Bathroom Waterproofing Guide for Australian Homes

The number one thing to keep in mind about waterproofing is this: without proper waterproofing, everything can fail. I have seen it far too many times; rotten timber framing behind the shower screen, black mould growing behind the plasterboard, and the repair bill that can dwarf the original renovation cost! I would like to think that most of this is preventable.
Here I have a step-by-step guide to help you learn how to waterproof a bathroom. We will cover Australian standards, ventilation, mould prevention and much more.

Title: Episode 5 Detailed Guide on Waterproofing Bathrooms AS3740 Upload Date: Apr 29, 2021 Description: This Australian builder's guide covers bathroom waterproofing to AS 3740 standards, demonstrating correct membrane application, surface preparation, and compliance requirements for domestic wet areas. Video Credit: Casabella Built

Why Waterproofing Matters: Understanding Australian Bathroom Moisture Challenges

Australian bathrooms get wet. A lot wetter.
You have hot water, summer humidity (particularly on the coast where the combination of heat, salt and high humidity can wreak havoc on materials), winter, and a changing climate.
Water doesn't stay on the surface. It moves through gaps, joints and penetrations that are poorly sealed into the wall and cavity, down into the frame or the floor. Without a bathroom waterproofing system or proper installation of a membrane system, water will get into places you won't see until it's too late (I have pulled apart walls that looked fine from the outside only to find the timber behind had turned to mush).
Typical structural repairs for water damage can easily cost between $5,000 to $20,000 (not counting bathroom re-do). The damage goes unnoticed until months have passed, or an obvious sign of it has shown.

Applying blue waterproofing membrane on bathroom floor edge with brush during renovation work.

Waterproofing Standards and Materials: Meeting Australian Building Code AS 3740

AS 3740 provides the requirements for waterproofing of domestic wet areas. This is not an optional standard, rather an Australian building code. It is a legal requirement that every shower, bath, and ablution area are effectively sealed and protected against moisture.
There are two methods commonly used, each with pros and cons:
Liquid membranes. Most common method for residential bathroom waterproofing. Waterproofer applies a polyurethane-based membrane to the surface of the bathroom using a brush or spray gun. Average cost is $30 to $50 per square metre. Conforms very well to irregular surfaces and penetrations.
Sheet membranes. PVC or rubber-based peel and stick waterproofing membranes. Average cost is $40 to $60 per square metre. More consistent in thickness and less application issues with thickness but difficult to install around corners and penetrations.
In terms of sealant material, you will find silicone ($5 to $10 per metre) will work better than a more standard acrylic caulk ($2 to $5 per metre). Silicone maintains flexibility better after a couple of months/years. Always require the waterproofer to provide you with a certificate of compliance on completion, this is essential in order to have a valid warranty and insurance!

Applying waterproofing resin to floor surface with roller and bucket during installation

Step-by-Step Waterproofing Application: Membranes, Sealants and Installation

The sequence of application is paramount for waterproofing success. Wrong and you will have wasted time and money!
The substrate or base must be completely clean, level and free of dust. Any cracks or gaps in the plasterboard must be fully sealed prior to water proofer starts; all surfaces must be sound and sanded smooth. The surface cannot be dusty or dirty because no membrane or sealant will adhere.
Primer (if required). The water proofer will apply a specific primer designed to the specific waterproofing product they intend on using. Don't mix brands of waterproofing with primers!
First application of liquid membrane system. The water proofer will start applying their membrane in the corners and around the penetrations of the shower area. Reinforcing fabric strips will be installed by the water proofer in all internal corners and the junction between the floor and the wet area. This is where most waterproofing failures occur.
After approximately 24 hours (check the waterproofing product specification) of curing you can apply the second coat. Don't rush the curing!
Apply the second coat of waterproofing. The second coat is usually applied in a cross-hatching method to ensure full coverage and consistent thickness.
Final step! Water proofer will complete the installation with sealant or silicone around all penetrations and movement joints.
A typical 5 square metres wet area will cost between $800 and $1,500 total including a water proofer's labour cost (average $80 to $120 per hour). I would say that's a pretty good estimate. After all, water damage repair is extremely expensive.

White modern bathroom ventilation fan mounted on tiled ceiling surface

How to Design Bathroom Ventilation to Prevent Moisture and Mould

Adequate bathroom ventilation is the other side of the moisture coin. You can achieve perfect waterproofing, but if you have bad bathroom ventilation then you will likely face mould and condensation.
6 to 8 changes of air per hour is a minimum standard, which is why a fan of 10 to 12 litres a second should be adequate for a 5 square metre bathroom. However, I recommend going larger, which will cost you slightly more but can make a large difference.
Adequate exhaust ducting should be no less than 100mm and ideally the length should be less than 3m. You also need to make sure the duct vents outside the building and not into roof cavities. I have seen many installations vent humid air into the ceiling and end up with other issues.
A humidity sensor switch is a great additional option, which can be programmed to automatically turn on when humidity is over 60 per cent. Also, good drainage like bath drains & plugs can help remove water from the areas as much as possible.

Hand with yellow sponge cleaning mouldy white ceramic bathroom tiles

Mould Prevention and Early Detection in Bathrooms

Even if you have proper waterproofing and ventilation, bathroom areas will still need ongoing attention to prevent and control mould in bathroom spaces.
Your exhaust fan should be on during and for 20 to 30 mins after a shower. A diluted white vinegar solution applied weekly will help prevent the growth of mould, more so than bleach which will destroy grout in the long run.
It is also important to be aware of any early signs like black spots in the grouting, discolouration of the silicone around built in bath tubs, shower bases and wet area floor surfaces and musty odours in the room. As always, the earlier it is caught, the easier the fix and in this case, surface treatment will suffice. Otherwise, mould can penetrate behind the tiles which requires an average of between $500 to $3,000 in professional remediation.
When replacing fixtures or bath hoses, check the condition of the silicone sealant used around these penetrations as the area should always be re-sealed as part of general bathroom maintenance. This preventative action usually costs around $50 to $100 a year. Compare that to remediation, right?

FAQs

Do I need a water proofer, or can I do it myself?

Most states across Australia requires wet area waterproofing to be carried out by a licensed professional. A waterproofing certificate is also needed to ensure building compliance and insurance, so better to get it right first time than have to deal with fixing it after.

How long can you expect a waterproofing membrane to last?

Typically a waterproofing membrane should last as long as the life of the bathroom, usually around 15 to 25 years. However, a good maintenance practice is to have the sealants reapplied every 5 to 10 years to protect the membrane and silicone sealant to check for deterioration and cracks.

What are the signs my waterproofing has failed?

Signs of failure are water marks visible on the ceiling directly below or in the immediate room below, musty odour that lingers, tiles that seem to feel spongy or soft on the surface and the return of mould in spots. Get a waterproofer to check the issue as early as possible, as it could determine a simple repair job or a total strip-out and re-waterproof.