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Interior Wet Area Waterproofing

Interior Wet Area Waterproofing

Bathrooms - the devil is in the detail

Questions regarding waterproofing of interior wet areas are one of the most commons topics of enquiry received by the HIA
Technical team. Everyone in the industry has heard horror stories about leaking showers or bathroom areas; and defects related to waterproofing are high on the list of findings from the NSW Building Commissioner auditing of apartment construction.

This article provides an overview of the provisions regarding internal wet area waterproofing within the National Construction Code (NCC), the relevant Australian Standard, and seeks to answer some of the most frequently asked questions related to bathrooms.

What is a wet area?

The NCC defines a wet area to include bathrooms, showers, laundries and sanitary compartments in all classes of building. It specifically excludes kitchens, bar areas, kitchenettes and domestic food and beverage preparation areas from the definition, so the wet area waterproofing requirements do not apply to these areas.

NCC Waterproofing Provisions

The wet area Performance Requirements of both Volume One and Two of the NCC (FP1.7 and P2.4.1 respectively), covering
all classes of building, require that water must be prevented from penetrating behind fittings and linings or into concealed

Functionally, buildings must be constructed to avoid dampness or overflow from wet areas creating unhealthy or dangerous conditions, or damage to building elements. This is to safeguard occupants from illness or injury, and to protect the structure of the building.

The general waterproofing requirements found in Table F1.7 of Volume One and Table of Volume Two are nearly
identical. However, following the table is not enough to demonstrate compliance – the installation must also comply
with Australian Standard AS 3740 – Waterproofing of domestic wet areas.

To apply the NCC tables and Australian Standard correctly, as a minimum:

  • all wall junctions and joints, wall-floor junctions and all penetrations must be waterproof in shower areas
  • the presence, location and type (enclosed or unenclosed) of shower, and whether or not it uses a hob or step-down (or used a preformed shower base) will dictate the extent of waterproofing required for the walls, floors and other horizontal surfaces in the shower area
  • the materials used for the structural floor of the wet area – timber-based flooring requires full floor waterproofing, unlike for concrete or fibre cement.

The type and location of any spas or baths will dictate what waterproofing is required in their vicinity – inset baths and
spas require more waterproofing of the wall and wall junctions than freestanding baths (which require water resistance), but
don’t require waterproofing of the floor underneath. Waterproofing of the floor beneath a freestanding bath depends on the flooring material used as noted above.

Standards requirements

The NCC sets the requirements for when and what elements must be provided with waterproofing protection but does
not provide the specific detail of how to provide the waterproofing to those elements.

Australian Standard AS 3740 – Waterproofing of domestic wet areas contains the detailed construction and material requirements for the waterproofing of wet areas. It also defines materials considered waterproof vs water resistant, as well as providing specific details for waterproofing around penetrations and junctions. Familiarity with the standard is a must for anyone installing or supervising/ inspecting waterproofing of wet areas.

Close attention must be paid to the presence and correct installation of water stops, puddle flanges, hobs (where specified), and fillets or bond-breakers. 

Common enquiries HIA receives questions on and are raised in construction audits include:

  • requirements for water stops and their correct installation 
  • correct installation of puddle flanges to allow correct water drainage
  • the correct design and detailing of hobs and niches
  • understanding the application and use of fillets or bond breakers, which are required at certain locations such as wallfloor junctions to allow the waterproofing membrane to be more resistant to building movement in these locations.

Other standards to be aware of when constructing wet areas include the ceramic tiling standards AS 3958.1 and AS 3958.2, which cover selection and installation of tiling systems. Membranes used should comply with AS/NZS 4858, and tile adhesives should comply with AS 2358 – check with your supplier or the manufacturer for details.

Frequently asked questions on bathrooms and wet areas:

Q: Why do I need 90 per cent coverage on tile adhesives?

A: Having less coverage may facilitate a failure with the tiling system because the load from the tile will be concentrated over a smaller area. This could even damage the waterproofing. Better coverage will also help to resist movement and cracking.

Q: Are falls to wastes outside the shower area required?

A: Floor wastes are only required by the Deemed-to-satisfy Provisions of the NCC, so you first need to determine if this applies to your project. 

These NCC provisions state floor wastes are required:

  • for shower areas, as per AS 3740 wet area waterproofing requirements in the NCC
  • in a room containing a urinal
  • in a Class 2, 3 or 4 part of a building when a bathroom or laundry is located above a sole occupancy unit or public space.
    The above parts are required by the NCC to be graded to the floor waste to help prevent water overflow from breaching the other units in the same building.
    Floor wastes are also installed in other areas – where specified under the plumbing code, or in the laundry of a Class 1 building. As neither of these is required waste as defined in the building code, a fall is not needed.


Q: Do I need to provide puddle flanges to a waste pipe with a concrete slab substrate?

A: ‘Yes’. Regardless of the substrate a puddle flange is required as per AS 3740. It is recommended to be rebated into the slab to be set at the lowest level otherwise water can pond around the flange.

Training and Qualifications

If you possess a full general building licence you are able to conduct your own waterproofing work in NSW. Contractors performing wet area waterproofing installations or supervisors of those installations must possess the correct
occupational licence for waterproofing as required by NSW Fair Trading. Generally, this will require completion of a Certificate
III in waterproofing as listed on the NSW Fair Trading licensing website.

If you already have experience in wet area waterproofing but need to refresh your skills and knowledge, HIA runs a two-day workshop on wet area waterproofing periodically – contact HIA Training at 02 9978 3333 for more information, or visit the Training page on the HIA website.


- Joshua Burg, HIA Assistant Director Building
Source: HIA Hunter Building News, Issue 1 2021