AS 3740 Explained

What is AS 3740?

AS 3740 is the Australian Standard for waterproofing of domestic wet areas. It sets out how bathrooms, ensuites, laundries and toilets in homes must be waterproofed, including which areas must be waterproofed, how far the waterproofing must extend, how the floor must fall and how penetrations, joints and corners must be treated. It is referenced by the National Construction Code, which means following it is a requirement for wet area work, not an optional extra.

For homeowners, AS 3740 is the reason a renovated shower should never leak. Most leaking showers we see in Morwell, Moe and Churchill homes were built before the standard was widely applied, or were built without following it.

This guide explains, in plain English, what AS 3740 covers and how it affects your renovation.

Why waterproofing matters

Tiles and grout are not waterproof. Grout is porous, and tiles can crack. Water that gets behind tiles soaks into the floor and wall framing, causing rot, mould, termite attraction, damaged plaster and, in homes on stumps, damage to the floor structure. The waterproofing membrane under the tiles is what stops this.

Wet areas and how they are treated

The standard identifies areas that must be waterproofed and those that must be water-resistant.

Showers

Inside a shower, the floor must be waterproofed, along with the walls to a set height. The junction between floor and wall must be waterproofed, as must any penetrations such as taps and shower arms.

Bathroom floors

Outside the shower, the bathroom floor must be waterproofed or water-resistant depending on the construction and whether there is a floor waste. Where the floor is timber or another moisture-sensitive material, the whole floor typically needs waterproofing.

Around baths

The walls around a bath must be waterproofed or protected to a set height above the bath. Where a bath is built in, the area beneath and around it must be treated.

Laundries and toilets

Laundries and toilets must be water-resistant or waterproofed in set areas, particularly where there is a floor waste.

Unenclosed showers

Showers without a full enclosure, such as walk-in showers with a fixed panel, require waterproofing to extend further, as water can spread across more of the floor.

Falls

The standard requires floors in showers and certain wet areas to fall towards the floor waste, so water drains rather than pooling. The fall must be enough to drain water but not so steep as to be unsafe underfoot.

Membranes

Waterproofing membranes come in several types, including liquid-applied membranes and sheet membranes. Each must be installed according to the manufacturer’s instructions, including primers, thickness, number of coats and curing time. Membranes must be compatible with the substrate, the tile adhesive and any heating mats.

Bond breakers and corners

Corners and joints in a bathroom move slightly as the building expands, contracts and settles. If the membrane is bonded tightly across these joints, it can tear. Bond breakers, such as a flexible sealant or tape, allow the membrane to stretch without splitting. This detail is one of the most important in the standard.

Penetrations

Every pipe or fitting that passes through the waterproofing, such as a shower mixer, shower arm, bath spout or floor waste, is a potential leak point. The standard requires these to be sealed with suitable flanges, collars or sealants.

Floor wastes and puddle flanges

The floor waste must connect to the membrane so water on top of the membrane drains away. A puddle flange around the waste provides this connection. Many older showers leak because there is no puddle flange.

Water stops

At the doorway or edge of a wet area, a water stop angle is installed to stop water escaping onto adjoining floors.

Who does the waterproofing?

Waterproofing is skilled work. We use licensed and insured waterproofers who follow AS 3740 and the manufacturer’s requirements.

Inspection and documentation

We photograph the membrane before tiling and provide documentation at handover. Keep these records with your house documents; they are useful for insurance and when selling.

How AS 3740 has changed

The standard has been revised over the years. Earlier versions required less waterproofing than the current version, particularly on bathroom floors outside the shower and around unenclosed showers. Many bathrooms built in Morwell, Moe and Churchill before these changes do not meet current requirements, even if they were built correctly for their time. When we renovate, we waterproof to the current version.

Common failures we see

When we open up old showers, we see the same faults again and again:

• No membrane at all, with tiles laid directly on a mortar bed or fibre-cement sheet.
• A membrane that stops short of the shower walls or does not extend up behind the hob.
• No bond breaker at the internal corners, so the membrane has torn.
• No puddle flange at the floor waste, so water runs down the outside of the pipe.
• Penetrations for taps and shower arms left unsealed.
• Floors with too little fall, or falling the wrong way, so water pools.
• Membranes applied too thinly, or tiled over before curing.

Each of these allows water into the structure, often for years before anyone notices.

The waterproofing sequence

In a renovation, waterproofing follows a set sequence.

1. The substrate is prepared: wall linings installed, floor screeded to falls, and surfaces cleaned and dried.

2. A primer is applied if the membrane system requires it.

3. Bond breakers are installed at internal corners and junctions between walls and floors.

4. Puddle flanges and penetration seals are installed.

5. The membrane is applied in the specified number of coats, with reinforcement at critical points.

6. The membrane cures for the time the manufacturer specifies.

7. The membrane is inspected and photographed.

8. Tiling begins.

Skipping or rushing any step can lead to a failure.

