What Is AS 3740 and Why Does It Matter?
AS 3740-2021 (Waterproofing of domestic wet areas) is the Australian Standard that sets out the minimum requirements for waterproofing in residential buildings. In Victoria, compliance with AS 3740 is referenced by the National Construction Code (NCC) and is mandatory for all new construction, renovations, and repairs involving wet areas.
For Melbourne homeowners, this standard is not optional. Whether you are building a new home in Brighton, renovating a bathroom in Richmond, or repairing a leaking shower in Brunswick, the waterproofing work must comply with AS 3740. Non-compliant work can void your building insurance, create structural damage worth tens of thousands of dollars, and leave you liable if you sell the property.
The 2021 revision of AS 3740 introduced several important changes, including updated terminology, clearer definitions of wet areas, and revised requirements for membrane termination heights. Any waterproofing work carried out in Melbourne today must comply with the 2021 version of the standard.
Scope of AS 3740-2021
AS 3740 applies to the waterproofing of wet areas in Class 1 and Class 10 buildings under the NCC. This includes detached houses, townhouses, duplexes, and associated structures such as garages and carports (where applicable).
The standard covers: - Internal wet areas: bathrooms, shower recesses, laundries, toilets, and any room with a floor waste or water-using fixture. - External wet areas: balconies, decks, terraces, and rooftop areas that are trafficable and exposed to weather. - Junctions and penetrations: all joints between floors and walls, pipe penetrations, and fixture connections within wet areas.
It does not cover below-ground waterproofing (which is addressed by AS 3740 Supplement 1 and AS 2870), nor does it cover commercial buildings of Class 2-9, which have additional requirements under the NCC.
For Melbourne properties, the most common applications are bathroom renovations, new shower installations, balcony waterproofing, and laundry fit-outs. Every one of these projects requires AS 3740-compliant membrane installation by a licensed waterproofer.
Wet Area Definitions Under AS 3740
AS 3740 defines specific zones within wet areas, each with different waterproofing requirements. Understanding these zones is critical for both homeowners and contractors.
A "wet area" is any area within a building that has a floor waste or is subject to water overflow, splashing, or hosing. The standard distinguishes between:
1. Shower areas: The floor and walls of a shower recess, plus the area within 1500mm of the showerhead when no shower screen is present. 2. Bath areas: The area beneath and surrounding a bathtub, extending to 150mm above the finished floor level at minimum. 3. Bathroom floors: The entire floor of a room containing a shower, bath, basin, or toilet, including the area beneath vanity units and behind toilets. 4. Laundry floors: The entire floor of a room containing a washing machine, laundry tub, or other water-using appliance. 5. Balconies and decks: Any external trafficable area attached to the building that is exposed to weather or has a fall to a floor waste.
In Melbourne, the most contentious wet area is often the bathroom floor outside the shower recess. Many homeowners assume only the shower needs waterproofing, but AS 3740 requires the entire bathroom floor and a minimum height up the walls to be waterproofed with a continuous membrane.
Membrane Requirements
AS 3740 specifies that waterproofing membranes must form a continuous, impervious barrier across the entire wet area floor and extend up the walls to the required height. The standard does not mandate a specific membrane product but requires all membranes to comply with AS 4858 (Wet area membranes).
Key membrane requirements include:
Bond-breakers: Membranes must incorporate bond-breaker tape at all internal corners (floor-to-wall junctions) and at any change of substrate. This allows the membrane to flex without cracking when the building moves.
Minimum thickness: Liquid-applied membranes typically require a minimum dry film thickness (DFT) of 1.0mm for bathrooms and wet areas. Some manufacturers specify higher thicknesses for specific applications.
Reinforcing: All internal corners, external corners, junctions, and penetrations must be reinforced with membrane tape or fabric embedded in the membrane. This is one of the most commonly failed inspection points in Melbourne.
Continuity: The membrane must be continuous across the entire wet area floor with no gaps, pinholes, or missed areas. It must extend through doorways to a height of at least 25mm above the finished floor level or to the bottom of any hob.
Compatibility: All membrane components (primer, membrane, reinforcing, sealants) must be from the same manufacturer or certified as compatible. Mixing products from different manufacturers is a common cause of membrane failure.
In Melbourne's climate, where temperatures can range from near-freezing to over 40 degrees Celsius, membrane flexibility and UV stability are particularly important for external applications like balconies and roof decks.
Wall Height Requirements
One of the most critical and frequently misunderstood aspects of AS 3740 is the required height of wall waterproofing. The 2021 revision updated several of these requirements.
