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Waterproofing Performance Solution: when the membrane detail is not in the Deemed-to-Satisfy provisions

The waterproofer has stopped, or the certifier has written: the window sill is inside the shower zone, the balcony door has no step-down, the carpark slab has no membrane, or the basement has no Australian Standard to follow. The design is not necessarily wrong. The Deemed-to-Satisfy (DtS) provisions do not describe it, so compliance is demonstrated against the Performance Requirement instead of looked up: a Performance Solution under NCC A2G2. We write them from Sydney, the Gold Coast and Melbourne.

What triggers a waterproofing Performance Solution

1. A window or glazing in a shower or wet area, with the sill below the height the wall must be waterproof to.
2. A balcony or terrace threshold below the DtS step-down, usually for level access.
3. Omitting or substituting the membrane on a slab or at a movement joint, typically a trafficked suspended slab.
4. A basement. Australia has no below-ground membrane standard, so there is no DtS path; the design is made to BS 8102:2022.

Unsure? Use our Performance Solution checker; cladding has its own page, facade Performance Solutions.

The Performance Requirements involved

Name the right requirement first.

• Internal wet areas: Volume One Part F2. F2P2 requires that water not penetrate behind fittings and linings or into concealed spaces; the DtS route is F2D2, Specification 26 and AS 3740-2021. Houses under Volume Two: H4P1 with Housing Provisions Part 10.2.
• External above-ground membranes: DtS is F1D5 in Part F1, materials to AS 4654.1 and design to AS 4654.2. The requirement answered is F3P1, weatherproofing, with F1P2 and F1P3 where drainage is part of the case.
• Below ground: the NCC asks for the result and leaves the method open; the relevant Part F1 Performance Requirement is answered with BS 8102:2022 as the reference.

Of the four A2G2 assessment methods (evidence of suitability, verification method, expert judgement, comparison with DtS), waterproofing solutions almost always run on expert judgement with comparison to DtS.

Case 1: a window in a shower or wet area

What the DtS says. Shower walls must be waterproof to 1,800 mm above the floor substrate: Specification 26 S26C3 for a Class 2 building, Housing Provisions 10.2.2 for a house. AS 3740-2021 clause 4.6.3(c) runs the membrane to 1,800 mm above the finished floor or 50 mm above the rose, whichever is higher. Glass is a deemed waterproof material; the aluminium frame is not.

Why the design departs. A sill below that line puts a frame where the DtS wall expects membrane and tiles. Check the datum: schedules dimension from the structural floor, the code from the substrate.

What the solution must show. Membrane over the sill, jambs and head of the reveal, bonded onto the frame around its full perimeter; a neutral-cure silicone bead at the tile-to-frame joint; a sill falling back into the shower at not less than 1:100; a sliding track kept draining outward; a wet-area substrate behind the tiles. The frame perimeter is treated as a sealed junction, like the doorway waterstop a membrane bonds to (see our cavity sliding door waterstop detail).

Case 2: a balcony or terrace threshold below the step-down

What the DtS says. F1D5 takes the balcony membrane to AS 4654.2, whose Table A1 sets the minimum upturn at the door line by site wind speed, cross-referenced to the AS 4055 class (derive the site class with the AS 4055 wind classification calculator or our termination height calculator):

Why the design departs. Level access and flush decks put the walking surface at the door line, so Table A1 cannot be measured from it.

What the solution must show. Table A1 assumes a solid surface. Over open-jointed decking or pavers on pedestals the water-shedding surface is the membrane below, so the solution keeps the Table A1 separation from the membrane's drainage surface to the top of the upturn and adds a full-width grated drain at the sill to stormwater, so water cannot stand against the door. The membrane runs under deck and drain, up into the sill rebate and behind the frame; doors are to AS 2047 at the project pressure. The deck stays open-jointed and the drains are cleaned six-monthly.

Case 3: no membrane on a slab, or a substituted joint detail

What the DtS says. F1D5 expects a membrane to AS 4654.2 across a suspended slab over occupied space. The structural drawings of a trafficked driveway or carpark deck show only the joint gap and slip joint.

Why the design departs. A full membrane under traffic is costly, easily damaged and hard to inspect. The case rests on engineering criteria answered against F1P2, F1P3 and F3P1, plus industry practice: shopping-centre carpark decks are built this way, and the NCC excludes open-deck carparks.

What the solution must show. Our criteria: no ponding, confirmed in writing by the principal contractor after a flood test or rainfall; durability through AS 3600 cover and crack control, backed by the structural certificate; every joint and drain junction waterproofed; cracks injected and sealed over promptly; the soffit left exposed; water-sensitive stock below kept covered. Joints and outlets get polyurea, and one typical joint detail serves movement and construction joints alike: a polyurea strip debonded over the joint under a vehicle-rated metal cover plate fixed on one side. Reglet, outlet and pipe collar details complete the set (see detailing around floor wastes).

Case 4: a basement

What the DtS says. Nothing. AS 3740 covers wet areas and AS 4654 membranes above ground; neither covers ground water. The reference is BS 8102:2022, which a basement Performance Solution in NSW will usually cite.

What the solution must show. The grade follows the use of the space (Table 2): 1a where seepage and damp are tolerable; 1b no seepage, damp tolerable; 2 damp from condensation only; 3 no water or damp, with ventilation or dehumidification, for habitable rooms. The water table class comes from the geotechnical report (clause 5.1.3). They set the protection: Type A, a barrier membrane; Type B, water-resisting concrete with waterstops at every joint; Type C, a drained cavity with maintained pumps. Clause 6.2.3 calls for combined protection where the risk or consequence of failure is high. With no geotechnical report we design for a high water table and make the measured level a hold point before the slab is poured. Try the BS 8102 basement waterproofing calculator, and read wet versus dry basements.

What the report contains, and the conditions on site

The performance-based design brief records the requirement, assessment method and acceptance criteria, with a signature page for each stakeholder. The report measures the departure, quotes the Performance Requirement, works the analysis limb by limb, and lists the conditions as "shall" clauses with the verification the certifier will check. Then the details for the case.

The conditions are where a solution holds or fails. Hold points: membrane inspected before tiling, with film thickness and photographs; upturn measured before the door frame goes in; water table confirmed before a basement slab is poured. Tests: a timed spray test at a window; a flood test before decking or finishes with thresholds and outlets inside the test area; written confirmation of no ponding on an unmembraned slab. Nothing is substituted on site without re-review.

NSW: the Design and Building Practitioners Act

Waterproofing is a building element under section 6(1) of the Act, so on a Class 2 building the waterproofing design is a regulated design needing a design compliance declaration. There is no waterproofing class of registration, so the declaration comes from a practitioner whose class covers the work: we declare external waterproofing and weatherproofing solutions under the Facade class of DEP0001529, and internal wet areas on a new build sit in the architectural package we prepare for the architect to incorporate. The Act contemplates declaration before the work is carried out, so the solution belongs in the declared set, not in a document written later to explain what was built. More on our waterproofing design declaration page and under waterproofing consultancy.

Fees and turnaround

Bands are the middle of our own fee book, indicative and ex GST. Every project is quoted on its brief; turnaround runs from the day the drawings and site data are in.

Common questions