Waterproofing Failures: Why Wet Areas Are the Defect That Costs the Most

The defect that costs the most and gets the least attention

Ask a homeowner what they worry about in a new build and you'll hear structure. Cracking. Slabs. Whether the frame is straight.

Almost nobody says waterproofing. And waterproofing is the one that gets them.

Wet area failures sit at or near the top of every residential defect list in NSW, year after year, and they have done for as long as I've been in this industry. Not because it's hard. Because it's invisible. A membrane is a few hundred dollars of product applied by someone for a couple of hours, buried under screed and tile within a week, and then nobody sees it again until the day a skirting board in the next room goes soft.

That's the whole problem in one sentence. Every other defect announces itself. This one waits.

And the waiting is the expensive part. A membrane that fails on day one and a membrane that fails in year three cost the same to fix, because the fix is identical: strip the tiles, strip the screed, strip the sheeting, redo it, retile. You're not repairing a membrane. You're demolishing a bathroom to get at it.

What AS 3740 actually requires, in plain English

AS 3740 is the Australian Standard for waterproofing domestic wet areas. It's referenced by the NCC, which means it isn't advisory. It's the rule.

The parts that matter to you as an owner, stripped of the jargon:

The shower floor is fully waterproofed, and the walls of the shower go up. The enclosure is treated as a wet area, not a splash zone. Height requirements apply to the shower walls, and the membrane has to turn up them — not stop at the floor line.

Junctions get more attention than flat areas. Wall-to-floor, wall-to-wall, around the waste, around the tap penetrations. Water does not attack the middle of a floor. It attacks the corners and the holes. Bond breakers and correct detailing at those junctions are where a compliant job and a cosmetic one separate.

Falls have to actually fall. The floor has to grade to the waste. This sounds obvious and it is routinely wrong. A shower floor that ponds is a shower floor whose membrane is permanently submerged, which is not what the membrane was designed for.

The membrane needs to be compatible with what goes over it and what goes under it. Substrate, primer, membrane, screed, adhesive — they're a system. Mixing brands and skipping the primer because the tiler had something in the van is how you get delamination.

None of this is exotic. It's a day's work done properly instead of half a day done fast.

Why it fails, and it's almost never the product

I want to be direct here, because there's a comfortable myth that waterproofing failures are about cheap materials. They mostly aren't. The membranes sold in this country are, by and large, fine.

The failures I see come from four things, and they're all human.

Rushing the cure. Membranes have coat counts and drying times printed on the tin. Two coats means two coats, and the second one goes on after the first has cured, not while it's still tacky because the tiler is booked for the morning. Programme pressure is the single biggest enemy of a good wet area. When a job is running late, this is the trade that gets squeezed, because it's the one nobody can inspect afterwards.

Penetrations added after the fact. Membrane goes down. Someone then decides the towel rail needs to be there, or the plumber comes back to move a waste 40mm. A screw through a cured membrane is a hole in the only thing standing between your shower and your bedroom floor.

Wrong substrate, or a substrate that moves. Standard plasterboard where water-resistant sheeting was required. Or a floor with too much deflection under it, so the membrane is being asked to bridge movement it was never rated for.

No bond breaker at the junctions. This is the technical one, and it's the most common genuine error. At a corner, the membrane needs to be able to move independently of both surfaces. If it's glued hard to both sides of a junction that then moves even slightly, it tears. You cannot see that it has torn. Everything looks perfect.

So when someone tells you their waterproofing failed because of a bad product, my honest reaction is scepticism. It usually failed because of a bad week.

The one photo that's worth more than the certificate

Here's the practical bit, and it's the reason I wanted to write this one.

You will be given a waterproofing certificate. Keep it — you want it, it matters for warranty, and a licensed applicator issuing one is putting their name against the work. But understand what a certificate is: a statement that someone says they did it correctly.

What's worth more is photographs of the membrane before it was covered.

Ask for them. Time-stamped, showing the full floor, the shower walls with the membrane turned up, the corners, and every penetration. Ask before the tiler starts, not after. On our jobs this is standard and it takes the site supervisor about ninety seconds.

The reason this matters isn't distrust. It's that if you ever do have a problem in year two, the difference between a fast resolution and a two-year argument is evidence. With photos, you can see whether the junction detailing was there. Without them, you're in an argument about what's under a floor nobody can look at without destroying it, and those arguments are miserable for everyone including the builder.

I'd rather hand over the photos and never need them.

What to actually ask for, before anyone tiles

Short list. Use it.

Ask who is doing the waterproofing and whether they hold the relevant licence — in NSW, waterproofing is licensed work, and the person applying it should be able to say so without hesitating.

Ask which membrane system is being used, and confirm the primer, membrane and screed are from a compatible system rather than assembled from whatever was cheapest that week.

Ask for the photos, as above, before tiling.

Ask whether there's a flood test. Not every job gets one and I'm not going to pretend it's universal, but on a second-storey wet area over living space below, I want one. Block the waste, fill it, leave it, check underneath. It's an afternoon.

Ask when the second coat went on relative to the first.

And if you're being told the tiler will 'just do the waterproofing too' — slow down and ask a few more questions. Plenty of tilers are properly licensed for it and do excellent work. Some aren't, and the ones who aren't are exactly the ones who'll be applying it at four in the afternoon so they can start setting out in the morning.

Your statutory warranty covers you here — waterproofing failures generally fall inside the non-structural period, and depending on the consequential damage, sometimes further. But warranty is a remedy after the fact. It gets you a repaired bathroom and a lot of lost weekends. Getting it right the first time costs a day of programme.

Where I'd spend the extra, if you only spend it once

If someone said to me, I've got a limited budget, where do I put it in a wet area, I wouldn't say tapware and I wouldn't say stone.

I'd say the membrane and the fall.

Everything visible in a bathroom can be changed later for money you can plan for. You can replace a vanity in a weekend. You can swap tapware in an hour. Those are decisions with an exit.

The membrane has no exit. It's the one component in that room where the only way to change your mind is to destroy everything sitting on top of it. So it's the one component where the cheap version isn't cheap, it's just deferred.

That's not a sales line. It's the same reason I care about substructure more than I care about splashbacks — the stuff you bury is the stuff you're stuck with. Get the buried work right and the rest is furniture.

If you're planning a build or a renovation with wet areas and you want the spec read properly before it's locked, that's what /advisory/land-assessment and a proper scope conversation are for. And if you're weighing up a renovation against starting again, the wet areas are usually a bigger part of that maths than people expect — worth reading alongside /insights/heritage-renovation-vs-rebuild-cumberland-2026.