Shower walls: The membrane must extend to a minimum of 1800mm above the finished floor level (FFL) on all shower walls, or to the full height of the wall if the showerhead is installed above 1800mm. Where a shower screen is not installed, the membrane must extend to 1800mm on all walls within 1500mm of the showerhead.
Bath surrounds: The membrane must extend to a minimum of 150mm above the rim of the bathtub on the wall adjacent to the bath. Where a shower-over-bath is installed, shower wall heights apply.
General bathroom walls: The membrane must extend to a minimum of 150mm above the finished floor level on all walls within a bathroom, including behind toilets, under vanity basins, and at doorways.
Laundry walls: The membrane must extend to a minimum of 150mm above the finished floor level on all walls. Behind a laundry tub or washing machine connection, the membrane should extend to at least 150mm above the water connection point.
These heights represent minimums. Many Melbourne waterproofers recommend exceeding these heights, particularly in shower recesses where steam and splash can drive moisture above the 1800mm mark. Full-height tiling over a full-height membrane is considered best practice.
Hob and Threshold Requirements
AS 3740 has specific requirements for hobs (also called step-ups or thresholds) at wet area doorways. These requirements prevent water from flowing out of the wet area into adjacent rooms.
Shower hobs: A minimum 5mm step-down (hob) is required at the shower entry when a shower screen is installed. Without a shower screen, the hob must be sufficient to contain water within the shower area, and the membrane must be continuous over the hob.
Bathroom doorway hobs: Where a bathroom floor is lower than the adjacent corridor or room, a minimum 25mm upstand or hob is required at the doorway. The membrane must extend over the top of the hob and down the outside face to at least the finished floor level of the adjacent room.
Alternatives to physical hobs: AS 3740 permits alternatives such as recessed thresholds or embedded water stops, provided they prevent water egress. In Melbourne renovations, this flexibility is particularly useful when dealing with existing floor levels that cannot accommodate a traditional hob.
Balcony hobs: At the junction between a balcony and the interior of a building, the membrane must extend to a minimum of 100mm above the finished floor level of the internal area, or a physical hob must be installed to prevent water ingress.
Incorrect hob installation is one of the top three reasons waterproofing inspections fail in Melbourne. Ensure your waterproofer understands and implements the correct hob detail for your specific situation.
Inspection and Testing Requirements
Under the NCC and Victorian building regulations, waterproofing in wet areas must be inspected before tiling or any covering is applied. This inspection is a mandatory hold point, meaning that no subsequent work can proceed until the waterproofing has been inspected and approved.
In Melbourne, waterproofing inspections are typically carried out by: - A registered building inspector or building surveyor - The council building inspector (for owner-builder permits) - A private building surveyor engaged by the builder or homeowner
The inspection checks for: 1. Membrane continuity across all floor and wall areas 2. Correct wall height coverage 3. Proper reinforcing at all corners, junctions, and penetrations 4. Correct hob installation and membrane termination 5. Adequate membrane thickness (flood testing may be used) 6. Correct fall to floor wastes (minimum 1:80 for floors, 1:60 for channels) 7. Compatibility of all membrane system components
Flood testing: While not mandatory under AS 3740, flood testing is recommended and commonly requested by Melbourne inspectors. This involves flooding the wet area to a depth of approximately 25mm for a minimum of 24 hours and checking for leaks from below. Many reputable Melbourne waterproofers include flood testing as standard practice.
If the inspection fails, the waterproofer must rectify the defects and request a re-inspection before any tiling can commence. Never allow a tiler to begin work before the waterproofing inspection has been passed.
Waterproofing Certificates and Documentation
Upon completion of compliant waterproofing work, a waterproofing certificate should be issued. This certificate is a critical document for Melbourne homeowners for several reasons:
What the certificate should include: - The name and licence number of the waterproofing contractor - The address and specific areas waterproofed - The membrane system used (manufacturer, product names, batch numbers) - The date of application and the date of inspection - Confirmation that the work complies with AS 3740-2021 - The certificate of compliance number - Photos of the completed membrane (best practice)
Why you need this certificate: 1. Building compliance: Required for building sign-off on new builds and renovations in Melbourne. 2. Insurance: Your building insurance may be voided if waterproofing was not certified as compliant. 3. Property sale: Buyers and their solicitors increasingly request waterproofing certificates during conveyancing. Missing certificates can delay or reduce a sale price. 4. Warranty: Most membrane manufacturers require certified application for their product warranty to be valid. 5. Dispute resolution: If waterproofing fails, the certificate provides evidence of the responsible contractor and the system used.
Always request and keep your waterproofing certificate. Store it with your other building documents, as you will need it if you sell the property, make an insurance claim, or need warranty work.