AS 3740:2021 is the Australian standard governing waterproofing of domestic wet areas. It is not a performance standard in the sense of specifying membrane chemistry; it is a coverage standard. It tells you where the membrane must go, how high it must run, and what conditions the substrate must meet before the membrane is applied. Most wet area failures investigated in defect claims are not membrane failures in the material sense. They are coverage failures, sequencing failures, or substrate failures that the membrane was never going to overcome.
This guide works through the standard's requirements zone by zone, then addresses the details that leak investigations consistently identify as the points of failure.
What AS 3740 Actually Specifies
AS 3740 divides wet areas into two broad categories based on water exposure. A shower recess is the highest-exposure zone. Bathrooms outside the shower, laundries and toilet rooms sit in a lower category, though they still require membrane coverage at defined locations.
For a shower recess, the standard requires:
- The entire floor area of the recess waterproofed
- Walls waterproofed to a minimum 1800 mm above the finished floor level on the three shower walls (or to the ceiling where the ceiling is lower)
- The floor-to-wall junction waterproofed continuously
- All penetrations through the membrane waterproofed
For bathrooms outside the shower, the requirement is more targeted:
- The floor within 150 mm of a bath or shower tray must be waterproofed
- The floor-to-wall junction must be waterproofed to a height of at least 150 mm on all walls, and to 1800 mm on any wall within 1500 mm of the shower rose
- The area around floor waste must be waterproofed
Laundries require waterproofing at the floor-to-wall junction to 150 mm on all walls, the floor within 150 mm of any trough or appliance connection, and the area around floor wastes.
These are minimum extents. A certifier or building surveyor may require more depending on the NCC deemed-to-satisfy pathway being used and any additional conditions in the relevant state's building regulations.
Substrate Preparation: The Step That Determines Everything Downstream
No membrane performs to its datasheet specification on a substrate that is not ready for it. AS 3740 requires the substrate to be sound, clean, dry and free from surface contamination. In practice, that means:
- Concrete or masonry substrates must be free from laitance, form release agents and curing compounds. Mechanical preparation, either grinding or scarifying, is typically required to achieve adequate bond.
- Fibre cement sheet substrates must be installed with the correct fastener pattern and joint treatment before the membrane is applied. Joints between sheets must be filled and reinforced with membrane-compatible tape.
- Screeds must be cured, structurally sound and not dusty. A screed laid the day before membrane application is not cured. Most membrane manufacturers require a minimum of 24 to 48 hours, and some require longer depending on screed thickness and ambient conditions. Check the product datasheet, not the programme.
- Moisture content matters. Cementitious substrates with elevated moisture content can cause blistering or adhesion failure in sheet membranes and some liquid membranes. A moisture meter reading is not a substitute for the manufacturer's specified test method, but it is a useful screening tool.
Surface defects, voids and sharp edges must be made good before membrane application. Liquid membranes bridge small cracks but are not designed to span structural movement or fill voids. If the substrate has honeycombing, spalling or delamination, those conditions need repair before waterproofing begins.
Movement Joints and Bond Breakers at Internal Corners
The floor-to-wall junction in a wet area is a movement joint location. Concrete slabs move. Walls move. They do not move together, and the junction between them is where the differential movement concentrates. A membrane applied continuously across an internal corner without accommodation for movement will crack at that corner under normal building movement, often within the first year.
AS 3740 requires that internal corners be treated with a coved fillet or a bond breaker, depending on the membrane system being used. The purpose is the same in both cases: to prevent the membrane from being bonded to two substrates moving independently of each other.
- Coved fillets are typically formed from a polymer-modified mortar or a compatible fillet compound. The fillet converts the sharp 90-degree internal corner into a radius, which distributes stress across a larger membrane area rather than concentrating it at the apex.
- Bond breakers are strips of material, often polyethylene tape or a compatible backing material, applied to the corner before the liquid membrane coat. The membrane bonds to the bond breaker rather than directly to both substrates, allowing relative movement without tearing the membrane.
Which approach is appropriate depends on the membrane system. Refer to the membrane manufacturer's installation instructions. Using a fillet compound that is not compatible with the membrane, or applying a bond breaker tape that the membrane cannot adequately wet, creates a detail that looks correct but is not.
The same principle applies at any internal corner where two substrates meet at different planes, including the junction between a hob and the shower floor, and the junction between a wall and a built-in bath.
