PERTHWet areas are the most commonly failed part of a new build
Waterproofing under AS 3740, who is licensed to do it in WA, the four places wet areas leak, and why Perth winters are when a marginal membrane gives up.
Building and Energy has published on this specifically, which tells you how often it goes wrong. Wet areas are the most commonly failed part of a new build, and the failure is expensive because it is buried under tiling by the time anyone notices.
The standard, and who is allowed to do the work
Waterproofing of domestic wet areas is governed by AS 3740, called up through the National Construction Code, which sets the performance requirement plainly: water must not penetrate from a wet area into other parts of the building or into the structure. The Building Act 2011 and the Building Regulations make that a legal requirement in WA, not a guideline.
In Western Australia the work has to be carried out by a licensed waterproofer. Not a tiler, not a general builder, and not the owner. When the membrane is done it is inspected and a certificate of compliance is issued, and no tiling should start until that certificate exists. If a renovation was done without one, that absence is itself a finding.
Where wet areas leak
Four places, in order of how often we see them.
The floor-to-wall junction. The membrane has to turn up the wall and the two planes have to be continuous. Where they are not, water tracks out along the floor and into the wall base, and the first sign is often a damp skirting in the next room rather than anything visible in the bathroom.
The fall to the floor waste. The floor has to slope to the drain. Where it does not, water ponds, sits on the membrane, and finds the one weakness it has. A spirit level on a bathroom floor tells you more than most people expect.
The shower recess. The most heavily wetted part of the house, and the junction of screen, hob and membrane is where it concentrates. AS 4349.1 makes the point that a dry recess on the day is not proof of a recess that does not leak, because a shower not used for a week reveals nothing.
Penetrations. Every tap, every waste, every fixing through the membrane is a hole that had to be sealed. Each is a potential path.
Why Perth makes it worse
Concentrated winter rainfall and sustained humidity from May to September keep wet areas wetter for longer than the drier months suggest, and that is the season a marginal membrane gives up. A shower that coped through summer starts staining the ceiling below in July.
What we look at
On a completed home we cannot see the membrane, so we read its effects: staining on the ceiling below an upstairs bathroom, damp meter readings at the base of wet-area walls, movement or drumminess in floor tiles, the condition of grout and silicone, and whether the floor actually falls to the waste. On a practical completion inspection the timing is better, because the waterproofing can be checked before the tiler covers it.
Where we find evidence of a leak, the finding is the water path, not just the stain, because the stain is the symptom and the junction is the cause.
If you are buying an older home
Anything built before about 2000 may have waterproofing done to a lower standard than AS 3740 now requires, or none at all behind an original bathroom. That is not a defect in the sense of something done wrong — it was built to its era — but it is a cost to plan for, and a pre-purchase inspection will flag the wet areas as an area to watch.
Why wet areas are the most common failure point
Wet-area waterproofing failure is one of the most common and most costly defects in residential building, and the reason is simple: a wet area is deliberately and repeatedly soaked, and everything depends on a hidden membrane doing its job perfectly, out of sight, for the life of the building. Unlike a roof leak, which announces itself in the first heavy rain, a wet-area failure is usually slow and concealed. Water gets past the membrane a little at a time, tracks into the surrounding structure, and does its damage in the floor and walls before anything shows on the surface. By the time there is visible staining, a musty smell, or a soft patch of floor, the water has been getting where it should not for a while, and the repair has grown accordingly.
What makes it worse is that the fix is rarely simple. The waterproofing sits beneath the tiles, the screed, and the fittings, so correcting a failed membrane usually means stripping the wet area back and rebuilding it. There is no surface repair for a membrane that was laid short or has failed at a junction; the wet area has to be reconstructed. This is why wet-area waterproofing is one of the defects most worth catching early, ideally at the waterproofing stage of a build when the membrane is visible and a problem is a re-do rather than a demolition, and why on an existing house the signs of a wet-area problem deserve serious attention rather than being dismissed as cosmetic.
The junctions where membranes fail
Waterproofing failures concentrate at the junctions and edges rather than in the middle of a floor, because the junctions are where the membrane has to do the hardest work. The membrane has to be turned up at the walls and at the shower hob so that water cannot escape at the edges, sealed carefully around every penetration where a pipe or a drain passes through, and lapped correctly so that water runs onto the membrane and to the waste rather than under it. Each of these details is a place where a rushed or careless installation leaves a weakness, and each is a place water will find if the detail is not right.
The classic failures are a membrane not turned up far enough at the wall junction, so that water rising in a splashed corner gets behind it; a poorly sealed penetration around a floor waste or a tap; a hob or threshold detail that lets water track out of the wet area entirely; and a membrane that stops short of where the water actually reaches. None of these is visible once the tiles are on, which is exactly why the waterproofing stage of a build, before the tiling, is the one practical opportunity to confirm the membrane is continuous, turned up, and sealed where it needs to be. It is also why a licensed waterproofer, working to the standard, matters: the work is concealed and unforgiving, and the difference between a membrane that lasts and one that fails is in details nobody can check later.
What an older Perth home is likely to show
In an older Perth home, the wet areas are one of the first places worth looking closely, because waterproofing standards and materials have improved markedly over the decades and an original wet area from an earlier era may never have had waterproofing to the standard now expected. Older showers and bathrooms are a common source of concealed water damage, particularly where the original waterproofing has aged past its service life or was marginal to begin with. The signs are worth knowing: staining or damage at the base of the walls adjacent to a wet area, a shower floor that holds water, lifting or drummy tiles, a persistent damp smell, or evidence of moisture in the floor or the room on the other side of a wet-area wall.
Where these signs are present, they point at a problem that is both concealed and expensive to fix properly, which is exactly the kind of thing a buyer wants identified before committing rather than discovered after. An inspection assesses the wet areas as far as a visual, non-invasive look allows, reading the surface signs and, where available, moisture readings, and flags where the evidence points to a waterproofing problem that warrants a closer look. On an older home in particular, the wet areas are worth treating as a known risk area rather than assuming that because they look tidy they are sound, because the failure that matters is the one happening behind the tiles.
Common questions
Who can waterproof a bathroom in WA?
A licensed waterproofer. In Western Australia wet area waterproofing must be carried out by a licensed practitioner, not a tiler or general builder, and a certificate of compliance is issued when the membrane is complete.
What standard applies to bathroom waterproofing?
AS 3740, called up through the National Construction Code. The Building Act 2011 and Building Regulations make compliance a legal requirement in WA.
Why does my shower only leak sometimes?
Detection depends on use and weather. AS 4349.1 notes that a wet area dry at the time of inspection is not proof it does not leak, because a recess unused for some time may reveal nothing.
This comes up on every practical completion (pci) inspection and pre-purchase building inspection we carry out.
Read next: perth ceilings that fall down on their own and metal roofs act like a wing. Tie-downs are what stop them leaving..
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.