with different exposure conditions and applicable standards:
- Bathrooms and ensuites — internal wet areas waterproofed to AS 3740:2021 (Specification 26 of NCC Volume One)
- Laundries — floor and wall junctions waterproofed where a floor waste is present
- Kitchens — water-resistant treatment required at sink locations and floor junctions where a floor waste is installed
- Balconies — external waterproofing to AS 4654.2, with particular attention to drainage design and door threshold detailing
- Roofs and roof terraces — external waterproofing to AS 4654.2, exposed to the full range of environmental conditions including UV, thermal cycling, and wind-driven rain
- Podiums — large horizontal external areas, often over habitable space or car parks, requiring robust membrane systems with drainage and overflow provisions
- Basements and below-ground structures — tanking systems designed to resist hydrostatic pressure, typically to AS 3735 for concrete water-retaining structures
Hold Points During Construction
Hold points are mandatory pauses where construction cannot proceed until the waterproofing has been inspected and signed off. For a Class 2 building, the standard hold points are:
- Hold Point 1: Post-substrate preparation — substrate surface, falls, and hob construction verified before primer application
- Hold Point 2: Post-membrane application and curing — membrane coverage, thickness, junction treatments, and penetration details verified before any protective screed, topping, or tiling begins
- Hold Point 3: Post-flood test — flood test completed and results documented before tiling or finishes proceed
If work proceeds past a hold point without inspection and sign-off, the membrane is concealed and can no longer be verified without destructive investigation. This is one of the most common — and most expensive — failures in waterproofing quality management.
Sign-Off and Compliance Certification
Once all waterproofing is installed, inspected, and tested, the compliance pathway involves:
- Waterproofing compliance certificate — issued by the licensed waterproofing contractor, certifying that the work was carried out in accordance with the specification and relevant standards
- Independent inspection report — prepared by the waterproofing consultant or engineer, documenting the inspection findings, test results, and compliance status
- DBPA building compliance declaration — lodged by the registered building practitioner on the NSW Planning Portal, confirming that the building work (including waterproofing) complies with the approved designs
- Occupation certificate — the certifying authority relies on the above documentation when issuing the occupation certificate under the Environmental Planning and Assessment Act 1979
When Things Go Wrong: Remedial Investigation
Waterproofing failures manifest as water ingress — staining, mould, efflorescence, timber rot, or active leaking in the unit below. When this occurs, the investigation process typically involves:
- Non-destructive assessment — moisture mapping using thermal imaging and electronic moisture meters to identify the extent of water penetration without opening up the structure
- Destructive investigation — targeted removal of tiles, screed, or wall linings to expose the membrane and identify the failure point
- Root cause analysis — determining whether the failure is due to a design deficiency, installation defect, material failure, or structural movement
- Remedial scope and specification — preparing a detailed rectification scope, with a new waterproofing specification for the affected areas
- Cost estimation — preparing a schedule of rates or lump sum estimate for the rectification work, which may be used in contractual claims or NCAT proceedings
Remedial waterproofing is invariably more expensive than getting it right the first time. The cost of stripping tiles, removing screed, remediating the membrane, and reinstating finishes can be five to ten times the cost of the original waterproofing work. For a building with systemic defects across multiple units, the total rectification cost can reach into the millions.
This is why the process matters. Design declarations, hold point inspections, independent testing, and structured documentation are not regulatory overhead — they are the mechanisms that prevent these outcomes.