Waterproofing on timber floors

Timber floors move more than concrete, so they need particular care. The substrate must be stiff and suitable for wet areas, with sheets fixed securely. Movement joints must be accommodated. Many older Morwell homes have timber floors under the bathroom, and these are where we most often find damage from past leaks.

Waterproofing and tiles

The tile adhesive must be compatible with the membrane. Some membranes require a specific adhesive, and using the wrong product can cause tiles to come loose. We follow the system manufacturer’s specification.

Waterproofing and heating

In-floor heating is laid over or under the membrane depending on the system. The membrane must not be damaged during installation, and the products must be compatible.

What homeowners can check

You do not need to be an expert to ask good questions.

• Which membrane system will be used?
• Who will apply the waterproofing, and are they licensed and insured?
• Will bond breakers and puddle flanges be used?
• How long will the membrane cure before tiling?
• Will you receive photographs and documentation?

A good renovator will be happy to answer.

Does AS 3740 apply to small repairs?

Any work that involves rebuilding the waterproofing in a wet area should follow the standard.

Can I see the membrane before tiling?

We photograph each stage, and you are welcome to see the membrane before tiles are laid.

Does a new shower tray avoid the need for waterproofing?

No. Shower trays still require waterproofing to the walls and surrounding floor, and correct connection to the waste.

How long should waterproofing last?

Correctly installed waterproofing is intended to last for the life of the bathroom, provided silicone joints are maintained.

AS 3740 across the district

We waterproof every bathroom, ensuite and laundry we build to AS 3740, in Morwell, Traralgon, Churchill, Moe, Newborough and the rural towns around them.

Waterproofing laundries and toilets

Laundries and separate toilets are wet areas too. Where a laundry has a floor waste, the floor must be treated, and walls behind tubs protected. A toilet room floor may also need treatment. Many older laundries in Morwell homes, especially in enclosed back verandahs, have no waterproofing, and water from overflowing machines or tubs has damaged the floor.

Waterproofing balconies and outdoor showers

AS 3740 covers internal wet areas. Balconies and external decks are covered by a different standard. Outdoor showers, popular on rural properties for washing off after farm work, need their own careful detailing.

Membranes over existing tiles

Some products claim to waterproof over existing tiles. These may suit minor sealing, but they do not replace a properly installed membrane under new tiles. When a shower is leaking, rebuilding it with a full system is usually the reliable fix.

Documentation for sale or insurance

Documentation showing waterproofing to AS 3740 is useful if you sell your home, as buyers and building inspectors may ask about it. It also helps with insurance claims.

Maintaining the waterproofing

Once the bathroom is complete, simple maintenance protects the waterproofing. Check silicone joints at the screen, bath and internal corners each year, and replace any that have split or come away. Keep the floor waste clear so water drains freely. Report any loose tiles or cracked grout promptly. These small steps keep water on the surface, where it belongs.

Common waterproofing failures we find

When we strip out leaking bathrooms in Morwell and the surrounding towns, the same faults appear again and again:

• No membrane at all, especially in homes built before the 1990s.
• A membrane that stops short of the wall junctions or the shower walls.
• Missing bond breakers, so the membrane tears when the house moves.
• No puddle flange at the floor waste, letting water escape around the drain.
• Unsealed tap and shower arm penetrations.
• Screens fixed through the membrane without sealing the fixings.
• Hobs and shower bases that were never waterproofed underneath.

Timber floors and movement

Homes on stumps move a little with the seasons as timbers swell in damp winters and shrink in dry summers. That movement stresses joints. A flexible membrane, correctly placed bond breakers, a stable substrate fixed at close centres and flexible sealant at junctions all help the waterproofing cope with movement. This is especially important in older Morwell homes on timber floors.

Is a waterproofing certificate provided?

We provide documentation of the waterproofing, including photographs taken before tiling, as part of your handover pack.

Questions to ask any renovator

Ask which membrane system will be used, whether it will be photographed before tiling and how junctions and penetrations are treated.

Book a design consultation

If you want to understand how your bathroom in Morwell or a nearby town will be waterproofed, book a design consultation. We will explain the waterproofing system we will use and show you how it meets AS 3740.

Our bathroom services

• Accessible Bathrooms
• Bathroom Design
• Bathroom Renovations
• Bathroom Tiling
• Ensuite Renovations
• Laundry Renovations
• Leaking Shower Repairs
• Powder Room Renovations
• Shower Conversions
• Small Bathroom Renovations
• Vanity and Storage
• Waterproofing