Waterproofing is the part of a bathroom renovation you never see once the tiles go down. It sits under the floor and behind the walls, quietly keeping water where it belongs. When it fails, the damage often spreads well beyond the shower. That is why it deserves more of your attention than the tile colour or the tapware.

This guide covers what waterproofing does, who is allowed to do it and how to spot early signs of trouble. It also explains what to sort out in writing before any work begins. Rules differ between states, so the examples below come from New South Wales and Queensland. Check with the building regulator in your own state for the details that apply to you.

What waterproofing actually does

A waterproof membrane is a barrier laid under tiles in wet areas. Its job is to stop water escaping from the shower or floor into the timber, plaster and concrete around it. Tiles and grout are not waterproof on their own. Water slowly works through grout lines, so the membrane underneath does the real work.

The NSW Government describes waterproofing as any protective treatment of a home to prevent water or moisture getting into it. It also covers wet areas such as showers, to stop the unwanted escape of water from those areas. Membranes can be solid sheets or liquids applied by brush or roller. One NSW council waterproofing certificate lists bathrooms, showers, laundries and toilets as wet areas.

The standards behind the work

Wet area waterproofing in homes is covered by the National Construction Code. A NSW council waterproofing certificate refers to Part 10.2 of the NCC 2022 Housing Provisions. They also name Australian Standard AS 3740 as an accepted way to comply. You do not need to read these documents yourself, but your installer should know them well.

Ask your contractor which standard they will work to and which product they plan to use. A good installer will answer without fuss. They should also follow the product manufacturer’s instructions closely. Many failures start when a product is applied in a way the maker never intended.

Who can do waterproofing work

In New South Wales, waterproofing is classed as trade work. You need a contractor licence to carry out, advertise or contract for residential waterproofing worth more than $5,000 in labour and materials, including GST. The details are set out on the NSW Government waterproofing licence page. Unlicensed work under the Home Building Act 1989 can lead to fines of $22,000 for individuals and $110,000 for companies.

In Queensland, the Queensland Building and Construction Commission says waterproofing is regulated work. It must be carried out by licensed professionals. Other states have their own licensing rules and thresholds. Always check a licence on your state regulator’s public register before signing anything.

Choosing the right people matters for every room you renovate. The same habits apply when you talk to a kitchen renovation expert. Ask about licences, insurance and past work. Then compare at least two written quotes.

Get the scope in writing

The QBCC makes a simple but useful point. A quote, even one you have accepted, is not the same as a contract. Before work starts you should have a written contract that is signed and dated by both parties. It should cover the scope of works and the areas to be waterproofed.

The contract should also name the products being used and give a timeframe. It needs to set out who is responsible for what. This helps a great deal if something goes wrong later. A clear paper trail makes any dispute far easier to settle.

Why waterproofing fails

Waterproofing keeps appearing on the list of common building defects. The QBCC reports that residential bathroom claims finalised under the Queensland Home Warranty Scheme in the 2024/25 year averaged almost $25,000 in rectification costs. That is a large bill for something hidden under a few square metres of tile. Most of the causes are avoidable.

• Poor surface preparation or poor detailing at corners, joins and drains.
• Rushing the job, so tiles go down before the membrane has properly set.
• Skipping inspection before the membrane is covered up.
• Later damage from drilling, DIY work or other trades piercing the membrane behind the tiles.
• Normal wear on silicone, grout and seals that protect the membrane.

The point about later damage is easy to overlook. Fixing a towel rail or shelf in the wrong spot can puncture a membrane you cannot see. If you hire tools for small jobs, think carefully before you drill into a tiled wall. Our guide to renting construction equipment covers choosing the right gear for a task.

Plan the bathroom as a whole

Good waterproofing works best as part of a well planned room. Think about the floor finish, the drain position and the fall of the floor before anything is installed. Where a room needs a hard wearing floor without grout lines, some owners look at coatings rather than tiles. Our article on epoxy floor coatings explains how those systems are used.

Storage is worth planning early too. Vanities and cabinets near the shower need materials that cope with steam and splashes. The same thinking applies in a kitchen, where water from the sink and dishwasher can damage cupboards. Our piece on custom kitchen cabinets looks at what you pay for with made to measure joinery.

Signs your waterproofing may be failing

Leaks behind tiles are often slow and quiet. The QBCC guide to waterproofing defects lists several warning signs for home owners. A persistent musty smell or mould in the bathroom, laundry or nearby rooms can point to trapped moisture. Peeling, bubbling or blistering paint is another clue.

Look closely at silicone joints that are peeling or changing colour. Stained ceilings and walls below or beside a bathroom deserve a closer look. Damp patches that get worse after a shower or rain are a strong hint. Loose tiles, crumbling grout and a white chalky residue can also show moisture moving through the surface.

Looking after a new bathroom

A sound membrane still needs some care from you over the years. The QBCC suggests replacing silicone joints every few years. Repair cracked grout and loose tiles promptly. Keep drains clear so water does not pond on the floor.

Avoid harsh or corrosive cleaning chemicals that can break down grout and sealants. Simple habits like these protect the layer underneath. Bathrooms also bring water and power close together. Any electrical work near a shower or bath should be left to a licensed electrician.

What to do if you suspect a defect

Start by recording the problem with photos and written notes. Then tell your licensed contractor in writing as soon as you can. This gives them a chance to inspect and fix the issue before more damage occurs. Keep copies of everything you send and receive.

If the contractor cannot or will not fix the work, you may have other options. In Queensland, the Home Warranty Scheme can cover defective residential work in that situation. Eligibility rules, timeframes and conditions apply. Other states have their own home building insurance and dispute processes, so contact your state regulator or fair trading office.

Frequently asked questions

Are tiles and grout waterproof on their own?

No, they are not. Water can work its way through grout over time. The membrane under the tiles is what stops it reaching the structure.

Do I need a licensed waterproofer for my bathroom?

In most cases, yes. In NSW a contractor licence is needed for residential waterproofing work over $5,000 including GST. Queensland treats waterproofing as regulated work for licensed professionals, and other states set their own rules.

How can I tell if my shower is leaking?

Watch for musty smells, mould, bubbling paint and stained walls or ceilings nearby. Damp patches that get worse after showers are a common sign. Loose tiles and crumbling grout are also worth checking.

How often should silicone in a bathroom be replaced?

The QBCC suggests replacing silicone joints every few years. Replace it sooner if it is peeling, cracked or discoloured. Fresh silicone helps protect the membrane underneath.

Building rules and licensing requirements differ between states and change over time. This article is general information only. Check with your state building regulator or a licensed professional before starting work.