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What waterproofing actually is
Waterproofing is a liquid membrane applied to the walls and floor of a bathroom before tiling. It dries to form a continuous, flexible barrier that stops water from reaching the building structure behind the tiles.
Without it, water works through grout joints, around fixtures and into the wall framing and floor substrate. Over months and years, this causes rot, mould, structural damage and — in multi-storey homes — leaks into the rooms below.
Why it is a legal requirement in Australia
In NSW, bathroom waterproofing must comply with the Building Code of Australia and Australian Standard AS 3740 — Waterproofing of Domestic Wet Areas. It is not optional and it is not a cosmetic step. A bathroom without compliant waterproofing fails inspection.
The standard sets minimum requirements: which surfaces must be waterproofed, how high the membrane must extend up the wall, what materials are acceptable, and how the membrane must be tested before tiling. A compliance certificate is issued by the applicator or an inspector to confirm the work meets the standard.
Where the membrane goes
The entire floor of a wet area must be waterproofed, including under the bath and behind the toilet. The membrane extends at least 150 millimetres up the walls from the floor. In a shower, the membrane goes at least 1,800 millimetres up the wall — typically to the full height of the tiled area.
Around penetrations — pipes, drains, taps — the membrane wraps into purpose-made collars that seal the junction between the pipe and the membrane. These junctions are the most common failure points in older bathrooms.
Our niche design guide covers how waterproofing works inside a recessed shelf, which is a wet area within a wet area.
How it is applied
The substrate — usually cement sheet or fibre cement board — is cleaned and primed. A primer coat helps the membrane bond to the surface. The first coat of membrane is applied by brush or roller, covering all surfaces.
After the first coat dries (typically overnight), a second coat is applied at right angles to the first. Two coats ensure there are no thin spots or pinholes. In corners and junctions, a reinforcing fabric tape is embedded into the wet membrane to bridge the joint.
The membrane cures for at least 24 hours before tiling can begin. Rushing this step risks trapping moisture under the membrane or applying tiles before the membrane has reached its full strength.
What can go wrong
The most common failures are at penetrations (pipes and drains), at internal corners (where wall meets floor), and at the shower hob (the step into the shower). These are the points where water pressure is highest and where movement can crack a rigid seal.
Poor application — too thin, one coat instead of two, rushing the cure time, or missing a corner — creates weak spots that may hold for a year but fail within five. By then, the tiles are on, and the failure is invisible until damage appears.
Our mould prevention guide covers what happens when waterproofing fails and moisture gets trapped behind tiles.
The compliance certificate
After the membrane is applied and cured, it is inspected. If it meets AS 3740, a compliance certificate is issued. This certificate is a legal document that confirms the waterproofing was done correctly.
Keep this certificate. It is required for any future insurance claim related to water damage, and it may be requested during a property sale. Our waterproofing certificate guide covers what the certificate contains and why it matters.
How ETR handles waterproofing
Every ETR bathroom is waterproofed to AS 3740 — primer, two coats, reinforced corners, sealed penetrations, full cure before tiling. A compliance certificate is issued for every job.
Adam Dawood, with over 25 years in the industry, inspects the membrane before tiling starts. The written quote includes waterproofing as a standard line item, not an optional extra. A free on-site measure anywhere in Sydney is the starting point.
Frequently asked questions
How long does bathroom waterproofing last?
A properly applied membrane to AS 3740 should last the life of the bathroom — typically 15 to 25 years. Failure usually happens at penetrations or corners due to poor application, not because the membrane material itself wears out.
Can I waterproof my own bathroom?
In NSW, waterproofing in a wet area must be done by a licensed waterproofer or under the supervision of a licensed builder. DIY waterproofing will not receive a compliance certificate and may void your home insurance for water damage claims.
How do I know if my existing waterproofing has failed?
Common signs are mould that keeps returning despite cleaning, soft or spongy patches in the wall, discolouration at the base of tiled walls, a musty smell, or water damage to the ceiling of the room below. Any of these warrants an inspection.
Is waterproofing included in a renovation quote?
It should be. At ETR, waterproofing is a standard line item in every bathroom renovation quote. If a quote does not mention waterproofing, ask — it is not something that can be skipped or done cheaply.
What is the difference between waterproofing and water resistance?
Waterproofing is a continuous membrane that stops water completely. Water resistance means a material can tolerate some moisture but is not impervious. Moisture-resistant plasterboard, for example, handles humidity better than standard plasterboard but it is not a waterproof barrier.