How to tell if your waterproofing has failed

Step by step

1. Look at the room below or next door

   Bubbling paint, a soft patch of plaster, a stain on the ceiling below, or skirting board lifting in the adjoining room. Water travels — the damage often shows up somewhere other than the bathroom.

2. Press on the floor and the shower base

   Any softness, flex or hollow sound under the tiles means water has got into the substrate. Tiles should feel completely solid underfoot.

3. Tap around the walls

   Tap gently across the tiled wall with a knuckle. A change from a solid sound to a hollow drummy one means the tile has debonded — usually because the sheeting behind it is wet.

4. Look at where grout keeps cracking

   Grout that cracks in the same joint repeatedly, especially at the floor-to-wall junction, is movement. Movement means the substrate is unstable, which usually means it is wet.

5. Check the smell

   A persistent musty smell that cleaning does not fix, particularly noticeable when the room has been closed up, means moisture is sitting somewhere it cannot dry out.

6. Watch the shower screen and the door frame

   Silicone that will not stay stuck, a screen that has moved, or a swollen timber door frame at floor level all point the same way.

The signs, roughly in the order they appear

Grout that stays dark. Grout in a working shower dries out between uses. Grout that is still visibly darker than the rest a day later is holding water in the bed underneath.

Efflorescence. A white, chalky, slightly crystalline bloom in the grout lines or at the base of the tiles. It is mineral salts carried out of the screed by water that should not be there. This is one of the earliest reliable signs.

A hollow sound. Tap along the floor tiles with a knuckle. A change from solid to hollow marks where the tile has debonded from the bed, usually because water sat under it.

Paint or plaster failing on the other side of the wall. Bubbling paint in the hallway behind the shower is not a painting problem.

Skirting boards or door frames swelling in the room next door, or a stain appearing on the ceiling below an upstairs bathroom. By this stage the water has been moving for a long time.

A silicone joint that keeps going mouldy no matter how often you replace it. Silicone that will not stay clean is often being fed moisture from behind.

What is actually failing

In older Melbourne bathrooms the answer is usually that there was never a compliant membrane in the first place. Wet areas built before the current standards frequently relied on the tiles and grout themselves to hold water out — and grout is not waterproof. It is porous by nature.

In newer work, the failures cluster in the same few places: the junction where the floor meets the wall, around the floor waste, at the penetrations for taps and the shower rail, and at the hob or the step-down. These are the details AS 3740 spends most of its length on, and they are the details a rushed job skips.

Why patching does not work

The instinct is to re-grout, re-silicone, or paint on a sealer. All three are treating the symptom on the visible side of a barrier that has failed on the invisible side. Water that is already tracking under the tile bed will keep tracking; sealing the surface can even trap it.

A real fix means removing the tiles in the affected area, drying the substrate properly, replacing damaged sheeting or timber, applying a compliant membrane and re-tiling. In practice, for a bathroom of typical size, this usually means the whole room — which is why catching it early matters so much.

What it costs, and what waiting costs

A rebuild of a failed bathroom is a full renovation, so you are back in the range a complete bathroom occupies — roughly $18,000 to $35,000 in Melbourne depending on size and what has to move.

The cost of waiting is not linear. While the failure is confined to the bathroom, that is what you are paying for. Once it reaches the joists, the flooring in the next room, or a ceiling below, you are paying for those as well — and if it has been wet long enough, sometimes for structural timber.

Insurance and paperwork

Home insurance commonly excludes gradual water damage, which is exactly what this is, and commonly excludes damage caused by unlicensed work. If your bathroom was renovated by someone else, the waterproofing certificate is the document that matters. If it was renovated within the warranty period by a registered builder, that is worth checking before you pay for anything yourself.

Common mistakes

• Regrouting repeatedly instead of investigating why it keeps cracking
• Assuming a ceiling stain below is a one-off
• Sealing over the symptoms with silicone
• Waiting until it is visible from the room below — by then the subfloor is usually affected

Questions people ask

Can I just re-grout and hope?

Re-grouting fixes cosmetic grout. It does nothing for a failed membrane underneath, because the water is not getting in through the grout surface — it is already in the bed. If the symptoms are efflorescence, hollow tiles or damp on the other side of the wall, re-grouting buys nothing but time you do not want to spend.

How long does waterproofing last?

A compliant membrane, correctly detailed at the junctions, should outlast the tiles above it. When waterproofing fails early it is almost always a detailing or preparation problem rather than the membrane wearing out.

Is a leaking shower always waterproofing?

No. A failed silicone joint, a cracked shower base, a loose tap penetration or a blocked floor waste can all present similarly and are cheaper to fix. That is exactly why it is worth having it looked at before assuming the worst — or the best.

Can I do the waterproofing myself when it is rebuilt?

Legally in Victoria, yes — it is not licensed plumbing work. It must comply with AS 3740. We would still argue against it: it is the one component that is invisible after the job, fails silently, and costs the entire room again when it goes.

This guide is general information, not advice for your particular house. Rules change — check the current position with the Building and Plumbing Commission, Energy Safe Victoria and WorkSafe Victoria before you rely on it. Plumbing, gas and electrical work must be done by a licensed practitioner. If your home was built or last renovated before 1990, assume asbestos until it is ruled out — see what a strip-out finds.