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Wet Area Waterproofing Documentation Under AS 3740

ParitySense team · September 2026 · 6 min read

AS 3740 governs the waterproofing of wet areas in residential buildings — which surfaces must be waterproofed or water-resistant, and how membranes, falls, junctions and drainage are to perform. Waterproofing documentation is compliant when those requirements are actually drawn: membrane extents marked, falls dimensioned to drainage points, setdowns shown in both the architectural and structural sets, and the specified system consistent across every document that touches the wet area. Most sets do not meet that bar. They invoke the standard in a note and draw almost none of it.

The "to AS 3740" note is not documentation

The most common wet-area documentation on residential projects is a single annotation: "Waterproofing to AS 3740 and NCC requirements." It reads as compliance and functions as delegation — every decision the drawings don't make (how far the membrane runs up the wall behind the vanity, whether the laundry floor is membraned, where the fall breaks between the shower and the door) is being made by the installer on the day. Some installers make them well. The defect statistics say plenty don't: waterproofing failures consistently rank among the most common and most expensive residential defects in Australian data, and NSW's building reforms singled waterproofing out as a category needing mandated design detail. The pattern the regulators are pushing toward is the right one anyway: draw it, don't cite it.

What the set must show, and which document shows it

• Designated wet areas. Every room the standard treats as a wet area, identified as such on plan — including the marginal ones (laundries, WCs) where the water-resistance vs waterproof distinction changes the answer. The standard specifies different treatment for different surfaces and areas; the drawings must say which regime applies where.
• Membrane extents. Floor extent and wall heights for each wet area, drawn or scheduled — shower walls, hob or hobless junctions, and the perimeter overlaps. "Extent as required by AS 3740" is the note that guarantees a site argument.
• Falls. Direction and target: floor falls to the floor waste, shower falls to the shower waste, dimensioned or specified as a ratio. The reconciliation trap is hydraulic waste locations that moved at a revision while the architectural fall arrows still point at the old position.
• Setdowns and thresholds. The structural set must show the slab setdown or rebate that the architectural wet-area detail assumes. A 25 mm setdown drawn on the architectural detail and absent from the structural slab layout is the single most expensive version of this defect: it is discovered at pour or after, and the fix is grinding, screeding to a compromised threshold, or a step the accessibility documents cannot accept — see accessibility documentation for why hobless accessible showers make this worse, not better.
• Junctions and penetrations. Bond breakers, wall-floor junctions, penetration sealing at wastes and taps — detail references on plan, details in the set, products in the spec, all pointing at the same system.
• The system itself. The specification names a membrane system and its certification; the details must be buildable with that system (a sheet membrane detail with a liquid-applied spec is a real and recurring mismatch).

The reconciliation checklist

1. List every wet area from the architectural plans. Confirm each has: a detail reference, a membrane extent (drawn or scheduled), falls to a drainage point, and a threshold treatment.
2. For each, check the structural set shows the setdown or rebate the detail assumes — value and location, not just presence.
3. Check each hydraulic waste location against the architectural fall direction, on current revisions of both.
4. Check the spec's membrane system against the drawn details and the finishes schedule (tile bedding and the membrane must be a compatible system; slip-rated finishes in wet areas are their own schedule check).
5. Re-run the comparison whenever the architectural, structural or hydraulic set revises. This is mechanical cross-referencing — exactly the work that belongs in an automated check that re-fires on revision, as in the ParitySense discipline checks — with a human deciding which flagged gaps are real and raising the RFIs.

Why the paperwork standard is rising

Waterproofing sits behind walls and under tiles; it is certified largely on documentation and inspection records, and litigated years later from the same paper. A set that actually shows extents, falls and setdowns protects three parties at once: the installer (who builds what is drawn instead of guessing), the certifier (who assesses what is documented instead of assumed), and the builder (whose defence in a defect claim is the drawing, the inspection record and the RFI trail). The cost of drawing it properly is hours of documentation time. The cost of not drawing it is the most common defect category in the country. The broader method for making these checks systematic is in the compliance checking guide.

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