AS3740-2021 waterproofing, explained.
AS3740 is the Australian Standard that governs waterproofing wet areas inside residential buildings. The 2021 update tightened membrane systems, hob details and shower-floor coverage. Here's what compliant work actually looks like on site.
What must be waterproofed
Shower floor plus 150mm up all walls, shower walls to 1800mm, entire bathroom floor if the door step is less than 25mm above finished floor level, and the full perimeter to 150mm on walls behind vanities and baths.
The membrane system we install
Primer, then bond-breaker at internal corners, then two liquid coats — perpendicular to each other — of a certified membrane. Total dry film thickness is verified before tiling.
How to check a certificate is real
A legitimate AS3740 certificate names the individual applicator, lists the product used, includes photographic evidence, and is signed on the day of application — not backdated.
Common non-compliance we see
The failures we most often uncover behind a leaking Melbourne shower: single-coat membranes (spec is two), missing bond-breaker at internal corners, membrane skipped behind hob timbers, screed laid without falls to the drain, and 'topical' membranes painted over existing tiles as a supposed fix. All of these void insurance claims.
Substrate preparation before membrane
AS3740 compliance starts before the membrane goes on. Timber substrates need compressed fibre-cement sheeting or equivalent, screwed at 200mm centres. Concrete slabs need primer suited to the moisture content, verified with a hygrometer. Corners need 45mm cove-flashing where walls meet floor. Skip any of these and even the best membrane will delaminate within five years.
Warranty implications
In Victoria, waterproofing carries a statutory six-year warranty. Our own workmanship warranty is six years on the membrane and repair area, backed by our $20M public liability insurance. Insist on both — a builder unable to warrant six years is telling you something about their confidence in the installation.