Wet Areas

Poor wet area design or installation can compromise safety, comfort and convenience for building users, and lead to significant structural damage.

Wet area floor structure

Wet area floors can be concrete, or timber or steel framed. Each has pros and cons.

On this page

  • Concrete floors
  • Framed floors

The floor structure is the rigid support that forms the substrate for the finished surface, or it may be the finished surface itself.

In addition to meeting the structural requirements of NZBC clauses B1 Structure and B2 Durability, where the floor is between separate tenancies it must also meet the requirements of clause G6 Airborne and impact sound and clause C Protection from fire.

The floor structure for wet areas may be:

  • concrete – slab-on-ground or suspended
  • framed in timber or steel.

Concrete floors

Both slab-on-ground and suspended concrete floors for wet areas:

  • provide a good substrate for a wide range of wet area finishes
  • can be laid with falls and set downs
  • can be insulated and heated (insulation is mandatory if heated)
  • require careful planning and coordination of construction to accommodate sanitary plumbing
  • must have sanitary drainage pipes installed before the concrete is poured
  • are not easy to alter or retrofit for sanitary plumbing and drainage.

Concrete suspended floors should also incorporate a floor waste and require the floor/ceiling space below to be deep enough to accommodate pipe falls and bends.

If a bathroom design includes an open shower, be aware of specific fall requirements:

  • Acceptable Solution E3/AS1 ( states that the floor of open showers must have a fall of no less than 1:50 towards the floor waste, within a radius of 1500 mm taken from a point vertically below the shower rose, or from any wall within that radius.
  • The Code of Practice for Internal Wet Area Membranes (referenced by E3/AS2) requires that with floor membranes, “Falls must be formed in the substrate (or in a screed incorporated into the substrate) and not by the membrane itself.”


Framed floors

Timber-framed floors:

  • provide a good substrate for a wide range of wet area finishes
  • require protection from water damage, e.g. use of treated timber
  • are difficult to lay with falls
  • easily accommodate sanitary plumbing and drainage changes
  • allow easy alteration or retrofitting of sanitary plumbing and drainage
  • can be insulated and heated.

BRANZ recommends that all timber framing for wet area floors is treated H1.2 (boric) as a minimum, regardless of Code requirements. BRANZ also recommends that, where there is risk of water damage to framing surrounding or supporting a shower, the framing should be treated to H3.1 to provide an additional level of safety in case the waterproofing is compromised.

Where the flooring will be covered with a waterproof membrane and tiles, the substrate should be compressed fibre-cement sheet, fibre-cement sheet tile underlay, or plywood (treated to H3.2 with CCA, not LOSP).

Flooring-grade particleboard is not permitted as a new substrate in any wet area under the membrane manufacturers’ Code of Practice for Internal Wet-area Membrane Systems. NZS 3602:2003 Timber use in building recommends H3 treated plywood rather than particleboard in this situation. Use of particleboard is at the building consent authority’s discretion.

If particleboard is used, it should be H3.1-treated, and particleboard manufacturers require the whole wet area floor to be waterproofed, including under baths and vanities.


E3/AS2 Internal Wet-area Membrane Systems

The Acceptable Solution references the membrane manufacturers’ Code of Practice for Internal Wet-area Membranes with a few amendments. Membranes installed in accordance with sections 1–4 of the Code of Practice (as modified by the Acceptable Solution) will comply with Building Code clauses E3.3.2 – E3.3.6.



Updated: 10 June 2022