Hobs, Thresholds and Penetrations
Hobs are a common source of water escape because they are a three-dimensional detail that requires membrane continuity across multiple planes. The membrane must cover the floor of the shower, turn up and over the hob, and return down the outside face. Any gap in that continuity, particularly at the internal corners where the hob meets the shower walls, is a path for water. The hob detail must be completed before any tile adhesive or tile work begins.
Thresholds without hobs, where the shower is level-access, require particular attention to falls and to the membrane extent beyond the shower boundary. The membrane must extend far enough beyond the shower edge that any water tracking under the tile adhesive bed is still captured. The falls in the substrate must direct water toward the waste, not toward the threshold.
Penetrations through the membrane, primarily waste outlets and any pipe penetrations through the floor or walls, must be waterproofed back to the membrane. For floor wastes, this typically means the waste flange is set at the correct height relative to the finished tile surface, the membrane is dressed into the waste collar, and the waste clamp ring is tightened to compress the membrane against the body of the waste. A waste set too high or too low relative to the tile surface makes a watertight connection at the membrane level difficult or impossible to achieve. This is a substrate and setting-out problem, not a membrane problem, but it causes membrane failures.
Screed Falls and the Waste Detail
The membrane, the screed falls and the waste detail are interdependent. A correctly applied membrane with poor falls will pool water on the membrane surface. Ponded water at the membrane level, rather than draining through the waste, extends the contact time between water and any vulnerable detail. Falls in the screed must direct water to the waste at a minimum gradient of 1:60 to 1:80 across the shower floor. Steeper falls, in the range of 1:50, are preferable in showers where the waste is positioned in a corner rather than centrally.
The waste outlet must be positioned so that the falls in the screed converge at the waste, not adjacent to it. A waste set 50 mm from the lowest point of the screed, because the pipe position was fixed before the screed was laid, means water pools beside the waste rather than entering it. That pooled water finds the next available path, which is often a penetration or a corner detail.
The Sequencing That Gets Skipped
Under programme pressure, the steps most commonly skipped or compressed are:
- Substrate cure time before membrane application. Screeds applied and membraned within 24 hours are a recurring finding in defect investigations.
- Multiple membrane coats applied wet-on-wet. Liquid membranes require each coat to cure before the next is applied. Applying both coats in a single visit, or applying the second coat before the first has dried, produces a membrane that has the appearance of two coats but the performance of one.
- Membrane cure before tile adhesive. Tiling onto a membrane that has not fully cured can disrupt the membrane film and reduce adhesion. The membrane manufacturer's minimum cure time before overcoating must be observed.
- Inspection hold points. In jurisdictions where wet area waterproofing is a mandatory inspection stage, the membrane must be inspected before it is covered. Where inspection is not mandatory, a photographic record of the completed membrane before tiling is basic risk management for the waterproofer and the builder.
Where Investigations Find the Failures
Leak investigations in wet areas consistently identify the same locations:
- Internal corners at the floor-to-wall junction, where bond breakers were omitted or fillets cracked
- The waste-to-membrane connection, where the membrane was not dressed into the waste collar or the clamp ring was not tightened
- The hob-to-wall junction, where the membrane was not returned continuously across the corner
- The threshold, where falls directed water toward the door rather than the waste
- Penetrations through walls, particularly shower rose outlets and tapware backing plates, where the membrane was cut back from the penetration rather than dressed around it
In most cases, the membrane material itself is intact where it was correctly applied. The failure is at a detail, not across the field of the membrane.
Specifying and Ordering the Right Products
The membrane system must be selected to suit the substrate type, the exposure conditions and the tile adhesive system being used. Compatibility between the membrane, the fillet compound, the reinforcing tape and the tile adhesive is not automatic. A membrane that is not compatible with the tile adhesive can cause adhesive failure, which looks like a tile adhesion failure but originates at the membrane surface.
Fosroc's waterproofing range, available through CSA, includes products suited to wet area applications across a range of substrates and exposure conditions. Product selection, coverage rates and application requirements are specified in the manufacturer's datasheets, which should be read before specifying, not after the product is on site.
For builders, waterproofers and inspectors looking to specify wet area waterproofing products or review the technical data for a specific membrane system, the product range and trade pricing are available at constructionsupplies.group/au. Trade account enquiries can be directed to the CSA trade desk.