How to Waterproof a Bathroom | Australian Standards Guide

Title: How to waterproof a bathroom Upload Date: Mar 23, 2023 Description: This video demonstrates the process of properly waterproofing a bathroom before tiling, outlining key steps and materials used. It aims to guide viewers through preparation techniques to ensure a durable and leak-resistant installation. Video Credit: Tile Warehouse

Why Waterproofing Is Non-Negotiable in Australian Bathrooms

Waterproofing failures go far beyond a little damp wall. They can mean timber rotting away behind the tiles, mould growing in places where you cannot see it, and potentially even voiding your home insurance. Repairs can easily run to between $10,000 and $30,000 plus the cost of the new tiles and fixtures!
In Australia, waterproofing of wet areas like a bathroom is a legal requirement of the National Construction Code. In most states, only a waterproofer with an appropriate licence can perform this work. This is definitely not a job for the DIY enthusiast.

Builder installing waterproofing sealing band at bathroom wall-floor junction

Australian Standards: What AS 3740 Actually Requires

The National Construction Code requires shower walls to be waterproofed to a minimum of 1800 mm above the floor substrate (NCC 2022, ABCB Housing Provisions, Part 10.2). The wall on the bathing side of the bath must be waterproofed to a height not less than 150 mm above the bath rim. The wall to floor junction must have a flashing minimum horizontal leg of 40 mm.
For unenclosed showers, Australian Standard AS 3740 specifies that the waterproofed floor area must extend no less than a 1500 mm horizontal radius from the shower rose connection (AS 3740:2021). The floor fall must have a gradient no less than 1:80 or greater than 1:50 towards the floor waste. Your how to waterproof a bathroom project can comply through the NCC Housing Provisions or through AS 3740:2021 directly. When the waterproofing is complete, the waterproofer must issue a certificate of compliance. Keep this document as your insurance company may require it.

Roller applying waterproof membrane at shower wall and floor corner

Membrane Types and Application Methods Explained

Liquid membranes are usually used when it comes to waterproofing residential bathrooms. Usually a liquid applied polyurethane or acrylic, these are brushed or rolled directly onto the substrate. Usually 2 coats are required, and the system may take 24 to 72 hours to cure before it can be tiled. I think this system is very popular for bathrooms because it conforms very well to areas around penetrations, corners and joints within the bathroom shower waterproofing areas.
A sheet membrane is a flexible membrane which is pre-manufactured and adhered directly to the substrate. The membrane will have the same thickness throughout its surface, and are generally preferred for larger floor areas and on substrate areas which have higher levels of movement. Each joint, corner, overlap and penetration in the membrane must be waterproofed in order to ensure a good waterproofing result. One missed overlap and you have a potential leak.
Both types of waterproofing need to conform to the Australian standards for how to waterproof a shower. A waterproofer will recommend the best system for your specific bathroom conditions.

Ceiling-mounted bathroom exhaust fan extracting steam and moisture

Ventilation and Moisture Management: The Other Half of the Equation

The waterproofing does a great job at keeping water out of walls and floors, but ventilation does a very good job at removing the moist air left after every shower! Without proper bathroom fan installation, you will be inviting condensation, mould and long term damage. The National Construction Code specifies that the minimum exhaust rate for bathrooms is 25 L/s (NCC 2022, ABCB Housing Provisions, Part 10.8), and most budget exhaust fans won't achieve that, so look at the rated extraction capacity before you buy. An exhaust fan must be installed by a licenced electrician and must be ducted to outside the home, not the roof cavity. A timer switch or humidity sensor that activates the fan for 10 to 15 minutes after you shower really does help. Think about where a bath cabinet might go in relation to these ventilation zones in your design, and consider that keeping cabinetry away from moisture extends its life.

Professional contractor documenting plumbing inspection findings on clipboard

Hiring a Waterproofer: What to Look For and What to Expect to Pay

In most Australian states, waterproofing is a licenced trade. Ask for the waterproofer's licence number, verify it with your state's licensing authority, and ask for references from recent projects. The typical cost for how to waterproof a bathroom ranges from $1,500 to $4,000, depending on the type of membrane used, the size of the wet area and where you are. Sydney and Melbourne are on the higher end.
For me, the compliance certificate is the single most important thing you'll get out of your entire renovation. It confirms that the work complies, provides you with protection against future problems, and adds value to the house when you sell. If a waterproofer won't give you a compliance certificate, walk away.
Once your waterproofing is done, you can move on to tiling, installation of fixtures and fittings, and fitting out your bathroom cabinet and 900mm vanity. Get three quotes, but look closely at exactly what's included: does the waterproofing quote include the construction of the shower hob, or will that be an extra?

References

National Construction Code 2022, ABCB Housing Provisions, Part 10.2 Wet Areas
AS 3740:2021 Waterproofing of Domestic Wet Areas, Standards Australia
National Construction Code 2022, ABCB Housing Provisions, Part 10.8 Condensation Management

FAQs

Do I really need to engage a licenced waterproofer?

In most Australian states, the answer is yes. Waterproofing a wet area is a piece of licenced work; if you undertake this yourself, you may find that your home insurance is not valid if something goes wrong. The cost of hiring an accredited waterproofer is just a fraction of what you'd be paying to fix a failed job.

How long does it take to cure waterproofing before we can tile over?

Most liquid-applied membranes need 24 to 72 hours of cure time between the final application and tiling, depending on the specific product and ambient conditions. Rushing this step is one of the most common reasons waterproofing fails.

What's the tell-tale sign of failing waterproofing in a bathroom I already own?

Look for tiles that have bubbled up or lifted up, for persistent mouldy smells, for damp stains on ceilings in rooms below, and for damp or spongy floorboards. If you spot any of these, get a licenced waterproofer in to investigate as soon as possible. Catching problems early can save you thousands of dollars in repair bills.