Bathroom waterproofing, explained properly

Updated 19 Aug 2026/8 min read/By Manttan Construction · QBCC 15030266

AS 3740:2021 and NCC housing provisions — key requirements

1800 mm	Minimum waterproofing height on shower walls above the finished floor level — or 50 mm above the shower rose if that is higher.
150 mm	Typical minimum height for waterproofing at wall junctions outside the shower, and up the wall around a built-in bath.
1:80 to 1:50	Permitted fall to a floor waste — between 12.5 mm and 20 mm of drop per metre.
Whole shower floor	Fully waterproofed, with the membrane continuous through junctions and penetrations.
Category 1, 2, 3	Wet area risk classes: high risk (shower areas and anything within reach of a hand-held rose), moderate risk (bathroom floor outside an unenclosed shower), low risk (wet areas with no shower).
No particleboard	Since the 2021 revision, particleboard sheeting is not acceptable as a substrate beneath a waterproofing layer.
Timber or upper floors	Where the substrate contains timber, or the bathroom is above ground floor, the whole floor is normally waterproofed rather than just the shower.

01 — Why it matters

Why does waterproofing fail so often?

Because it is invisible, fast to skip and slow to expose. A membrane applied too thin, tiled over before it cured, or run short at a junction will hold water for a year or two and then start showing up as mould that returns, hollow tiles, a musty smell, or staining on the wall in the next room.

By then the fix is not a patch — it is stripping the tiles, replacing the substrate and starting the wet area again. The cost of doing it properly the first time is a rounding error against that.

02 — The standard

What does AS 3740:2021 actually require?

The standard sets out materials, design and installation for domestic wet areas, and classifies areas by risk. The headline requirements a home owner can check:

• Shower walls waterproofed to at least 1800 mm above finished floor level, or 50 mm above the rose if higher.
• The entire shower floor waterproofed, with falls to the waste between 1:80 and 1:50.
• Junctions between floors and walls, and around baths, sealed with flexible materials and bond breakers.
• Penetrations — wastes, mixer bodies, pipes — sealed with proper collars or flanges.
• A vertical water stop where a shower sits close to the wet area exit.
• Membrane inspected before any tiling goes over it.

The 2021 revision tightened substrate rules, membrane grading and compatibility between products. Mixing a primer from one system with a membrane from another is a common and avoidable failure.

03 — Sequence

What does the process look like on site?

The order that keeps water out

• Strip out to substrate; assess framing, any rot or termite damage and the existing fall.
• Rough-in plumbing and electrical, then screed the floor to establish the fall to the waste.
• Prepare and prime the substrate — clean, dry, sound and compatible with the membrane.
• Apply bond breakers to all junctions and internal corners.
• Two or more membrane coats at the specified thickness, with full curing time between each.
• Inspection and, where required, a flood test before tiling.
• Tile, then grout and seal with the right silicone at movement joints.

04 — Diagnosis

How do I tell if my existing bathroom is leaking?

The membrane is hidden under tiles, so diagnosis is a matter of reading the symptoms:

• Mould that keeps returning in the same spot no matter how often you clean.
• Water stains or bubbling paint on the other side of a shower wall, or on the ceiling below.
• Tiles that sound hollow when tapped, or grout that keeps cracking out.
• Water pooling in the shower instead of draining away — a fall problem.
• A persistent musty smell, or swollen skirtings or door frames nearby.

Sometimes a leaking shower can be repaired without a full strip-out. Often, in a bathroom built to the older standard, the honest answer is that a re-do is better value than a patch — and we will tell you which one you are looking at.

05 — Paperwork

What documentation should I get?

Ask for the waterproofing certificate or installer declaration naming the products used, the areas covered and the date. Keep it with your approval documents and any inspection reports.

That paperwork does two jobs: it supports a warranty claim if something fails, and it answers the question a buyer or their solicitor asks when they see a recently renovated bathroom.

06 — Choices

Which decisions have the biggest long-term effect?

• Linear drain vs centre waste. A linear drain lets you run large-format tiles with a single-direction fall and no awkward cuts.
• Niche vs shelf. A recessed niche must be waterproofed as carefully as the floor — it is a common leak point when rushed.
• Frameless glass. Beautiful, but the screen fixings and the hob or hobless detail have to be planned before the membrane goes on, not after.
• Epoxy grout. Costs more, stays cleaner in a family bathroom, and is worth it in the shower floor.
• Ventilation. An exhaust fan ducted outside — not into the roof space — does more for mould than any cleaning product.

Questions

Waterproofing FAQ

Still unsure how it applies to your place? Call 0402 847 873 and we will tell you straight.

How high must shower walls be waterproofed in Australia?+

Under AS 3740:2021 the shower wall lining must be waterproofed to a minimum of 1800 mm above the finished floor level, or 50 mm above the shower rose if that is higher.

What fall does a bathroom floor need?+

Falls to a floor waste must be between 1:80 and 1:50 — that is, between 12.5 mm and 20 mm of drop per metre. Too little and water pools; too much and it feels wrong underfoot and tiles are hard to lay.

How long should waterproofing cure before tiling?+

Follow the membrane manufacturer instructions for the conditions on the day — it varies by product, temperature and humidity, and is usually a matter of days rather than hours. Tiling over an uncured membrane is one of the most common causes of failure.

Do I need council approval to renovate a bathroom?+

A like-for-like refit in the same footprint often does not, but moving fixtures needs plumbing approval and a licensed plumber, and structural changes need building approval. We confirm the pathway before demolition — see our council approvals guide.

Bathroom renovations→ Council approvals explained→ Licences, contracts & insurance→ All guides→

Get advice & a quote → 0402 847 873