Bathroom Waterproofing Guide: Australian Standards and Best Practice

What the Law Requires

Under AS 3740 (Waterproofing of Domestic Wet Areas) and adopted into the National Construction Code, waterproofing must be applied in bathrooms, showers, and laundries. The work must be done by a licensed waterproofer, and compliance certificates must be issued. This is not optional — it's a building code requirement.

Minimum Coverage Requirements

• 1 Shower floor: full membrane to drain, minimum 100mm up the wall
• 2 Shower walls: minimum 1800mm height from floor
• 3 Adjacent floor area (bathroom floor outside shower): full membrane
• 4 Bath surrounds: if tiled — full wall coverage behind bath
• 5 Around the toilet: 300mm each side and behind the pan on the floor

Substrate Requirements

The wall substrate behind tiles in wet areas must be cement-based board (fibre cement sheet, villaboard, or compressed fibre cement) — never standard plasterboard/drywall. Standard plasterboard absorbs moisture even when tiled, eventually deteriorating behind the tiles and leading to de-bonding.

The Flood Test

Before tiling, the waterproofing should be tested by plugging the drain and filling the shower to 25mm depth. Leave for 24 hours and check for any reduction in water level. This simple test can identify failures before tiles are laid — fixing a waterproofing failure before tiling costs hundreds; fixing it after tiling costs thousands.