Is waterproofing legally required in a Sydney bathroom?

Yes. Waterproofing of domestic wet areas is covered by the National Construction Code, which calls up Australian Standard AS 3740. Any new bathroom, ensuite or laundry, and any renovation that disturbs the existing wet area, has to be waterproofed to that standard. It is not optional and it is not a matter of preference.

How much does bathroom waterproofing cost in Sydney?

As a standalone trade, waterproofing a standard Sydney bathroom typically runs between $600 and $1,200 depending on the size of the wet area, the condition of the substrate and the membrane system used. On a full bathroom renovation with us it is included in the project rather than quoted separately.

Can I waterproof my own bathroom?

In New South Wales, waterproofing is licensed work. Beyond the licensing question, a membrane that fails is hidden behind tiles and does not announce itself for months or years, by which time the damage is in the framing and the floor below. It is the single worst place in a bathroom to save a few hundred dollars.

How long does waterproofing take before tiling can start?

Most membranes need multiple coats with drying time between each, then a full cure before tiles go down. Allow 2 to 3 days in normal conditions. Rushing this is the most common cause of a failed wet area, because tiling over a membrane that has not cured traps moisture underneath it.

Bathroom Waterproofing: A Complete Guide for Sydney Homeowners

Waterproofing is the part of a bathroom nobody sees and nobody thinks about, right up until the ceiling below starts staining. It is also the part where cutting a corner costs the most later. A failed membrane does not mean a repair. It usually means stripping the tiles, the screed and the sheeting back to frame and starting again.

This guide covers what the standard actually requires, how the membrane systems differ, what it costs, and how to tell whether the wet area you already have is in trouble.

What waterproofing actually is

A waterproof membrane is a continuous, flexible layer applied over the substrate before tiling. Its job is to stop water that gets past the tiles and grout from reaching the structure behind.

That last point surprises people. Tiles are not waterproof, and grout certainly is not. Grout is porous by design. Water moves through the tiled surface constantly in a shower, and the membrane underneath is what stops it going anywhere it should not. The tiles are the wearing surface and the finish. The membrane is the actual waterproofing.

This is why "the tiles look fine" tells you nothing about whether a bathroom is watertight.

What AS 3740 requires

The Australian Standard for waterproofing domestic wet areas is AS 3740. It is called up by the National Construction Code, so meeting it is a compliance requirement, not a best-practice suggestion.

In broad terms the standard covers where the membrane has to go:

The entire shower floor, without exception.
Shower walls waterproofed up to a substantial height above the floor, typically 1800mm in the shower enclosure.
Wall and floor junctions throughout the wet area, because that junction moves and is where failures concentrate.
Walls outside the shower to a minimum height above the floor, commonly 150mm.
Around baths and vessels, including the vertical surfaces adjacent to them.
Hobs, steps and niches, fully, including their internal corners.

The heights and extents vary with the type of construction, whether the room is on a timber or concrete floor, and whether the shower is enclosed or open. That is exactly why it is worth having someone who works to the standard daily rather than working from a diagram found online. The AS 3740 requirements are worth understanding in more detail if you are planning a renovation.

The standard also deals with falls to the waste, so water actually leaves the room, and with the compatibility between the membrane and everything bonded to it.

The membrane systems

Not all membranes are the same, and the right one depends on the substrate and the situation.

Liquid-applied membranes are the most common in domestic bathrooms. They are rolled or brushed on in multiple coats, and they conform to every corner, penetration and awkward junction without seams. Most are polyurethane or acrylic based. Their weakness is application: the thickness has to be right, which is why coats and cure times matter so much.

Sheet membranes are supplied in rolls and bonded down. They give a guaranteed, consistent thickness because the sheet is made in a factory rather than spread by hand. The trade-off is the joins, which have to be lapped and sealed properly, and the difficulty of dressing them into complex shapes.

Cementitious systems are rigid and used where movement is minimal. They are less common in domestic bathrooms with timber floors, because timber moves.

Whichever system, the detail work at corners, penetrations and the waste is where a wet area is won or lost. The differences between membrane types are covered properly here.

Where waterproofing actually fails

After enough years of stripping out other people's bathrooms, the failures repeat:

Not enough coats, or coats applied too thick at once. A membrane specified at a given dry film thickness will not achieve it in one heavy pass. It skins over, traps solvent underneath and never fully cures.
Tiling before the membrane has cured. The most common one, and always driven by trying to finish faster.
Corners and junctions skipped. These need bond breakers or reinforcing fabric so the membrane can move with the building instead of tearing.
Penetrations not detailed. The waste, the tap penetrations, the shower rail fixings. Every hole through a membrane is a potential leak unless it is dressed properly.
Falls that do not drain. Water that sits will eventually find a way through anything.
The wrong primer, or none at all. Adhesion failure means the membrane separates from the substrate and water tracks underneath it.

None of these are visible once the tiles are on. That is the whole problem.

Signs an existing bathroom has failed

If you are not renovating yet but something feels wrong, the warning signs are worth knowing. The short version: staining or bubbling paint on the ceiling below, a musty smell that does not clear, swelling or softness at the base of the wall sheeting, loose or drummy tiles in the shower, efflorescence in the grout, or skirting boards lifting in an adjoining room.

Any one of those means water is getting somewhere it should not.

What it costs

Waterproofing a standard Sydney bathroom as a standalone trade generally runs $600 to $1,200. What moves the number is the size of the wet area, whether the substrate needs preparation or replacement, how many penetrations and junctions there are, and which membrane system suits the job.

On a full renovation, waterproofing is part of the project rather than a line item you shop around on. For what a complete bathroom costs, see our bathroom renovation cost guide.

The certificate

In New South Wales, waterproofing is licensed work, and you should receive documentation for it. Ask for the licence number of whoever does it and keep the paperwork. It matters when you sell, it matters if there is ever an insurance claim, and it matters if a problem appears later. How waterproofing certificates work is covered here.

The one thing worth remembering

Waterproofing is roughly 5 percent of a bathroom renovation budget and close to 100 percent of the risk. A tiler who is casual about cure times or vague about which membrane they use is telling you something important.

If you want it done properly, we waterproof every wet area we tile, to the standard, with the paperwork to match.