Is a Waterproof Membrane Required Beneath Timber Flooring in Wet Areas?

The recent updates to the New Zealand Building Code Clause E3 (Internal Moisture) can be tricky to navigate, and one of the top questions we get is “When is a full waterproof membrane genuinely necessary?” 

Whether it’s a standalone home or a multi-unit development, here’s the clear answer backed by CodeMark certification. 

When did this change Happen?  

Clause E3 has long required building elements in wet areas to resist moisture. In 2021, the introduction of Amendment 7 to Acceptable Solution E3/AS1 revised the compliance pathway for installing timber flooring in “wet areas” (including kitchens, bathrooms and laundries). There has been some misinterpretation of these changes, which have caused specifiers to default to E3/AS2— a waterproof membrane beneath all subfloors—even when it isn’t strictly required. 

So why is this an issue? Many waterproof membrane systems aren't designed to work with timber flooring adhesives, and timber flooring installers aren't generally approved to apply membrane systems. Therefore, using a membrane with timber flooring can add various layers of complexity and unnecessary cost.  

When a Membrane Is Not Required 

  If your specification meets all three of the following criteria, you may adopt Forté’s E3 Alternative Solution: 

• Forté’s Alternative Solution can be used in any wet areas that exclude a bath or shower e.g., kitchens, toilets, and laundries.  

• In multi-residential, the flow rate of tap must not exceed the tap-overflow capacity. 

3. Specify an Approved Substrate 

New timber subfloor:

• Permitted: H3.2-treated plywood or H1.2 Solid Pinus only 

• Not Permitted: Particleboard, OSB or non-H3 ply. (If these are present, a full E3/AS2 membrane is mandatory.) 

Existing Solid-Wood Subfloor:

• Apply a two-component epoxy moisture barrier (per Forté specifications) before laying flooring.
  

Concrete slab:

• No membrane is required– follow our general Timber Flooring Design Guide 

Other substrates:

• Contact us for tailored guidance 

What is Forté’s E3 Alternative Solution? 

Our Alternative Solution is a tested, CodeMark-approved system that allows compliance when installing timber flooring in wet areas without the need for a traditional waterproof membrane.  

Benefits include: 

• Designed for Timber flooring: Many waterproof membranes are NOT designed for timber flooring use 

• Fewer trades: A single installer can complete the entire process—no separate waterproofing contractor is required. 

• Cost savings: Eliminate membrane application expenses, saving both time and money. 

• Compliance confidence: CodeMark certification guarantees E3 compliance , streamlining the specification process.  

When This Guidance Does Not Apply 

• Non-treated subfloors: If the subfloor beneath your wet area is particleboard, oriented strand board, or untreated plywood, you’ll need to use the E3/AS2 compliance pathway and specify a waterproof membrane as per Forté’s Timber Flooring Design Guide.. 
• Non wet areas: For any non-wet areas, this will be outside of E3 requirements and can be installed directly without Alternative solution or E3/AS2. 

On-Site Implementation: What to Expect 

When using Forté’s Alternative Solution, the following steps must be carried out during installation: 

1. Glue Down installation: Timber flooring is fully glued down to the approved substrate. 

1. Sealing Joints: Apply a water-resistant D3 PVA sealer to all joints located within 1.5 metres of any sanitary fixture or appliance. 

1. Caulking Perimeter: Use water-resistant silicone gap filler around the entire room perimeter and around any fixed items. 

Additional requirements: 

• Moisture Testing: All Forté Timber Flooring products have successfully passed the E3 moisture test. 

• Existing Solid Timber Subfloor: A two-component epoxy moisture barrier must be applied before installation. 

How to Specify the Alternative Solution 

2. Timber Flooring Design Guide

3. E3 Compliance Pack  

If you need further clarification or want to discuss a specific project, Forté’s technical team is always available to assist.