When it comes to building or renovating, waterproofing is one of those behind-the-scenes jobs most home owners never think about — until it fails. A waterproofing system is meant to protect your home’s structure from moisture damage, but when it’s rushed, done poorly or skipped altogether, the results can be disastrous and expensive.
Leaky showers, damp walls, rotting timbers and swelling cabinetry are all symptoms of waterproofing gone wrong. And while the problem might start small, the cost of fixing it often isn’t.
Why waterproofing matters
In any home, water will always find the path of least resistance. Bathrooms, laundries, kitchens and balconies are all “wet areas” where moisture is constantly present. The role of waterproofing is to create a sealed barrier beneath tiles or flooring that stops water from seeping into walls, floors or the structure below.
When that barrier fails, even in a single spot, water can travel far from the original leak. You might see damp stains in an adjoining room, swollen skirting boards, or mould on a ceiling below. By the time these signs appear, the damage beneath the surface is often already serious.
Waterproofing isn’t just a finishing touch; it’s a structural safeguard. Under AS 3740 (Waterproofing of domestic wet areas), waterproofing must be applied in specific zones — including the full shower recess, walls up to at least 1.8m high, and floors that connect to a floor waste. These details are crucial to ensuring the waterproofing system performs for the life of your home.
How corner-cutting happens
The most common waterproofing failures aren’t caused by bad luck — they’re caused by shortcuts. A rushed job might look fine when it’s tiled over, but small errors at this stage can lead to big repair bills later.
Some of the most frequent mistakes include:
- Using the wrong product for the substrate or location (for example, applying a membrane designed for internal use on an exposed balcony).
- Skipping primer or not allowing proper curing time between coats.
- Inconsistent membrane thickness, leaving weak spots where water can break through.
- Tiling too soon before the membrane has fully cured.
- Failing to seal penetrations around drains, pipes and corners.
Even something as simple as not cleaning dust or debris before applying a membrane can cause it to delaminate and lose adhesion.
The problem is that waterproofing defects are invisible once the room is finished. You can’t see a bad job until months later when the damage begins to show — and by then, it’s too late for a quick fix.
The true cost of failure
Waterproofing failures are one of the top three causes of building defects in Australia. According to industry data, fixing a failed shower or balcony can easily cost between $10,000 and $30,000 — and in severe cases, even more.
Why so expensive? Because repairs usually mean stripping everything back to the bare structure. Tiles, screeds, cabinetry, plasterboard, insulation and sometimes even framing all have to be removed and replaced. Then the area has to be completely dried out before new waterproofing can be installed and retiled.
There are also hidden costs many home owners don’t factor in:
- Lost use of the room during repairs (especially painful if it’s your only bathroom).
- Temporary accommodation or facilities if extensive drying is needed.
- Ongoing moisture damage in adjoining areas that weren’t immediately obvious.
- Potential insurance complications, as most policies don’t cover faulty workmanship.
Cutting corners might save a few hundred dollars during construction, but the long-term risk far outweighs the short-term saving.
Compliance and the law
Waterproofing isn’t optional — it’s a mandatory part of the National Construction Code and must be carried out by a licensed professional. In most states and territories, a waterproofing licence is required for anyone applying membranes in residential construction.
The installer must also issue a certificate of compliance confirming the work meets AS 3740 and relevant state regulations. Without that certificate, you may find it difficult to sell your home or make an insurance claim if problems arise.
If you’re managing your own renovation, always check that your waterproofing contractor is licensed and insured. Ask which products they’re using, whether they’re compatible with your tile adhesive and grout, and how long the curing process will take. A good tradesperson will be happy to explain their process — and if they’re not, that’s a red flag.
Common problem areas
While showers top the list for waterproofing failures, other areas often go overlooked:
- Balconies and decks: These are exposed to both rain and UV light, so membranes need to be UV-resistant and protected with proper drainage layers. Failure here can cause leaks into rooms below.
- Laundry floors: If the washing machine overflows, the water should flow to a properly installed floor waste — but many laundries don’t have one, leading to underfloor water damage.
- Kitchens: Splash zones near sinks or dishwashers can suffer from long-term moisture exposure if joints and kickboards aren’t sealed correctly.
- Basements: Poor external waterproofing or blocked subsoil drainage can let water seep through retaining walls, creating damp smells or mould.
Each of these areas has its own set of standards and best-practice details under AS 3740 and AS/NZS 4654.2 (External above-ground waterproofing membranes).
Prevention is cheaper than repair
The key to reliable waterproofing is simple: Do it right the first time. That means using the correct products, following manufacturer instructions and allowing sufficient curing time. Rushing to tile too early is one of the biggest causes of premature failure.
Here are a few practical steps home owners can take to prevent issues:
- Hire a licensed waterproofer and keep their compliance certificate on file.
- Ask for photos before tiling, so you can see the full membrane coverage.
- Check falls to drains before tiles are laid — water should always flow to the waste, not toward walls or corners.
- Inspect balconies and decks for cracks or ponding water after rain.
- Re-seal grout and joints in wet areas every few years to maintain protection.
A small investment that protects your home
Waterproofing might not be the most exciting part of a renovation, but it’s one of the most important. It protects your home’s structure, keeps mould and damp at bay and saves you from major repair bills down the track.
When you’re budgeting your next project, resist the temptation to go cheap or fast on waterproofing. Those few extra days and dollars spent now could be the difference between a dry, durable home and a costly, stressful disaster later.
